"...... The fine nets which oft we woven seeOf scorched dew,"
"...... The fine nets which oft we woven seeOf scorched dew,"
says Spenser. Another, fellow to it, and equally absurd, was that adopted by a learned man and good natural philosopher, and one of the first fellows of the Royal Society, Robert Hooke, the author ofMicrographia. "Much resembling a cobweb," says he, "or a confused lock of these cylinders, is a certain white substance which, after a fogg, may be observed to fly up and down the air: catching several of these, and examining themwith my microscope, I found them to be much of the same form, looking most like to a flake of worsted prepared to be spun; though by what means they should be generated or produced is not easily imagined: they were of the same weight, or very little heavier than the air;and 'tis not unlikely, but that those great white clouds, that appear all the summer time, may be of the same substance[524]." So liable are even the wisest men to error when, leaving fact and experiment, they follow the guidance of fancy. Some French naturalists have supposed that thesefils de la Vierge, as they are called in France, are composed of the cottony matter in which the eggs of the Coccus of the vine (C. Vitis) are enveloped[525]. In a country abounding in vineyards this supposition would not be absurd; but in one like Britain, in which the vine is confined to the fruit-garden, and the Coccus seldom seen out of the conservatory, it will not at all account for the phænomenon. What will you say, if I tell you that these webs (at least many of them) are air-balloons—and that the aëronauts are not
"Lovers who may bestride the gossamerThat idles in the wanton summer air,And yet not fall"—
"Lovers who may bestride the gossamerThat idles in the wanton summer air,And yet not fall"—
butspiders, who long before Montgolfier, nay, ever since the creation, have been in the habit of sailing through the fields of ether in these air-light chariots!This seems to have been suspected long ago by Henry Moore, who says,
"As light and thin as cobwebs that do flyIn the blew air, caus'd by the autumnal sun,That boils the dew that on the earth doth lie,May seem this whitish rag then is the scum;Unless that wiser men make't the field-spider's loom[526]."
"As light and thin as cobwebs that do flyIn the blew air, caus'd by the autumnal sun,That boils the dew that on the earth doth lie,May seem this whitish rag then is the scum;Unless that wiser men make't the field-spider's loom[526]."
Where he also alludes to the old opinion of scorched dew. But the first naturalists who made this discovery appear to have been Dr. Hulse and Dr. Martin Lister—the former first observing that spiders shoot their webs into the air; and the latter, besides this, that they were carried upon them in that element[527]. This last gentleman, in fine serene weather in September, had noticed these webs falling from the heavens, and in them discovered more than once a spider, which he named thebird. On another occasion, whilst he was watching the proceedings of a common spider, the animal suddenly turning upon its back and elevating its anus, darted forth a long thread, and vaulting from the place on which it stood, was carried upwards to a considerable height. Numerous observations afterwards confirmed this extraordinary fact; and he further discovered, that while they fly in this manner, they pull in their long thread with their fore feet, so as to form it into a ball—or, as we may call it, air-balloon—of flake. The height to which spiders will thus ascend he affirms is prodigious. One day in the autumn, when the air was full of webs, he mounted to the top of the highest steeple of York minster, from whence he could discern the floating webs still veryhigh above him. Some spiders that fell and were entangled upon the pinnacles he took. They were of a kind that never enter houses, and therefore could not be supposed to have taken their flight from the steeple[528]. It appears from his observations, that this faculty is not confined to one species of spider, but is common to several, though only in their young or half-grown state[529]; whence we may infer, that when full-grown their bodies are too heavy to be thus conveyed. One spider he noticed that at one time contented itself with ejaculating a single thread, while at others it darted out several, like so many shining rays at the tail of a comet. Of these, in Cambridgeshire in October, he once saw an incredible number sailing in the air[530]. Speaking of hisAr. subfuscus minutissimis oculis, &c. he says, "Certainly this is an excellent rope-dancer, and is wonderfully delighted with darting its threads: nor is it only carried in the air, like the preceding ones; but it effects itself its ascent and sailing: for, by means of its legs closely applied to each other, it as it were balances itself, and promotes and directs its course no otherwise than as if nature had furnished it with wings or oars[531]." A later but equally gifted observer of nature, Mr. White, confirms Dr. Lister's account. "Every day in fine weather in autumn," says he, "do I see these spiders shooting out their webs, and mounting aloft: they will go off from the finger, if you will take them into your hand. Last summer onealighted on my book as I was reading in the parlour; and running to the top of the page and shooting out a web, took its departure from thence. But what I most wondered at was, that it went off with considerable velocity in a place where no air was stirring; and I am sure that I did not assist it with my breath. So that these little crawlers seem to have while mounting some locomotive power without the use of wings, and move faster than the air in the air itself[532]." A writer in the last number of Thomson'sAnnals of Philosophy[533], under the signature of Carolan, has given some curious observations on the mode in which some geometric spiders shoot and direct their threads, and fly upon them; by which it appears, that as they dart them out they guide them as if by magic, emitting at the same time a stream of air, as he supposes, or possibly some subtile electric fluid. One which was running upon his hand, dropped by its thread about six inches from the point of his finger, when it immediately emitted a pretty long line at a right angle with that by which it was suspended. This thread, though at first horizontal, quickly rose upwards, carrying the spider along with it. When it had ascended as far above his finger as it had dropped before below it, it let out the thread by which it had been attached to it, and continued flying smoothly upwards till it nearly reached the roof of the room, when it veered on one side and alighted on the wall. In flying, its motion was smoother and quicker than when a spider runs along its thread. He observes, that as the line lengthens behind them, the tendency of spiders to rise increases.—I have myself more than once observed these creaturestake their flight, and find the following memorandum with respect to their mode of proceeding. "The spider first extends its thighs, shanks, and feet, into a right line, and then elevating its abdomen till it becomes vertical, shoots its thread into the air, and flies off from its station." It is not often, however, that an observer can be gratified with this interesting sight, since these animals are soon alarmed. I have frequently noticed them—for at the times when these webs are floating in the air they are very numerous—on the vertical angle of a post, or pale, or one of the uprights of a gate, with the end of their abdomen pointing upwards, as if to shoot their thread previously to flying off; when, upon my approaching to take a nearer view, they have lowered it again, and persisted in disappointing my wish to see them mount aloft. The rapidity with which the spider vanishes from the sight upon this occasion and darts into the air, is a problem of no easy solution. Can the length of web that they dart forth counterpoise the weight of their bodies; or have they any organ analogous to the natatory vesicles of fishes[534], which contributes at their will to render them buoyant in the air? Or do they rapidly ascend their threads in their usual way, and gather them up, till having collected them into a mass of sufficient magnitude, they give themselves to the air, and are carried here and there in these chariots? I must here give you Mr. White's very curious account of a shower of these webs that he witnessed. On the 21st of September 1741, intent upon field diversions, he rose before day-break; but on going out, he found the whole face of the country covered with a thick coat of cobweb,drenched with dew, as if two or three setting-nets had been drawn one over the other. When his dogs attempted to hunt, their eyes were so blinded and hoodwinked that they were obliged to lie down and scrape themselves. This appearance was followed by a most lovely day. About nine A. M. a shower of these webs (formed not of single floating threads, but of perfect flakes, some near an inch broad, and five or six long,) was observed falling from very elevated regions, which continued without interruption during the whole of the day;—and they fell with a velocity which showed that they were considerably heavier than the atmosphere. When the most elevated station in the country where this was observed was ascended, the webs were still to be seen descending from above, and twinkling like stars in the sun, so as to draw the attention of the most incurious. The flakes of the web on this occasion hung so thick upon the hedges and trees, that baskets-full might have been collected. No one doubts, he observes, but that these webs are the production of small spiders, which swarm in the fields in fine weather in autumn, and have a power of shooting out webs from their tails, so as to render themselves buoyant and lighter than the air[535]. In Germany these flights of gossamer appear so constantly in autumn, that they are there metaphorically called "Der fliegender Sommer" (the flying or departing summer); and authors speak of the web as often hanging in flakes like wool on every hedge and bush throughout extensive districts.
Here we may inquire—Why is the ground in these serene days covered so thickly by these webs, and whatbecomes of them? What occasions the spiders to mount into the air, and do the same species form both the terrestrial and aërial gossamer?—And what causes the webs at last to fall to the earth? I fear I cannot to all these queries return a fully satisfactory answer; but I will do the best I can. At first one would conclude from analogy, that the object of the gossamer which early in the morning is spread over stubbles and fallows—and sometimes so thickly as to make them appear as if covered with a carpet, or rather overflown by a sea, of gauze, presenting, when studded with dew-drops, as I have often witnessed, a most enchanting spectacle—is to entrap the flies and other insects as they rise into the air from their nocturnal station of repose, to take their diurnal flights. But Dr. Strack's observations render this very doubtful; for he kept many of the spiders that produce these webs in a large glass upon turf, where they spun as when at liberty, and he could never observe them attempt to catch or eat—even when entangled in their webs—the flies and gnats with which he supplied them; though they greedily sucked water when sprinkled upon the turf, and remained lively for two months without other food[536]. As the single threads shot by other spiders are usually their bridges, this perhaps may be the object of the webs in question: and thus the animals may be conveyed from furrow to furrow or straw to straw less circuitously, and with less labour, than if they had travelled over the ground. As these creatures seem so thirsty, may we not conjecture that the drops of dew, with which they are always as it were strung, are a secondaryobject with them? So prodigious are their numbers, that sometimes every stalk of straw in the stubbles, and every clod and stone in the fallows, swarms with them. Dr. Strack assures us that twenty or thirty often sit upon a single straw, and that he collected about 2000 in half an hour, and could have easily doubled the number had he wished it: he remarks, that the cause of their escaping the notice of other observers, is their falling to the ground upon the least alarm.
As to what becomes of this immense carpeting of web there are different opinions. Mr. White conjectures that these threads, when first shot, might be entangled in the rising dew, and so drawn up, spiders and all, by a brisk evaporation, into the region where the clouds are formed[537]. But this seems almost as inadmissible as that of Hooke, before related. An ingenious and observant friend, thinking the numbers of the flying spiders not sufficient to produce the whole of the phenomenon in question, is of opinion that an equinoctial gale, sweeping along the fallows and stubbles coated with the gossamer, must bring many single threads into contact, which, adhering together, may gradually collect into flakes; and that being at length detached by the violence of the wind, they are carried along with it: and as it is known that such winds often convey even sand and earth to great heights, he deems it highly probable that so light a substance may be transported to so great an elevation, as not to fall to the earth for some days after, when the weather has become serene, or to descend upon ships at sea, as has sometimes happened. This, which is in part adopted from the German authors, is certainlya much more reasonable supposition than the other; but some facts seem to militate against it: for, in the first place, though gossamer often occurs upon the ground when there is none in the air, yet the reverse of this has never been observed; for gossamer in the air, as in the instance recorded by Mr. White, is always preceded by gossamer on the ground. Now, since the weather is constantly calm and serene when these showers appear, it cannot be the wind that carries the web from the ground into the air. Again, it is stated that these showers take place afterseveralcalm days[538]: now, if the web was raised by the wind into the air, it would begin to fall as soon as the wind ceased. Whence I am inclined to think that the cause assigned by Dr. Lister is the real source of the whole phenomenon. Though ordinary observers have overlooked them, he noticed these spiders in the air in such prodigious numbers, that he deemed them sufficient to produce the effect. I shall not, however, decide positively; but, having stated the different opinions, leave you to your own judgement.
The next query is, What occasions the spiders to mount their chariots and seek the clouds? Is it in pursuit of their food? Insects, in the fine warm days in which this phenomenon occurs, probably take higher flights than usual, and seek the upper regions of the atmosphere; and that the spiders catch them there, appears by the exuviæ of gnats and flies, which are often found in the falling webs[539]. Yet one would suppose that insects would fly high at all times in the summer in serene warm weather. Perhaps the flight of some particular species constituting a favourite food of our littlecharioteers—the gnats, for instance, which we have seen sometimes rise in clouds into the air[540]—may at these times take place; or the species of spiders that are most given to these excursions, may not abound in their young state—when only they can fly—at other seasons of the year.
Whether the same species that cover the earth with their webs produce those that fill the air, is to be our next inquiry. Did the appearance of the one always succeed that of the other, this might be reasonably concluded:—but the former, as I lately observed to you, often occurs without being followed by the latter. Yet, since it should seem that the aërial gossamer, though it does not always follow it, is always preceded by the terrestrial, this warrants a conjecture that they may be synonymous. Two German authors, Bechstein[541]and Strack[542], have described the spider that produces gossamer in Germany under the name ofAranea obtextrix[542]. But it is not clear, unless they have described it at different ages, when spiders often greatly change their appearance, that they mean the same species. The former describes his as of the size of a small pin's head, with its eight eyes disposed in a circle, having a black-brown body and light-yellow legs: while Dr. Strack represents hisA. obtextrixas more than two lines in length; eyes four in a square, and two on each side touching each other; thorax deep brown with paler streaks; abdomen below dull white, above dark copper brown, with a dentated white spot running longitudinally down the middle.The first of these, if distinct, as I suspect they are, agrees very well with the young of one which Lister observed as remarkable for taking aërial flights[543]; and which I have most usually seen so engaged. The other may possibly be that before noticed, which he found in such infinite numbers in Cambridgeshire[544]. If this conjecture be correct, it will prove that the same species first produce the gossamer that covers the ground, and then, shooting other threads, mount upon them into the air.
My last query was, What causes these webs ultimately to fall to the earth? Mr. White's observation will I think furnish the best answer. "If the spiders have the power of coiling up their webs in the air, as Dr. Lister affirms, then when they become heavier than the air they will fall[545]." The more expanded the web, the lighter and more buoyant, and the more condensed, the heavier it must be.
I trust you will allow from this mass of evidence, that the EnglishArachnologists—may I coin this term?—were correct in their account of this singular phenomenon; and think, with me, that Swammerdam (who however admits that spiders sail on their webs), and after him De Geer, were rather hasty when they stigmatized the discovery that these animals shoot their webs into the air, and so take flight, as a strange and unfounded opinion[546]. The fact, though so well authenticated, is indeed strange and wonderful, and affords another proof of the extraordinary powers, unparalleled in the higher orders of animals, with which the Creator has gifted the insect world. Were indeed man and the larger animals, with their present propensities, similarly endowed, the whole creationwould soon go to ruin. But these almost miraculous powers in the hands of these little beings only tend to keep it in order and beauty. Adorable is that Wisdom, Power, and Goodness, that has distinguished these next to nothings by such peculiar endowments for our preservation as if given to the strong and mighty would work our destruction.
After the foregoing marvellous detail of the aërial excursions of our insect air-balloonists, I fear you will think the motions of those which fly by means ofwingsless interesting. You will find, however, that they are not altogether barren of amusement. Though the wings are the principal instruments of the flight of insects, yet there are others subsidiary to them, which I shall here enumerate, considering them more at large under the orders to which they severally belong. These are wing-cases (Elytra,Tegmina, andHemelytra); winglets (Alulæ); poisers (Halteres); tailets (Caudulæ); hooklets (Hamuli); base-covers (Tegulæ), &c. Besides, theirtails,legs, and evenantennæ, assist them in some instances, in this motion.
Aswingsare common to almost the whole class, I shall consider their structure here. Every wing consists of two membranes, more or less transparent, applied to each other: the upper membrane being very strongly attached to the nervures (Neuræ), and the lower adhering more loosely, so as to be separable from them. The nervures[547]are a kind of hollow tube,—above elastic, horny, and convex; and flat and nearly membranaceousbelow,—which take their origin in the trunk, and keep diminishing gradually, the marginal ones excepted, to their termination. The vessels contained in the nervures consist of a spiral thread, whence they appear to be air-vessels communicating with the tracheæ in the trunk.—The expansion of the wing at the will of the insect, is a problem that can only be solved by supposing that a subtile fluid is introduced into these vessels, which seem perfectly analogous to those in the wings of birds; and that thus an impulse is communicated to every part of the organ, sufficient to keep it in proper tension. We see by this, that a wing is supported in its flight like a sail by its cordage[548]. It is remarkable that those insects which keep the longest on the wing, the dragon-flies (Libellulina) for instance, have their wings most covered with nervures. The wings of insects in flying, like those of other flying animals, you are to observe, move vertically, or up and down.
In considering the flight of insects, I shall treat of that of each order separately, beginning with theColeopteraor beetles. Their subsidiary instruments of flight are their wing-cases (Elytra), and in one instance, winglets (Alulæ). The former[549], which in some are of a hard horny substance, and in others are softer and more like leather, though they are kept immoveable in flight, are probably, by their resistance to the air, not without their use on this occasion. The winglets are small concavo-convex scales, of a stiff membranaceous substance, generally fringed at their extremity[550]. I know at presentof only one coleopterous insect that has them (Dytiscus marginalis). They are placed under the elytra at their base. Their use is unknown; but it may probably be connected with their flight. The wings of beetles[551]are usually very ample, often of a substance between parchment and membrane. The nervures that traverse and extend them, though not numerous, are stronger and larger than those in the wings of insects of the other orders, and are so dispersed as to give perfect tension to the organ. When at rest—except inMolorchus,Atractocerus,Necydalis, and some other genera—they are folded transversely under the elytra, generally near the middle, with a lateral longitudinal fold, but occasionally near the extremity[552]. When they prepare for flight, their antennæ being set out, the elytra are opened so as to form an angle with the body and admit the free play of the wings, and they then fly off, striking the air by the vertical motion of these organs, the elytra all the while remaining immoveable. During their flight the bodies of insects of this order, as far as I have observed them, are always in a position nearly vertical, which gives to the larger sorts, the stag-beetle for instance, a very singular appearance. Olivier, probably having some of the larger and heavier beetles in his eye, affirms that the wings of insects of this order are not usually proportioned to the weight of their bodies, and that the muscular apparatus that moves them is deficient in force. In consequence of which, he observes, they take flight with difficulty, and fly verybadly. The strokes of their wings being frequent, and their flight short, uncertain, heavy, and laborious, they can use their wings only in very calm weather, the least wind beating them down. Yet he allows that others, whose body is lighter, rise into the air and fly with a little more ease; especially when the weather is warm and dry, their flights however being short, though frequent. He asserts also, that no coleopterous insect can fly against the wind[553]. These observations may hold perhaps with respect to many species; but they will by no means apply generally. The cockchafer (Melolontha vulgaris), if thrown into the air in the evening, its time of flight, will take wing before it falls to the ground. The common dung-chafer (Geotrupes stercorarius)—wheeling from side to side like the humble-bee—flies with great rapidity and force, and, with all its dung-devouring confederates, directs its flight with the utmost certainty, and probably often against the wind, to its food. The root-devourers or tree-chafers (Melolontha,Hoplia, &c.) support themselves, like swarming bees, in the air and over the trees, flying round in all directions. TheBrachypteraandDonaciæ, in warm weather, fly off from their station with the utmost ease;—their wings are unfolded, and they are in the air in an instant, especially the latter, as I have often found when I have attempted to take them. None are more remarkable for this than theCicindelæ, which, however, taking very short flights, are as easily marked down as a partridge, and affords as much amusement to the entomologist, as the latter to the sportsman.—It is to be observed that many insects in this order have nowings, and the female glow-worms neither wings nor elytra.
Many persons are not aware that the insects of the next order, theDermaptera, can fly: but earwigs (Forficula), their size considered, are furnished with very ample and curious wings, the principal nervures of which are so many radii, diverging from a common point near the anterior margin. Between these are others which, proceeding from the opposite margin, terminate in the middle of the wing[554]. These organs, when at rest, are more than once folded both transversely and longitudinally.
Wings equally ample, forming the quadrant of a circle, and with five or six nervures diverging from their base, distinguish thestrepsipteroustribe. When unemployed, these are folded longitudinally[555].
Probably in the next order (Orthoptera), theTegmina, or wing-covers—since they are usually of a much thinner substance than elytra—assist them in flying. They are however quite covered by irregular reticulations, produced by various nervures sent forth by the longitudinal ones, and running in all directions. When at rest, the inner part of one laps over that of the other[556]: but in different genera there is a singular variation in this circumstance. Thus inBlatta,Phasma, and maleAcridæ, and generally speaking, but not invariably, inLocustaandTruxalis,—the left elytrum laps over theright: but inMantis;Mantispa; some femaleAcridæ;Gryllus; andGryllotalpa; the right is laid over the left. The wings in this order, though always ample and larger than the tegmina, do not invariably form a quadrant of a circle, falling often short of it. They are extended by means of nervures, which, like so many rays, diverge from the base of the wing, and are intersected alternately by transverse ones, which thus form quadrangular areas, arranged like bricks in a wall. When at rest, they are longitudinally folded. The flight of these insects, as far as it has been observed, much resembles, it is said, that of certain birds. Ray tells us that both sexes of the house-cricket (Gryllus domesticus) fly with an undulating motion, like a woodpecker, alternately ascending with expanded wings, and descending with folded ones[557]. The field- and mole-crickets (Gryllus campestrisandGryllotalpa vulgaris), as we learn from Mr. White[558],—and, since the structure of their wings is similar, probably the otherOrthoptera,—fly in the same way.
Hemipterousinsects, with respect to theirHemelytra, may be divided into two classes. Those in which they are all of the same substance—varying from membrane to a leathery or horny crust[559]—and those in which the base and the apex are of different substances; the first being generally corneous, and the latter membranaceous[560]. The former or homopterous division includes theCicadariæ, Latr.;Aphis;Chermes;Thrips; andCoccus;—and the latter the heteropterous division, comprehending besides theGeocorisæ, Latr.,Notonecta;Sigara;Nepa;Ranatra; andNaucorisof Fabricius. The posterior tibiæ of some of this last division (Lygæus phyllopus,foliaceus, &c. F.) are furnished on each side with a foliaceous process—which may act the part of outriggers, and assist them in their flight[561]. I can give you no particular information with respect to the aërial movements of the insects of this order: the British species that belong to it are generally so minute that it is not easy to trace them with the naked eye; and unless some kind optician, which is much to be wished, would invent a telescope by which the proceedings of insects could be examined at a distance, there is no other way of studying them.
The four wings of the next order, theTrichopteraor case-worm flies, both in their shape and nervures resemble those of many moths[562]; only instead of scales they are usually covered with hairs, and the under wings, which are larger than the upper, fold longitudinally. Some of these flies, I have observed, move in a direct line, with their legs set out, which makes them look as if they were walking in the air. In flying they often apply their antennæ to each other, stretching them out straight, and thus probably are assisted in their motion.
TheLepidopteravary so infinitely in the shape, comparative magnitude, and appendages of their wings, that I should detain you too long did I enlarge upon so multifarious a subject. I shall therefore only observe, that one species is described, both by Lyonet and De Geer[563](Lobophora hexaptera), as having six wings; for besidesthe four ordinary ones, it has a winglet (Alula) attached to the base of the lower one, and placed, when the wings are folded, between it and the upper. These organs in this order you know are covered with scales of various shapes[564]. Their nervures are diverging rays, which issue either from a basal area or from the base itself, and terminate in the exterior margin[565]. The wings of many male butterflies, hawk-moths, and moths, are distinguished by a remarkable apparatus, noticed by De Geer, and since by many other naturalists[566]for keeping them steady and underanged in their flight. The upper wings, on their underside near their base, have a minute process, bent into a hook (Hamus), and covered with hairs and scales. In this hook one or more bristles (Tendo), attached to the base of the under wing, have their play. When the fly unfolds its wings, the hook does not quit its hold of the bristle, which moves to and fro in it as they expand or close. The females, which seldom fly far, often have the bristles but never the hook. The hairy tails of some insects,Sesia, belonging to the hawk-moth tribe, are expanded when they fly, so as to form a kind of rudder, which enables them to steer their course with more certainty.
The insects of this, and of every other order, except theColeoptera, fly with their bodies in a horizontal position, or nearly so. As their wings are usually so ample, we need not wonder that theLepidopteraare excellent fliers. Indeed they seem to flit untired from flower to flower, and from field to field; impelled at one while by hunger, and at another by love or maternal solicitude.—Thedistance to which some males will fly is astonishing. That of one of the silk-worm moths (Attacus Paphia) is stated to travel sometimes more than a hundred miles in this way[567].—Our most beautiful butterfly, the purple emperor (Apatura Iris), when he makes his first appearance fixes his throne on the summit of some lofty oak, from whence in sunny days, unattended by his empress, who does not fly, he takes his excursions. Launching into the air from one of the highest twigs, he mounts often to so great a height as to become invisible. When the sun is at the meridian his loftiest flights take place; and about four in the afternoon he resumes his station of repose[568].—The large bodies of hawk-moths (Sphinx, F.) are carried by wings remarkably strong both as to nervures and texture, and their flight is proportionably rapid and direct. That of butterflies is by dipping and rising alternately, so as to form a zigzag line with vertical angles, which the animal often describes with a skipping motion, so that each zigzag consists of smaller ones. This doubtless renders it more difficult for the birds to take them as they fly; and thus the male, when paired, often flits away with the female.
Amongst theNeuropteroustribes the most conspicuous insects are the dragon-flies (Libellulina), which—their metamorphosis, habits, mode of life, and characters considered—form a distinct natural order of themselves. Their four wings, which are nearly equal in size, are a complete and beautiful piece of net-work, resembling the finest lace, the meshes of which are usuallyfilled by a pure, transparent, glassy membrane. In two of the genera belonging to this tribe, the wings, when the animal is at rest, are always expanded, so that they can take flight in an instant, no previous unfolding of these organs being necessary. InAgrion, the other genus of the tribe, the wings when they repose are not expanded. I have observed of these insects, and also of several others in different orders, that without turning they can fly in all directions—backwards, and to the right and left, as well as forwards. This ability to fly all ways, without having to turn, must be very useful to them when pursued by a bird. Leeuwenhoek once saw a swallow chasing an insect of this tribe, which he calls aMordella, in a menagerie about a hundred feet long. The little creature flew with such astonishing velocity—to the right—to the left—and in all directions—that this bird of rapid wing and ready evolution was unable to overtake and entrap it; the insect eluding every attempt, and being generally six feet before it[569]. Indeed, such is the power of the long wings by which the dragon-flies are distinguished, particularly inÆshnaandLibellula, and such the force of the muscles that move them, that they seem never to be wearied with flying. I have observed one of the former genus (Anax Imperator, Leach) sailing for hours over a piece of water—sometimes to and fro, and sometimes wheeling from side to side; and all the while chasing, capturing, and devouring the various insects that came athwart its course, or driving away its competitors—without ever seeming tired, or inclined to alight. Another species (Æshna variegata), very common in lanes and along hedges, which flies, like theOrthoptera, in a waving line, is equally alert and active after its prey. This however often alights for a moment, and then resumes its gay excursive flights. The species of the genusAgrioncut the air with less velocity; but so rapid is the motion of their wings, that they become quite invisible. Hawking always about for prey, the Agrions, from the variety of the colours of different individuals, form no uninteresting object during a summer stroll. With respect to the mode of flight of the other neuropterous tribes I have nothing to remark; for that of theEphemeræ, which has been most noticed, I shall consider under another head.
The next order of insects, theHymenoptera, attract also general attention as fliers, and from our earliest years. The ferocious hornet, with its trumpet of terror; the intrusive and indomitable wasp; the booming and pacific humble-bee, the frequent prey of merciless schoolboys; and that universal favourite, the industrious inhabitant of the hive,—all belonging to it,—are familiar to every one. And in summer-time there is scarcely a flower or leaf in field or garden, which is not visited by some of its numerous tribes. The four wings of these insects, the upper pair of which are larger than the under, vary much in their nervures. From the saw-flies (Serrifera), whose wings are nearly as much reticulated as those of someNeuroptera, to the minuteChalcisandPsilus, in which these organs are without nervures, there is every intermediate variety of reticulation that can be imagined[570]. It has been observed that the nervures of the wings are usually proportioned to the weight of the insect. Thus the saw-flies have generally bodies thickerthan those of most otherHymenoptera, while those that have fewer nervures are more slender. This, however, does not hold good in all cases—so that the dimensions and cut of the wings, the strength of their nervures, and the force of their muscles, must also be taken into consideration. The wings of many of these insects when expanded, are kept in the same plane by means of small hooks (Hamuli) in the anterior margin of the under wing, which lay hold of the posterior margin of the upper[571]. Another peculiarity also distinguishes them. Base covers (Tegulæ), or small concavo-convex shields, protect the base of the wings from injury[572], or displacement.
The most powerful fliers in this order are the humble-bees, which, like the dung-chafers (Geotrupes), traverse the air in segments of a circle, the arc of which is alternately to right and left. The rapidity of their flight is so great, that could it be calculated, it would be found, the size of the creature considered, far to exceed that of any bird.—The aërial movements of the hive-bee are more direct and leisurely. When leaving the hive for an excursion, I have observed that as soon as they come out they turn about as if to survey the entrance, and then wheeling round in a circle, fly off. When they return to the hive, they often fly from side to side, as if to examine before they alight. When swarming, the heads of all are turned towards the group at the mouth of their dwelling; and upon rising into the air these little creatures fly so thick in every direction, as to appear like a kind of net-work with meshes of every angle. The queenalso, upon going forth, when her object is to pair, after returning to reconnoitre, begins her flight by describing circles of considerable diameter, thus rising spirally with a rapid motion[573]. The object of these gyrations is probably to increase her chance of meeting with a drone.—I have not much to tell you with respect to the flight of other insects of this order, except that a spider-wasp (Pompilus viaticus) whose sting is redoubtable, and which often, when we are in the vicinity of sandy sunny banks, accompanies our steps, has a kind of jumping movement when it flies.
The next order, theDiptera, consists altogether of two-winged flies:—but to replace the under wings of the tetrapterous insects, they are furnished with poisers, and numbers of them also with winglets. The poisers (Halteres) are little membranaceous threads placed one under the origin of each wing, near a spiracle, and terminated by an oval, round, or triangular button, which seems capable of dilatation and contraction. The animal moves these organs with great vivacity, often when at rest, and probably when flying. Their winglets (Alulæ) are different from those ofDytiscus marginalis, and the moth before noticed. Like them, they are of rigid membrane, and fringed; but they consist generally of two concavo-convex pieces (sometimes surrounded by a nervure), situated between the wing and the poisers, which, when the insect reposes, fold over each other like the valves of a bivalve shell; but when it flies they are extended. The use of neither of these organs seems to have been satisfactorily ascertained. Dr. Derham thinks they are for keeping the body steady in flight;and asserts, that if either a poiser or winglet be cut off, the insect will fly as if one side overbalanced the other, till it falls to the ground; and that if both be cut off, they will fly awkwardly and unsteadily, as if they had lost some very necessary part[574]. Shelver cut off the winglets of a fly, leaving both wings and poisers, but it could no longer fly. He next cut off the poisers of another, leaving the wings and winglets, and the same result followed. He found, upon removing one of these organs, that they were not properly compared to balancers. Observing that a common crane-fly (Tipula crocata) moved the knee of the hinder tibia in connexion with the wing and poiser, he cut it off, and it could no longer fly: this last experiment, however, seems contradicted by the fact, which has been often observed, that the insects of this genus will fly when half their legs are gone. He afterwards cut off both its poisers, when it could neither fly nor walk. Hence he conjectures that the poisers are connected with the feet, and are air-holders[575]. I have often seen flies move their poisers very briskly when at rest, particularlySeioptera vibrans, before mentioned. This renders Shelver's conjecture—that they are connected with respiration—not improbable. Perhaps by their action some effect may be produced upon the spiracle in their vicinity, either as to the opening or closing of it.
There are three classes of fliers in this order, the form of whose bodies, as well as the shape and circumstances of their wings, is different. First are the slender flies—the gnats, gnat-like flies, and crane-flies (Tipulariæ).The bodies of these are light, their wings narrow, and their legs long, and they have no winglets. Next are those whose bodies, though slender, are more weighty—theAsilidæ,Conopsidæ, &c.; these have larger wings, shorter legs, and very minute and sometimes even obsolete winglets. Lastly come the flies, theMuscidæ, &c., and their affinities, whose bodies being short, thick, and often very heavy, are furnished not only with proportionate wings and shorter legs, but also with conspicuous winglets. From these comparative differences and distinctions, we may conjecture in the first place—since the lightest bodies are furnished with the longest legs, and the heaviest with the shortest—that the legs act as poisers and rudders, that keep them steady while they fly, and assist them in directing their course[576]; and in the next—since the winglets are largest in the heaviest bodies, and altogether wanting in the lightest—that one of their principal uses is to assist the wings when the insect is flying.
The flight of the Tipularian genera is very various. Sometimes, as I have observed, they fly up and down with a zigzag course; at others in vertical curves of small diameter, like some birds; at others, again, in horizontal curves:—all these lines they describe with a kind of skipping motion. Sometimes they would seem to flit in every possible way—upwards, downwards, athwart, obliquely, and sometimes almost in circles. The common gnat (Culex pipiens) seems to sail along also in various directions. The motion of its wings, if it does not flylike a hawk, is so rapid as not to be perceptible. When the crane-fly (Tipula oleracea) is upon the wing, its fore-legs are placed horizontally, pointing forwards, and the four hind ones stretched out in an opposite direction, the one forming the prow and the other the stern of the vessel, in its voyage through the ocean of air. The legs of another insect of this tribe (Hirtæa Marci) all point towards the anus in flight, the long anterior pair forming an acute angle with the body:—thus, perhaps, it can better cut the air.
I have often been amused in my walks with the motions of the hornet-fly (Asilus crabroniformis), belonging to the second division just mentioned. This insect is carnivorous, living upon small flies. When you are taking your rambles, you may often observe it alight just before you;—as soon as you come up, it flies a little further, and will thus be your avant-courier for the whole length of a long field. This usually takes place, I seem to have observed, when a path lies under a hedge; and perhaps the object of this manœuvre may be the capture of prey. Your motions may drive a number of insects before you, and so be instrumental in supplying it with a meal. Other species of the genus have the same habit.
The aërial progress of the fly tribes, including the gad-flies (Œstridæ); horse-flies (Tabanidæ); carrion-flies (Muscidæ), and many other genera—which constitute the heavy horse amongst our two-winged fliers—is wonderfully rapid, and usually in a direct line. An anonymous observer in Nicholson'sJournal[577]calculates that, in its ordinary flight, the common house-fly (Musca domestica) makes with its wings about 600 strokes, whichcarry it five feet, every second. But if alarmed, he states their velocity can be increased six- or seven-fold, or to thirty or thirty-five feet, in the same period. In this space of time a race-horse could clear only ninety feet, which is at the rate of more than a mile in a minute. Our little fly, in her swiftest flight, will in the same space of time go more than the third of a mile. Now compare the infinite difference of the size of the two animals (ten millions of the fly would hardly counterpoise one racer), and how wonderful will the velocity of this minute creature appear! Did the fly equal the race-horse in size, and retain its present powers in the ratio of its magnitude, it would traverse the globe with the rapidity of lightning.
It seems to me, that it is not by muscular strength alone that many insects are enabled to keep so long upon the wing. Every one who attends to them must have noticed, that the velocity and duration of their flights depend much upon the heat or coolness of the atmosphere: especially the appearance of the sun. The warmer and more unclouded his beam, the more insects are there upon the wing, and every diurnal species seems fitted for longer or more frequent excursions. As these animals have no circulating fluid except the air in their tracheæ and bronchiæ, their locomotive powers, with few exceptions, must depend altogether upon the state of that element. When the thermometer descends below a certain point they become torpid, and when it reaches a certain height they revive; so that the air must be regarded, in some sense, as their blood, or rather the caloric that it contains; which when conveyed by the air, it circulates quickly in them, invigorates all their motions,enters into the muscles and nervures of their wings, maintaining their tension, and by the greater or less rapidity of its pulsations accelerating or diminishing their action.
Having given you all the information that I can collect with respect to the motions of perfect insects in theair, I must next say something concerning their modes of locomotion in or upon thewater. These are of two kinds,swimmingandwalking. Observe—I call that movement swimming, in which the animal pushes itself along by strokes—while in walking, the motion of the legs is not different from what it would be if they were on land. Most insects thatswimhave their posterior legs peculiarly fitted for it, either by a dense fringe of hairs on the shank and foot, as in the water-beetles (Dytiscus)[578], or the water-boatmen (Notonecta); or by having their terminal joints very much dilated—as in the whirlwig (Gyrinus)—so as to resemble the paddle of an oar[579]. When the Dytisci rise to the surface to take in fresh air—a silver bubble of which may often be seen suspended at their anus—they ascend, as it should seem, merely in consequence of their being specifically lighter than the water; but when they descend or move horizontally, which they do with considerable rapidity, it is by regular and successive strokes of their swimming legs. While they remain suspended at the surface, these legs are extended so as to form a right angle with their body. The water-boatmen swim upon their back,which enables them to see readily and seize the insects that fall upon the water, which are their prey.Sigara, however, a cognate genus separated fromNotonectaby Fabricius, swims in the ordinary way. As the Gyrini are usually in motion at the surface, whirling round and round in circles, it is probable that their legs are best adapted to this movement. They dive down, however, with great ease and velocity when alarmed. The common water-bug (Gerris lacustris), though it never goes under water, will sometimes swim upon the surface, which it does by strokes of the intermediate and posterior legs[580]. These, however, are neither fringed nor dilated, but very long and slender, with claws, not easily detected, situated under the apex of the last joint of the foot, which covers and conceals them. The underside of their body—as is the case withElophorus, and many other aquatic insects—is clothed with a thick coat of gray hairs like satin, which in certain lights have no small degree of lustre, and protect its body from the effects of the water. Some insects, that are not naturally aquatic, if they fall into the water will swim very well. I once saw a kind of grasshopper (Acrydium), which by the powerful strokes of its hind legs pushed itself across a stream with great rapidity.
Other insectswalk, as it were, in the water, moving their legs much in the same way as they would do on the land. Many smaller species of water-beetles, belonging to the generaHydrophilus,Elophorus,Hydræna,Parnus,Limnius, &c. thus win their way in the waves.—Thus also the water-scorpion (Nepa) pursues its prey; and the little water-mites (Hydrachna) may be seen inevery pool thus working their little legs with great rapidity, and moving about in all directions.—Some spiders also will not only traverse the surface of the waters, but, as you have heard with respect to one[581], descend into their bosom. There are other insects moving in this way that are not divers. Of this kind are the aquatic bugs (Gerris lacustris,Hydrometra Stagnorum,Velia Rivulorum, &c. Latr.). The first can walk, run, and even leap, which it does upon its prey, as well as swim upon the surface. The second, remarkable for its extreme slenderness, and for its prominent hemispherical eyes—which, though they are really in the head, appear to be in the middle of the body—rambles about in chase of other insects, in considerable numbers, in most stagnant waters. TheVeliais to be met with chiefly in running streams and rivers, coursing very rapidly over their waves[582]. The two last species neither jump nor swim.
I am next to say a few words upon the motions of insects thatburrow, either to conceal themselves or their young. Though burrowing is not always a locomotion, I shall consider it under this head, to preserve the unity of the subject. Many enter the earth by means of fore-legs particularly formed for the purpose. The flat dentated anterior shanks, with slender feet, that distinguish the chafers (Petalocera)—most of which in their first states live under ground, and many occasionally in their last—enable them to make their way either into the earth or out of it. Two other genera of beetles (ScaritesandClivina, Latr.)[583]have these shanks palmated, or armed with longer teeth at their extremity, for thesame purpose. But the most remarkable burrower amongst perfect insects is that singular animal the mole-cricket (Gryllotalpa vulgaris)[584]. This creature is endowed with wonderful strength, particularly in its thorax and fore legs. The former is a very hard and solid shell or crust, covering like a shield the trunk of the animal; and the latter are remarkably fitted for burrowing, both by their strength and construction. The shanks are very broad, and terminate obliquely in four enormous sharp teeth[585], like so many fingers: the foot consists of three joints—the two first being broad and tooth-shaped, and pointing in an opposite direction to the teeth of the shank; and the last small, and armed at the extremity with two short claws. This foot is placed inside the shank, so as to resemble a thumb and perform the office of one[586]. The direction and motion of these hands, as in moles, is outwards; thus enabling the animal most effectually to remove the earth when it burrows. By the help of these powerful instruments, it is astonishing how instantaneously it buries itself. This creature works under ground like a field-mouse, raising a ridge as it goes; but it does not throw up heaps like its name-sake the mole. They will in this manner undermine whole gardens; and thus in wet and swampy situations, in which they delight, they excavate their curious apartments, before described. The field-cricket (Gryllus campestris) is also a burrower, but by means of different instruments; for with its strong jaws, toothed like the claws of a lobster, but sharper, in heaths and other dry situations it perforates and rounds its curious and regular cells. The house-cricket (G. domesticus),which, on account of the softness of the mortar, delights in new-built houses, with the same organs, to make herself a covered-way from room to room, burrows and mines between the joints of the bricks and stones[587].
But of all the burrowing tribes, none are so numerous as those of the orderHymenoptera. Wherever you see a bare bank, of a sunny exposure, you always find it full of the habitations of insects belonging to it;—and besides this, every rail and old piece of timber is with the same view perforated by them. Bees; wasps; bee-wasps (Bembex); spider-wasps (Pompilus); fly-wasps (Mellinus,Cerceris,Crabro), with many others, excavate subterranean or ligneous habitations for their young. None is more remarkable in this respect than the sand-wasp (Ammophila), or as it might be better named—since it always commits its eggs to caterpillars which it inhumes—the caterpillar-wasp. It digs its burrows, by scratching with its fore legs like a dog or a rabbit, dispersing with its hind ones, which are particularly constructed for that purpose, the sand so collected[588].
Since most of these burrows are designed for the reception of the eggs of the burrowers, I shall next describe to you the manner in which one of the long-legged gnats, or crane-flies (Tipula variegata,)—a proceeding to which I was myself a witness—oviposits. Choosing a south bank bare of grass, she stood with her legs stretched out on each side, and kept turning herself half round backwards and forwards alternately. Thus the ovipositor, which terminates her long cylindrical pointed abdomen, made its way into the hard soil, and deposited hereggs in a secure situation. All, however, were not committed to the same burrow; for she every now and then shifted her station, but not more than an inch from where she bored last. While she was thus engaged, I observed her male companion suspended by one of his legs on a twig, not far from her. The common turf-boring crane-fly (T. oleracea), when engaged in laying eggs, moves over the grass with her body in a vertical position, by the help—her four anterior legs being in the air—of her two posterior ones, and the end of her abdomen, which performs the office of another. Whether in boring, likeT. variegata, she turns half round and back, does not appear from Reaumur's account[589].
I now come to motions whose object seems to besportand amusement rather than locomotion. They may be considered as of three kinds—hovering—gyrations—and dancing.
You have often in the woods and other places seen flies suspended as it were in the air, their wings all the while moving so rapidly as to be almost invisible. Thishovering, which seems peculiar to the aphidivorous flies, has been also noticed by De Geer[590]. I have frequently amused myself with watching them; but when I have endeavoured to entrap them with my forceps, they have immediately shifted their quarters, and resumed their amusement elsewhere. The most remarkable insects in this respect are the sphinxes, and from this they doubtless took their name ofhawk-moths. When they unfold their long tongue, and wipe its sweets from any nectariferous flower, they always keep upon the wing, suspending themselves over it till they have exhausted them,when they fly away to another. The species called by collectors the humming-bird (Macroglossa Stellatarum), and by some persons mistaken for a real one, is remarkable for this, and the motion of its wings is inconceivably rapid[591].
Thegyrationsof insects take place either when they are reposing, or when they are flying or swimming.—I was once much diverted by observing the actions of a minute moth upon a leaf on which it was stationed. Making its head the centre of its revolutions, it turned round and round with considerable rapidity, as if it had the vertigo, for some time. I did not, however, succeed in my attempts to take it.—Scaliger noticed a similar motion in the book-crab (Chelifer cancroides)[592].
Reaumur describes in a very interesting and lively way the gyrations of the Ephemeræ before noticed[593], round a lighted flambeau. It is singular, says he, that moths which fly only in the night, and shun the day, should be precisely those that come to seek the light in our apartments. It is still more extraordinary that these Ephemeræ—which appearing after sunset, and dying before sun-rise, are destined never to behold the light of that orb—should have so strong an inclination for any luminous object. To hold a flambeau when they appeared was no very pleasant office; for he who filled it, in a few seconds had his dress covered with the insects, which rushed from all quarters to him. The light of the flambeau exhibited a spectacle which enchanted every one that beheld it. All that were present, eventhe most ignorant and stupid of his domestics, were never satisfied with looking at it. Never had any armillary sphere so many zones, as there were here circles, which had the light for their centre. There was an infinity of them—crossing each other in all directions, and of every imaginable inclination—all of which were more or less eccentric. Each zone was composed of an unbroken string of Ephemeræ, resembling a piece of silver lace formed into a circle deeply notched, and consisting of equal triangles placed end to end (so that one of the angles of that which followed touched the middle of the base of that which preceded), and moving with astonishing rapidity. The wings of the flies, which was all of them that could then be distinguished, formed this appearance. Each of these creatures, after having described one or two orbits, fell upon the earth or into the water, but not in consequence of being burned[594]. Reaumur was one of the most accurate of observers; and yet I suspect that the appearance he describes was a visual deception, and for the following reason. I was once walking in the day-time with a friend[595], when our attention was caught by myriads of small flies, which were dancing under every tree;—viewed in a certain light they appeared a concatenated series of insects (as Reaumur has here described his Ephemeræ) moving in a spiral direction upwards;—but each series upon close examination, we found was produced by the astonishingly rapid movement of a single fly. Indeed, whenwe consider the space that a fly will pass through in a second, it is not wonderful that the eye should be unable to trace its gradual progress, or that it should appear present in the whole space at the same instant. The fly we saw was a small male Ichneumon.
Other circular motions of sportive insects take place in the waters. Linné, in his Lapland tour, noticed a black Tipula which ran over the water, and turned round like a whirlwig, orGyrinus[596]. This last insect I have often mentioned;—it seems the merriest and most agile of all the inhabitants of the waves. Wonderful is the velocity with which they turn round and round, as it were pursuing each other in incessant circles, sometimes moving in oblique, and indeed in every other direction. Now and then they repose on the surface, as if fatigued with their dances, and desirous of enjoying the full effect of the sun-beam: if you approach they are instantaneously in motion again. Attempt to entrap them with your net, and they are under the water and dispersed in a moment. When the danger ceases they reappear, and resume their vagaries. Covered with lucid armour, when the sun shines they look like little dancing masses of silver or brilliant pearls[597].
But the motions of this kind to which I particularly wish to call your attention, are the choral dances of males in the air; for the dancing sex amongst insects is the masculine, the ladies generally keeping themselves quiet at home. These dances occur at all seasons of the year, both in winter and summer, though in the former seasonthey are confined to the hardy Tipulariæ. In the morning before twelve, theHopliæ, root-beetles before mentioned, have their dances in the air, and the solstitial and common cockchafer appear in the evening—the former generally coming forth at the summer solstice—and fill the air over the trees and hedges with their myriads and their hum. Other dancing insects resemble moving columns—each individual rising and falling in a vertical line a certain space, and which will follow the passing traveller—often intent upon other business, and all unconscious of his aërial companions—for a considerable distance.
Towards sun-set the common Ephemeræ (E. vulgata), distinguished by their spotted wings and three long tails (Caudulæ), commence their dances in the meadows near the rivers. They assemble in troops, consisting sometimes of several hundreds, and keep rising and falling continually, usually over some high tree. They rise beating the air rapidly with their wings, till they have ascended five or six feet above the tree; then they descend to it with their wings extended and motionless, sailing like hawks, and having their three tails elevated, and the lateral ones so separated as to form nearly a right angle with the central one. These tails seem given them to balance their bodies when they descend, which they do in a horizontal position. This motion continues two or three hours without ceasing, and commences in fine clear weather about an hour before sun-set, lasting till the copious falling of the dew compels them to retire to their nocturnal station[598]. Our most common species,which I have usually taken for theE. vulgata, varies from that of De Geer in its proceedings. I found them at the end of May dancing over the meadows, not over the trees, at a much earlier hour—at half-past three—rising in the way just described, about a foot, and then descending, at the distance of about four or five feet from the ground. Another species, common here, rises seven or eight feet. I have also seen Ephemeræ flying over the water in a horizontal direction. The females are sometimes in the air, when the males seize them, and they fly paired. These insects seem to use their fore-legs to break the air; they are applied together before the head, and look like antennæ.—Hilara maura, a little beaked fly, I have observed rushing in infinite numbers like a shower of rain driven by the wind, as before observed[599], over waters, and then returning back.