"... a river and a seaAre a dish of tea,And a kingdom bread and butter,"
"... a river and a seaAre a dish of tea,And a kingdom bread and butter,"
would prefer the geographical treasures of Saxton or Speed, in spite of their ink and alum, to the freshest rind of the flax plant.—The larva of a little fly (Oscinis cellaris), whose economy, as I can witness from my own observations, is admirably described by Mentzelius[703], disdains to feed on any thing but wine or beer, which like Boniface in the play it may be said both to eat and drink, though, unlike its toping counterpart, indifferent to the age of its liquor, which whether sweet or sour is equally acceptable.
A diversity of food almost as great may be boasted by the insects which feed on animal substances. Some (flesh-flies, carrion-beetles, &c.) devour dead carcases only, which they will not touch until imbued with thehaut goutof putridity. Others, like Mr. Bruce's Abyssinians, preferring their meat before it has passed through the hands of the butcher, select it from living victims, and may with justice pride themselves upon the peculiar freshness of their diet. Of these last, different tribes follow different procedures. The Ichneumons devourthe flesh of the insects into which they have insinuated themselves. Some of the Œstri, fixed in a spacious apartment beneath the skin of an ox or deer, regale themselves on a purulent secretion with which they are surrounded. Others of the same tribe, partial to a higher temperature, attach themselves to the interior of the stomach of a horse, and in a bath of chyme of 102 degrees of Fahrenheit revel on its juices. The various species of horse-flies dart their sharp lancets into the veins of quadrupeds, and satiate themselves in living streams; while the gnat, the flea, the bug, and the louse, plunge their proboscis even into those of us lords of the creation, and banquet on "the ruddy drops which warm our hearts." Some make their repast upon birds only, as the fly of the swallow, and otherOrnithomyiæ, and the bird-louse; insects nearly allied, though one is dipterous and the other apterous. And a most singular animal belonging to the latter tribe (Nycteribia Vespertilionis) revenges upon the bat its ravages of the insect world[704]. Another numerous class kill their prey outright, either devouring its solid parts, as the predaceous and rove-beetles, &c., or imbibing its juices only, as the infinite hordes of the field-bug tribe. And the larvæ of the gnat, chameleon (Stratyomis), and other flies aquatic in that state, the leviathans of the world of animalcules, swallow whole hosts of these minute inhabitants of pools and ponds at a gulp, causing with their oral apparatus a vortex in the water, down which myriads of victims are incessantly hurried into their destructive maw.
But not only animals themselves, almost every animal substance that can be named is the appropriate food ofsome insect. Multitudes find a delicious nutriment in excrements of various kinds. Matters apparently so indigestible as hair, wool, and leather, are the sole food of many moths in the larva state (Tinea tapetzella,pellionella, &c.). Even feathers are not rejected by others; and the grub of a beetle (Anthrenus Musæorum), with powers of stomach which the dyspeptic sufferer may envy, will live luxuriously upon horn[705].
For the most part, insects feeding upon animal substances will not touch vegetables, andvice versâ. You must not however take the rule without exceptions. Many caterpillars (as those ofThyatira derasa,Chariclea Delphinii, &c.), though plants are their proper food, will occasionally devour other caterpillars, and sometimes even their own species. The large green grasshopper (Acrida viridissima), and probably others of the order, will eat smaller insects as well as its usual vegetable food[706]; so also will the larvæ of manyPhryganeæ.Allantus marginellus, as I was last summer amused by witnessing, like manyScatophagæ, sips the nectar of umbelliferous plants only till a fly comes within its reach, pouncing upon which it gladly quits its vegetable for an animal repast.Anobium paniceum, which ordinarily feeds upon wood, was, as I before mentioned, once found by Mr. Sheppard in great abundance living upon the dried Cantharides (Cantharis vesicatoria) of the shops. On the other hand,Necrophorus mortuorum, which subsists on carcases, and many other carnivorous species, will make a hearty meal of a putrid fungus;Ptinus Furdevours indifferently dried birds or plants, not refusing even tobacco; and from the impossibility that one of a million of the innumerableswarms of gnats which abound in swampy places, particularly in regions which but for them would be lost to sensitive existence, should ever taste blood, it seems clear that they are usually contented with vegetable aliment. Indeed the males, as well as those of the horse-fly of which even the females readily imbibed the sugared fluid offered to them by Reaumur[707], never suck blood at all; so that they must either feed on vegetable matter, which in fact I have observed them to do, or fast during their whole existence in the perfect state.
Though insects, generally considered, have thus a much more extensive bill of fare than the larger animals, each individual species is commonly limited to a more restricted diet. Many both of animal and vegetable feeders are absolutely confined to one kind of food, and cannot exist upon any other. The larva ofGasterophilus Equican subsist no where but in the stomach of the horse or ass, which animals therefore this insect might boast with some show of reason to have been created for its use rather than for ours, being to us useful only, but to it indispensable. The larvæ ofSyrphus Pyrastriaccording to De Geer eat no other Aphis but that of the rose[708]. Most Ichneumons andSphecinaprey each upon a single species of insect only, which therefore they would seem to have been formed for the express purpose of keeping within due limits. Reaumur mentions having once found in a parcel of decaying wood the nests of six different kinds of the latter tribe, each of which was filled with flies of a distinct species[709].Cerceris auritusandPhilanthus lætusin the larva state feed solely on the weevil tribe ofColeoptera, the latter being restricted evento the short-rostrum'd family, asOtiorhynchus raucus, &c.[710], whileBembex rostrata, another hymenopterous insect, selects flies, asMusca Cæsar, &c.[711]
A very large proportion of species, however, are able to subsist on several kinds of food. Amongst the carnivorous tribes, it is indifferent to most of those which prey upon putrid substances from what source they have been derived: and the predaceous insects, such as theLibellulina,Telephorus,Empis, theAraneidæ, &c. will attack most smaller insects inferior to them in strength, not excepting in many instances their own species. The wax-moth larva (Galleria Cereana) will for want of wax eat paper, wafers, wool, &c.[712]: another Tinea described by Reaumur, and before adverted to, attacks chocolate[713], which cannot have been its natural food, even selecting that most highly perfumed; and the Tineæ which devour dressed wool, but happily for the farmer and wool-stapler refuse it when unwashed, must have existed when no manufactured wool was accessible.—The vegetable feeders are under greater restrictions, yet probably the majority can subsist on different kinds of food. This is certainly true of most lepidopterous larvæ, several of which as well as manyColeoptera(Haltica oleracea, &c.) are polyphagous, eating almost every plant. It is worthy of remark, however, that when some of these have fed for a time on one plant they will die rather than eat another, which would have been perfectly acceptable to them if accustomed to it from the first[714]. Here too it must be borne in mind, that by far the greater part of insects feed upondifferent substances in their different states of existence, eating one kind of food in the larva and another in the imago state. This is the case with the whole OrderLepidoptera, which in the former eat plants chiefly, in the latter nothing but honey or the sweet juices of fruit, which they have often been observed to imbibe; and the same rule obtains also in regard to most dipterous and hymenopterous insects. Those which eat one kind of food in both states, are chiefly of the remaining orders.
I have said that insects, like other animals, draw their subsistence from the vegetable or animal kingdoms. But I ought not to omit noticing that some authors have conceived that several species feed upon mineral substances[715]. Not to dwell upon Barchewitz's idle tale of East Indian ants which eat iron[716], or on the stone-eating caterpillars recorded in the Memoirs of the French Academy[717], which are now known to erode the walls on which they are found, solely for the purpose of forming their cocoons; Reaumur and Swammerdam have both stated the food of the larvæ ofEphemeræto be earth, that being the only substance ever found in their stomachs and intestines, which are filled with it. This supposition, which if correct renders invalid the definition by which Mirbel (and my friend Dr. Alderson of Hull long before him) proposed to distinguish the animal and vegetable kingdoms, is certainly not inadmissible; for, though we might not be inclined to give much weight to Father Paulian's history of a flint-eater who digested flints and stone[718], the testimony of Humboldt seems to prove that the human race is capable of drawing nutriment from earth,which, if the odious Ottomaques can digest and assimilate, may doubtless afford support to the larvæ of Ephemeræ. Yet after all it is perhaps more probable that these insects feed on the decaying vegetable matter intermixed with the earth in which they reside, from which after being swallowed it is extracted by the action of the stomach: like the sand that, from being found in a similar situation, Borelli erroneously supposed to be the food of manyTestacea, though in fact a mere extraneous substance.
The majority of insects, either imbibing their food in a liquid state, or feeding on succulent substances, require no aqueous fluid for diluting it. Water, however, is essential to bees, ants, and some other tribes, which drink it with avidity; as well as in warm climates to manyLepidoptera, which are there chiefly taken in court yards, near the margins of drains, &c. Even some larvæ which feed upon juicy leaves have been observed to swallow drops of dew; and one of them (Odenestis potatoria), which (according to Goedart) after drinking lifts up its head like a hen, has received its name from this circumstance. That it is not the mere want of succulency in the food which induces the necessity of drink, is plain from those larvæ which live entirely on substances so dry that it is almost unaccountable whence the juices of their body are derived. The grub of an Anobium will feed for months upon a chair that has been baking before the fire for half a century, and from which even the chemist's retort could scarcely extract a drop of moisture; and will yet have its body as well filled with fluids as that of a leaf-fed caterpillar.
By far the greater part of insects always feed themselves. The young however of those which live in societies,as the hive- and humble-bees, wasps, ants, &c. are fed by the older inhabitants of the community, which also frequently feed each other. Many of these last insects are distinguished from the majority of their race, which live from day to day and take no thought for the morrow, by the circumstance of storing up food. Of those which feed themselves, the larger proportion have imposed upon them the task of providing for their own wants; but the tribe of Spheges, wild bees, and some others, are furnished in the larva state by the parent insect with a supply of food sufficient for their consumption until they have attained maturity.
As to theirtimeof feeding, insects may be divided into three great classes: the day-feeders, the night-feeders, and those which feed indifferently at all times. You have been apt to think, I dare say, that when the sun's warmer beams have waked the insect youth, and
"Ten thousand forms, ten thousand different tribes,People the blaze,"
"Ten thousand forms, ten thousand different tribes,People the blaze,"
you see before you the whole insect world. You are not aware that a host as numerous shun the glare of day, and, like the votaries of fashion, rise not from their couch until their more vulgar brethren have retired to rest. While the painted butterfly, the "fervent bees," and the quivering nations of flies, which sport
"Thick in yon stream of light, a thousand ways,Upward and downward thwarting and convolved,"
"Thick in yon stream of light, a thousand ways,Upward and downward thwarting and convolved,"
love to bask in the sun's brightest rays, and search for their food amidst his noon-tide fervor, an immense multitude stir not before the sober time of twilight, and eat only when night has overshadowed the earth. Thenonly, the vast tribe of moths quit their hiding-places; "the shard-born[719]beetle with his drowsy hum," accompanied by numerous others of his order, sallies forth; the airy gnat-flies institute their dances; and the solitary spider stretches his net. All these retire into concealment at the approach of light.—Some few larvæ (Agrotis exclamationis, &c.) have similar habits, and those of one singular genus before adverted to (Nycterobius) are remarkable for providing in the night a store of food which they consume in the day; but to the generality of these the period of feeding is indifferent, and most of them seem to eat with little intermission night and day.
Insects like other animals take in their food by the mouth (inChermesandCoccus, indeed, the rostrum seems to be, but really is not, inserted in the breast, between the fore-legs), but there is one exception to this rule. The singularUropoda vegetans, which is such a plague to some beetles, derives its nutriment from them by means of a filiform pedicle or umbilical cord attached to itsanus; and what increases the singularity, sometimes several of these mites form a kind of chain, of which the first only is fixed by its pedicle to the beetle, each of the remainder being similarly connected with the one that precedes it; so that the nutriment drawn from the beetle passes to the last through the bodies and umbilical cords of the individuals which are intermediate[721]. Some have regarded these bodies as true eggs; and their analogy with the pedunculated eggs ofTrombidium aquaticum, which also seem to derive nourishment from the water-boatmen, &c. to which they are fixed, and still more the circumstance of their ultimately losing their pedicle and detaching themselves from the infested beetles, give plausibility to the idea. Yet these animals are certainly furnished with feet, and have according to De Geer[722]a part resembling a mouth—characters which cannot be attributed to any egg.
In the variety of their instruments of nutrition, which you must bear in mind are often quite different in the larva and perfect states, insects leave all other animals far behind. In common with them, a vast number (the ordersColeoptera,Hymenoptera, andOrthoptera, and the larvæ ofLepidoptera, someDiptera, &c.) are furnished with jaws, but of very different constructions, and all admirably adapted for their intended services; some sharp, and armed with spines and branches for tearing flesh; others hooked for seizing, and at the same time hollow for suction; some calculated like shears for gnawing leaves; others more resembling grindstones, of a strength and solidity sufficient to reduce the hardest wood to powder: and this singularity attends the major part of theseinsects, that they possess in fact two pairs of jaws, an upper and an under pair, both placed horizontally, not vertically, the former apparently in most cases for the seizure and mastication of their prey; the latter, when hooked, for retaining and tearing, while the upper comminute it previously to its being swallowed[723].
To the remainder of the class of insects, a mighty host, jaws would have been useless. Their refined liquid food requires instruments of a different construction, and with these they are profusely furnished. The innumerable tribes of moths and butterflies eat nothing but the honey secreted in the nectaries of flowers, which are frequently situated at the bottom of a tube of great length. They are accordingly provided with an organ exquisitely fitted for its office—a slender tubular tongue, more or less long, sometimes not shorter than three inches, but spirally convoluted when at rest, like the main spring of a watch, into a convenient compass. This tongue, which they have the power of instantly unrolling, they dart into the bottom of a flower, and, as through a siphon, draw up a supply of the delicious nectar on which they feed. A letter would scarcely suffice for describing fully the admirable structure of this organ. I must content myself therefore with here briefly observing that it is of a cartilaginous substance, and apparently composed of a series of innumerable rings, which, to be capable of such rapid convolution, must be moved by an equal number of distinct muscles; and that, though seemingly simple, it is in fact composed of three distinct tubes, the two lateral ones cylindrical and entire, intended, as Reaumur thinks, for the reception of air; and the intermediate one, throughwhich alone the honey is conveyed, nearly square, and formed of two separate grooves projecting from the lateral tubes; which grooves, by means of a most curious apparatus of hooks like those in the laminæ of a feather, inosculate into each other, and can be either united into an air-tight canal, or be instantly separated, at the pleasure of the insect[724].
Another numerous race, the whole of the orderHemiptera, abstract the juices of plants or of animals by means of an instrument of a construction altogether different—a hollow grooved beak, often jointed, and containing three bristle-formed lancets, which, at the same time that they pierce the food, apply to each other so accurately as to form one air-tight tube, through which the little animals suck up[725]their repast; thus, forming a pump, which, more effective than ours, digs the well from which it draws the fluid[726].
A third description of insects, those of the orderDiptera, comprising the whole tribe of flies, have a sucker formed on the same general plan as that last described, but of a much more complicated and varied structure. It is in like manner composed of a grooved case and several included lancets; but the case, although horny, rigid and beak-like in some, is in others fleshy, flexible,and more resembling the proboscis of an elephant, and terminates in two turgid liplets: and the accompanying lancets are themselves included in an upper hollow case, in connexion with which they probably compose an air-tight tube for suction. The number and form of these instruments is extremely various. In some genera (Musca) there is but one, which resembles a sharp lancet. Others (Empis,Asilus,) have three, the two lateral ones needle-shaped, that in the middle like a scymetar; together forming so keen an apparatus, that De Geer has seen an Asilus pierce with it the elytra of a lady-bird; and I have myself caught them with not only anElaterand weevil, but even aHisterin their mouths. In many horse-flies we find four; two precisely resembling lancets, and two, even to the very handles, buck-hafted carving-knives[727]. The blood-thirsty gnat has five, some acutely lanced at the extremity, and others serrated on one side. The flea, the spider, the scorpion have all instruments for taking their food of a construction altogether different[728]. But it is impossible here to attempt even a sketch of the variations in these organs which take place in the apterous genera, and in many of the dipterous larvæ. Suffice it to say that they all manifest the most consummate skill in their adaptation to the purposes of the insects which are provided with them, and which can often employ them not only as instruments for preparing food, but as weapons of offence and defence, as tools in the building of their nests, and even as feet.
Some insects in their perfect state, though furnished with organs of feeding, make no use of them, and consume no food whatever. Of this description are themoth which proceeds from the silk-worm, and several others of the same order; the different species of gad-flies, and the Ephemeræ, insects whose history is so well known as to afford a moral or a simile to those most ignorant of natural history. All these live so short a time in the perfect state as to need no food. Indeed it may be laid down as a general rule, that almost all insects in this state eat much less than in that of larvæ. The voracious caterpillar when transformed into a butterfly needs only a small quantity of honey; and the gluttonous maggot, when become a fly, contents itself with a drop or two of any sweet liquid.
While in the state of larvæ the quantity of food consumed by insects is vastly greater in proportion to their bulk than that required by larger animals. Many caterpillars eat daily twice their weight of leaves, which is as if an ox, weighing sixty stone, were to devour every twenty-four hours three quarters of a ton of grass—a power of stomach which our graziers may thank their stars that their oxen are not endowed with. A probable proximate cause for this voracity in the case of herbivorous larvæ has been assigned by John Hunter, who attributes it to the circumstance of their stomach not having the power of dissolving the vegetable matters received into it, but merely of extracting from them a juice[729]. This is proved both by their excrement, which consists of coiled-up and hardened particles of leaf, that being put into water expand like tea; and by the great proportion which the excrement bears to the quantity of food consumed. From experiments, with a detail of which he has favoured me, made by Colonel Machell on thecaterpillars ofEuprepia Caja, he ascertained that, though a larva weighing thirty-six grains voided every twelve hours from fifteen to eighteen grains weight of excrement, it did not increase in weight in the same period more than one or two grains. On the other hand, many carnivorous larvæ increase in weight in full proportion to the food consumed, and that in an astonishing degree. Redi found that the maggots of flesh-flies, of which one day, twenty-five or thirty did not weigh above a grain, the next weighed seven grains each; having thus in twenty-four hours become about two hundred times heavier than before[730].
Some insects have the faculty of sustaining a long abstinence from all kinds of food. This seems to depend upon the nature of their habits. If the insect feeds on a substance of a deficiency of which there is not much probability, as on vegetables, &c. it commonly requires a frequent supply. If, on the contrary, it is an insect of prey, and exposed to the danger of being long deprived of its food, it is often endowed with a power of fasting, which would be incredible but for the numerous facts by which it is authenticated. The ant-lion will exist without the smallest supply of food, apparently uninjured, for six months; though, when it can get it, it will devour daily an insect of its own size. Vaillant, whose authority may be here taken, assures us that he kept a spider without food under a sealed glass for ten months, at the end of which time, though shrunk in size, it was as vigorous as ever[731]. And Mr. Baker, so well known for his microscopical discoveries, states that he kept a darkling beetle (Blaps mortisaga) alive for three years withoutfood of any kind[732]. Some insects, not of a predaceous description, are gifted with a similar power of abstinence. Leeuwenhoek tells us that a mite, which he had gummed alive to the point of a needle and placed before his microscope, lived in that situation eleven weeks[733].
In some cases the very want of food, however paradoxical the proposition, seems actually to be a mean of prolonging the life of insects. At least one such instance has fallen under my own observation. The aphidivorous flies, such asSyrphus Pyrastri, &c. live in the larva state ten or twelve days, in the pupa state about a fortnight, and as perfect insects sometimes possibly as long—the whole term of their existence in summer not exceeding at the very utmost six weeks. But one[734], which I put under a glass on the 2d of June, 1811, when about half grown, and, after supplying it with Aphides once or twice, by accident forgot, I found to my great astonishment alive three months after; and it actually lived until the June following without a particle of food. It had therefore existed in the larva state more than eight times as long as it would have lived in all its states, if it had regularly undergone its metamorphoses—which is asextraordinary a prolongation of life as if a man were to live 560 years. It is true that its existence was not worth having even to the larva of a fly. For the last eight months it remained without motion, attached by its posterior pair of tubercles to the paper on which it was placed, manifesting no other symptoms of life than by moving the fore part of the body when touched, and replacing itself on its belly if turned upon its back. But this was quite enough to prove it still alive.—I can attribute this singular result to no other circumstance than its having been deprived of a sufficient quantity of food to bring it into the pupa state, though provided with enough for the attainment of nearly its full growth as larva. Possibly the same remote cause might act in this case, as operates to prolong the term of existence of annual plants that have been prevented from perfecting their seed; and it would almost seem to favour the hypothesis of some physiologists, who contend that every organised being has a certain portion of irritability originally imparted to it, and that its life will be long or short as this is slowly or rapidly excited—no great consolation this for the advocates for fast-living, unless they are in good earnest in their affected preference of a "short life and a merry one:" though it must be admitted that they would have the best of the argument were the alternative such a state of torpid insensibility as that with which our larva purchased the prolongation of its existence.
After this general view of the food of insects, and of circumstances connected with it, I proceed to give you an account of some peculiarities in their modes of procuring it.
The vegetable feeders have for the most part but littledifficulty in supplying their wants. In the larva state they generally find themselves placed by the parent insect upon the very plant or substance which is to nourish them: and in their perfect state their wings or feet afford a ready conveyance to the banquet to which by an unerring sense they are directed. All nature lies before them, and it is only when their numbers are extraordinarily increased, or in consequence of some unusual destruction of their appropriate aliment, that they perish for want. The description of their food renders unnecessary those artifices to which many of the carnivorous insects are obliged to have recourse: and none of them, if we except the white-ants, whose cunning mode of insinuating themselves into houses in tropical climates has been detailed in a former letter, can be said to use stratagem in obtaining their food.
Of the carnivorous species, the greater proportion attack their prey by open violence, such as the predaceous beetles, the Ichneumons, burrowing wasps, and true wasps; the praying insects (Mantis); the bugs (GeocorisæLatr.); dragon-flies (Libellulina), &c.; which have been before adverted to. But a very considerable number, chiefly, however, of one tribe, that of spiders, provide their sustenance solely by artifice and stratagem, the singularity of which, and the admirable adaptation of the instruments by which they take their prey to the end in view, afford a most wonderful instance of the power and wisdom of the Creator, and have attracted admiration in all ages. A description of these, however, which will require a detailed survey, I must refer to another letter.
I am, &c.
Thestratagemsof insects in obtaining their food are now to engage our attention. I shall not dwell on those inartificial modes of surprising their prey, of which examples may be found amongst almost every order of insects, such as watching behind a leaf or other object affording concealment until its approach; but shall proceed to describe the various artifices of the race of spiders, of which there are several hundred distinct species differing essentially from each other both in characters and manners.
Many of these are constantly under our eyes; and were it not that we are accustomed to neglect what is the subject of daily occurrence, we should never behold a spider's web without astonishment. What, if we had not witnessed it, would seem more incredible than that any animal should spin threads; weave these threads into nets more admirable than ever fowler or fisherman fabricated; suspend them with the nicest judgement in the place most abounding in the wished-for prey; and there concealed watch patiently its approach? In this case, as in so many others, we neglect actions in minute animals, which in the larger would excite our endless admiration. How would the world crowd to see a foxwhich should spin ropes, weave them into an accurately-meshed net, and extend this net between two trees for the purpose of entangling a flight of birds? Or should we think we had ever expressed sufficient wonder at seeing a fish which obtained its prey by a similar contrivance? Yet there would, in reality, be nothing more marvellous in their procedures than in those of spiders, which, indeed, the minuteness of the agent renders more wonderful.
All spiders do not spin webs. A considerable number adopt other means for catching insects. Of these I shall speak hereafter. At present I shall endeavour to give you a clear idea of the operations of theweavers, explaining successively the instruments by which they spin—the mode of forming their nets, together with the various descriptions of them—and the manner in which they entrap and secure their prey.
The thread spun by spiders is in substance similar to the silk of the silk-worm and other caterpillars, but of a much finer quality. As in them, it proceeds from reservoirs, into which it is secreted in the form of a viscid gum: but in the mode of its extrication is very dissimilar, issuing not from the mouth but the hinder part of the abdomen. If you examine a spider, you will perceive in this part four or six little teat-like protuberances or spinners. These are the machinery through which, by a process more singular than that of rope-spinning, the thread is drawn. Each spinner is furnished with a multitude of tubes, so numerous and so exquisitely fine, that a space often not much bigger than the pointed end of a pin, is furnished, according to Reaumur[735], with athousand of them. From each of these tubes, consisting of two pieces, the last of which terminates in a point infinitely fine, proceeds a thread of inconceivable tenuity, which, immediately after issuing from it, unites with all the other threads into one. Hence from each spinner proceeds a compound thread; and these four threads, at the distance of about one-tenth of an inch from the apex of the spinners, again unite, and form the thread we are accustomed to see, which the spider uses in forming its web. The threads, however, are not all of the same thickness, for Leeuwenhoek observed that some of the tubes were larger than others, and furnished a larger thread. Thus a spider's thread, even spun by the smallest species, and when so fine that it is almost imperceptible to our senses, is not, as we suppose, a single line, but a rope composed of at least four thousand strands. How astonishing! But to feel all the wonder of this fact we must follow Leeuwenhoek in one of his calculations on the subject. This renowned microscopic observer found by an accurate estimation that the threads of the minutest spiders, some of which are not larger than a grain of sand, are so fine that four millions of them would not equal in thickness one of the hairs of his beard. Of such tenuity it is utterly beyond the power of the imagination to conceive: the very idea overwhelms our faculties, and humbles us under a sense of their imperfection.—Ofthe probable accuracy of this calculation you may any day in summer convince yourself, by taking one of the large field spiders (Epeira Diadema), and after pressing its abdomen against a leaf or other substance, so as to attach the threads to the surface—the same preliminary step which the spider adopts in spinning—drawing it gradually to a small distance. You will plainly perceive that the proper thread of the spider is formed of four smaller threads, and these again of threads so fine and numerous, that there cannot be fewer than a thousand issue from each spinner; and if you pursue your researches with the microscope, you will find that precisely the same takes place in the minutest species that spins.—You will inquire what can be the end of machinery so complex? One probable reason is, that it was necessary for drying the gum sufficiently to form a tenacious line, that an extensive surface should be exposed to the air; which is admirably effected by dividing it at its exit from the abdomen into such numerous threads. But the chief cause, perhaps, is the occasion (hereafter to be adverted to) which the spider sometimes has to employ its threads in their finer and unconnected state before they unite to form a single one.—The spider is gifted by her Creator with the power of closing the orifices of the spinners at pleasure, and can thus, in dropping from a height by her line, stop her progress at any point of her descent: and, according to Lister[736], she is also able to retract her threads within the abdomen; but this is doubted, and with apparent reason, by De Geer[737].
The only other instruments employed by the spider in weaving are her feet, with the claws of which sheusually guides, or keeps separated into two or more, the line from behind; and in many species these are admirably adapted for the purpose, two of them being furnished underneath with teeth like those of a comb, by means of which the threads are kept asunder. But another instrument was wanting. The spider in ascending the line by which she has dropped herself from an eminence, winds up the superfluous cord into a ball. In performing this the pectinated claws would not have been suitable. She is therefore furnished with athirdclaw between the other two[738], and is thus provided for every occasion.
The situations in which spiders place their nets are as various as their construction. Some prefer the open air, and suspend them in the midst of shrubs or plants most frequented by flies and other small insects, fixing them in a horizontal, a vertical, or an oblique direction. Others select the corners of windows and of rooms, where prey always abounds; while many establish themselves in stables and neglected out-houses, and even in cellars and desolate places in which one would scarcely expect a fly to be caught in a month. It is with the operations of these last especially, that we are accustomed to associate the ideas of neglect and desertion by man—associations which both in painting and allegory have been often happily applied. Hogarth, when he wished to produce a speaking picture of neglected charity, clothed the poor's box in one of his pieces with a spider's web: and the Jews, in one of the fables with which they have disfigured the records of holy writ, have not less ingeniously availed themselves of the same idea.They relate that the reason why Saul did not discover David and his men in the cave of Adullam[739]was, that God had sent a spider which had quickly woven a web across the entrance of the cave in which they were concealed; which being observed by Saul, he thought it useless to investigate further a spot bearing such evident proofs of the absence of any human being[740].
The most incurious observer must have remarked the great difference which exists in the construction of spiders' webs. Those which we most commonly see in houses are of a woven texture similar to fine gauze, and are appropriately termed webs; while those most frequently met with in the fields are composed of a series of concentric circles united byradiidiverging from the centre, the threads being remote from each other. These last, which in their simple state, or still more when studded with dew drops, you must have a thousand times admired, are with greater propriety termednets; and the insects which form them proceeding on geometrical principles may be calledgeometricians, while the former can aspire only to the humbler denomination ofweavers. I shall endeavour to describe the process followed in the construction of both, beginning with the latter.
The weaving spider which is found in houses, having selected some corner for the site of her web, and determined its extent, presses her spinners against one of the walls, and thus glues to it one end of her thread. She then walks along the wall to the opposite side, and there in like manner fastens the other end. This thread, which is to form the outer margin or selvage of her web,and requires strength, she triples or quadruples by a repetition of the operation just described; and from it she draws other threads in various directions, the interstices of which she fills up by running from one to the other, and connecting them by new threads until the whole has assumed the gauze-like texture which we see. Books of natural history, all copying from one another, have described these kinds of web as fabricated of a regular warp and woof, or of parallel longitudinal lines crossed at right angles by transverse ones glued to them at the points of intersection. This, however, is clearly erroneous, as you will see by the slightest examination of a web of this kind, in which no such regularity of texture can be discovered.
The webs just described present merely a simple horizontal surface, but others more frequently seen in outhouses and amongst bushes possess a very artificial appendage. Besides the main web, the spider carries up from its edges and surface a number of single threads often to the height of many feet, joining and crossing each other in various directions. Across these lines, which may be compared to the tackling of a ship, flies seem unable to avoid directing their flight. The certain consequence is, that in striking against these ropes they become slightly entangled, and, in their endeavours to disengage themselves, rarely escape being precipitated into the net spread underneath for their reception, where their doom is inevitable.
But the net is still incomplete. It is necessary that our hunter should conceal her grim visage from the game for which she lies in wait. She does not therefore station herself upon the surface of her net, but in asmall silken apartment constructed below it, and completely hidden from view. "In this corner," to use the quaint translation of Pliny by Philemon Holland, Doctor in Physic[741], "with what subtiltie doth she retire making semblance as though she meant nothing less than that she doth, and as if she went about some other business! nay, how close lieth she, that it is impossible to see whether any one be within or no!" But thus removed to a distance from her net and entirely out of sight of it, how is she to know when her prey is entrapped? For this difficulty our ingenious weaver has provided. She has taken care to spin several threads from the edge of the net to that of her hole, which at once inform her by their vibrations of the capture of a fly, and serve as a bridge on which in an instant she can run to secure it.
You will readily conceive that thegeometricalspiders, in forming their concentric circled nets, follow a process very different from that just described, than which indeed it is in many respects more curious. As the net is usually fixed in a perpendicular or somewhat oblique direction, in an opening between the leaves of some shrub or plant, it is obvious that round its whole extent will be required lines to which can be attached those ends of theradiithat are furthest from the centre. Accordingly the construction of these exterior lines is the spider's first operation. She seems careless about the shape of the area which they inclose, well aware that she can as readily inscribe a circle in a triangle as in a square, and in this respect she is guided by the distance or proximity of the points to which she can attach them. She spares no pains, however, to strengthen and keep them in aproper degree of tension. With the former view she composes each line of five or six or even more threads glued together; and with the latter she fixes to them from different points a numerous and intricate apparatus of smaller threads. Having thus completed the foundations of her snare[742], she proceeds to fill up the outline. Attaching a thread to one of the main lines, she walks along it, guiding it with one of her hind feet that it may not touch in any part and be prematurely glued, and crosses over to the opposite side, where by applying her spinners she firmly fixes it. To the middle of this diagonal thread, which is to form the centre of her net, she fixes a second, which in like manner she conveys and fastens to another part of the lines encircling the area. Her work now proceeds rapidly. During the preliminary operations she sometimes rests, as though her plan required meditation. But no sooner are the marginal lines of her net firmly stretched, and two or three radii spun from its centre, than she continues her labour so quickly and unremittingly that the eye can scarcely follow her progress. The radii to the number of about twenty, giving the net the appearance of a wheel, are speedily finished. She then proceeds to the centre, quickly turns herself round, and pulls each thread with her feet to ascertain its strength, breaking any one that seems defective and replacing it by another. Next, she glues immediately round the centre five or six small concentric circles, distant about half a line from each other, and then four or five larger ones, each separatedby a space of half an inch or more. These last serve as a sort of temporary scaffolding to walk over, and to keep the radii properly stretched while she glues to them the concentric circles that are to remain, which she now proceeds to construct. Placing herself at the circumference, and fastening her thread to the end of one of the radii, she walks up that one, towards the centre, to such a distance as to draw the thread from her body of a sufficient length to reach to the next. Then stepping across and conducting the thread with one of her hind feet, she glues it with her spinners to the point in the adjoining radius to which it is to be fixed. This process she repeats until she has filled up nearly the whole space from the circumference to the centre with concentric circles distant from each other about two lines. She always, however, leaves a vacant interval around the smallest first spun circles that are nearest to the centre, but for what end I am unable to conjecture. Lastly, she runs to the centre and bites away the small cotton-like tuft that united all the radii, which being now held together by the circular threads have thus probably their elasticity increased; and in the circular opening resulting from this procedure she takes her station and watches for her prey.
In the above description, which is from my own observations, I have supposed the spider to fix the first and main line of her net to points from one of which she could readily climb to the other, dragging it after her; and many of these nets are placed in situations where this is very practicable. They are frequently, however, stretched in places where it is quite impossible for the spider thus to convey her main line—between the branchesof lofty trees having no connection with each other; between two distinct and elevated buildings; and even between plants growing in water. Here then a difficulty occurs. How does the spider contrive to extend her main line, which is often many feet in length, across inaccessible openings of this description?
With the view of deciding this question, to which I could find no very satisfactory answer in books, I made an experiment, for the idea of which I am indebted to a similar one recorded by Mr. Knight[743], who informs us that if a spider be placed upon an upright stick having its bottom immersed in water, it will, after trying in vain all other modes of escape, dart out numerous fine threads so light as to float in the air, some one of which attaching itself to a neighbouring object furnishes a bridge for its escape. It was clear that if this mode is pursued by the geometric spiders, it would go considerably towards furnishing a solution of the difficulty in question. I accordingly placed the large field spider (Epeira Diadema) upon a stick about a foot long, set upright in a vessel containing water. After fastening its thread (as all spiders do before they move) at the top of the stick, it crept down the side until it felt the water with its fore feet, which seem to serve as antennæ: it then immediately swung itself from the stick (which was slightly bent) and climbed up by the thread to the top. This it repeated perhaps a score times, sometimes creeping down a different part of the stick, but more frequently down the very side it had so often traversed in vain. Wearied with this sameness in its operations, I left the room for some hours. On my return I was surprised to find myprisoner escaped, and not a little pleased to discover, on further examination, a thread extended from the top of the stick to a cabinet seven or eight inches distant, which thread had doubtless served as its bridge. Eager to witness the process by which the line was constructed, I replaced the spider in its former position. After frequently creeping down and mounting up again as before, at length it let itself drop from the top of the stick, not as before by a single thread, but bytwo, each distant from the other about the twelfth of an inch, guided as usual by one of its hind feet, and one apparently smaller than the other. When it had suffered itself to descend nearly to the surface of the water, it stopped short, and, by some means which I could not distinctly see, broke off close to the spinners the smallest thread, which still adhering by the other end to the top of the stick floated in the air, and was so light as to be carried about by the slightest breath. On approaching a pencil to the loose end of this line, it did not adhere from mere contact. I therefore twisted it once or twice round the pencil, and then drew it tight. The spider, which had previously climbed to the top of the stick, immediately pulled at it with one of its feet, and, finding it sufficiently tense, crept along it, strengthening it as it proceeded by another thread, and thus reached the pencil[744].
That this therefore is one mode by which the geometric spiders convey the main line of their nets between distant objects, there can be no doubt, but that it is theonlyone is not so clear. If the position of the main line be thus determined by the accidental influence of the wind, we might expect to see these nets arranged with great irregularity, and crossing each other in every direction; yet it is the fact, that however closely crowded they may be, they constantly appear to be placed not by accident but design, commonly running parallel with each other at right angles with the points of support, and never interfering. Another objection too presents itself. From the experiment related, it is clear that the main line of the net can never be longer than the height of the object from which the spider dropped in forming it. But it is no uncommon thing to see nets in which these lines are a yard or two long, fastened to twigs of grass not a foot in height, and yet separated by obstacles effectually precluding the possibility of the spiders havingdraggedthe lines from one to the other. Here therefore some other process must have been used.
Both these difficulties would be removed by adopting the explanation of an anonymous author in theJournal de Physique[745], founded as he asserts on actual observation. He says that he saw a small spider, which he had forced to suspend itself by its thread from the point of afeather, shoot out obliquely in opposite directions other smaller threads, which attached themselves in the still air of a room, without any influence of the wind, to the objects towards which they were directed. He therefore infers that spiders have the power of shooting out threads and directing them at pleasure towards a determined point, judging of the distance and position of the object by some sense of which we are ignorant. Something like this manœuvre I once myself witnessed in a male of the small garden spider (Epeira? reticulata). It was standing midway on a long perpendicular fixed thread, and an appearance caught my eye of what seemed to be the emission of threads from its projected spinners. I therefore moved my arm in the direction in which they apparently proceeded, and, as I suspected, a floating thread attached itself to my coat, along which the spider crept. As this was connected with the spinners of the spider, it could not have been formed in the same way with the secondary thread ofE. Diademaabove described.
Probably in this case, as in so many others, we bewilder ourselves by attempting to make nature bend to generalities to which she disdains to submit. Different spiders may lay the foundations of their net in a different manner; some on the plan adopted byE. Diadema; others, as Lister long ago conjectured[746], by shooting out threads in the mode of the flying species, as in the instances recorded by the anonymous observer, and Mr. Knight. Nor is it improbable that the same species has the power of varying its procedures according to circumstances.
How far these suppositions are correct it is impossible to determine without further experiments, which it is somewhat strange should not before now have been instituted. Pliny thought it nothing to the credit of the philosophers of his day, that while they were disputing about the number of heroes of the name of Hercules, and the site of the sepulchre of Bacchus, they should not have decided whether the queen bee had a sting or not[747]; but it seems much more discreditable to the entomologists of ours, that they should yet be ignorant how the geometric spiders fix their nets. One excuse for them is, that these insects generally begin their operations in the night, so that, though it is very easy to see them spinning their concentric circles, it is seldom that they can be caught laying the foundations of their snares. Yet doubtless the lucky moment might be hit by an attentive observer, and I shall be glad if my attempt to describe their more ordinary operations should induce you to aim at signalizing yourself by the discovery. If you failed in solving every difficulty, you would at least be rewarded by witnessing their industry, ingenuity, and patience.
For the latter virtue they have no small occasion. Incapable of actively pursuing their prey, they are dependent upon what chance conducts into their toils, which, especially those spread in neglected buildings, often remain for a long period empty. Even the geometrical spiders, which fix themselves in the midst of a well-peopled district in the open air, have frequently to sustain a protracted abstinence. A continued storm of wind and rain will demolish their nets, and preclude thepossibility of reconstructing them for many days or sometimes weeks, during which not a single gnat regales their sharp-set appetites. And when at length formed anew or repaired, an unlucky bee or wasp, or an overgrown fly, will perversely entangle itself in toils not intended for insects of its bulk, and in disengaging itself once more leave the net in ruin.—All these trials move not our philosophic race. They patiently sit in their watching-place in the same posture, scarcely ever stirring but when the expected prey appears. And however repeatedly their nets are injured or destroyed, as long as their store of silk is unexhausted, they repair or reconstruct them without loss of time.
The web of a house spider will, with occasional repairs, serve for a considerable period; but the nets of the geometric spiders are in favourable weather renewed either wholly, or at least their concentric circles, every twenty-four hours, even when not apparently injured. This difference in the operations of the two tribes depends upon a very remarkable peculiarity in the conformation of their snares. The threads of the house spider's web are all of the same kind of silk, and flies are caught in them from their claws becoming entangled in the fine meshes which form the texture. On the other hand the net of the garden spider is composed of two distinct kinds of silk; that of the radii not adhesive, that of the circles extremely viscid[748]. The cause of this difference, which, when it is considered that both sorts ofsilk proceed from the same instrument, is truly wonderful, may be readily perceived. If you examine a newly formed net with a microscope, you will find that the threads composing the outline and the radii are simple, those of the circles closely studded with minute dew-like globules, which from the elasticity of the thread are easily separable from each other. That these are in fact globules of viscid gum, is proved by their adhering to the finger and retaining dust thrown upon the net, while the unadhesive radii and exterior threads remain unsoiled. It is these gummed threads alone which retain the insects that fly into the net; and as they lose their viscid properties by the action of the air, it is necessary that they should be frequently renewed.
In this renewal, as above hinted, the geometrical spiders are constantly regulated by the future probable state of the atmosphere, of which they have such a nice perception, that M. Q. D'Isjonval, to whom we are indebted for the fact, has proposed them as most accurate barometers. He asserts that if the weather be about to be variable, wet and stormy, the main threads which support the net will be certainly short; but if fine settled weather be on the point of commencing, these threads will be as invariably very long[749]. Without going the length with M. D'Isjonval of deeming his discoveries important enough to regulate the march of armies, or the sailing of fleets, or of proposing that the first appearance of these barometrical spiders in spring should be announced by the sound of trumpet, I have reason to suppose from my own observations that his statementsare in the main accurate, and that a very good idea of the weather may be formed from attending to these insects.
The spiders which form geometrical nets differ from the weavers also with respect to the situation in which they watch for their prey. They do not conceal themselves under their net, but are placed in the centre with their head downwards, and retire to a little apartment formed on one side under some leaf of a plant, only when obliged by danger or the state of the weather. The moment an unfortunate fly or other insect touches the net, the spider rushes towards it, seizes it with her fangs, and if it be a small species at once carries it to her little cell, and, having there at leisure sucked its juices, throws out the carcase. If the insect be larger and struggle to escape, with surprising address she envelops it with threads in various directions, until both its wings and legs being effectually fastened, she carries it off to her den. If the captured insect be a bee or a large fly so strong that the spider is sensible it is more than a match for her, she never attempts to seize or even entangle it, but on the contrary assists it to disengage itself, and often breaks off that part of the net to which it hangs, content to be rid of such an unmanageable intruder at any price.—When larger booty is plentiful, these spiders seem not to regard smaller insects. I have observed them in autumn, when their nets were almost covered with the Aphides which filled the air, impatiently pulling them off and dropping them untouched over the sides, as though irritated that their meshes should be occupied with such insignificant game.—A species of spider described by Lister, (Epeira conica,) more provident than its brethren, suspends itsprey in the meshes above and below the centre, and it is not uncommon to see its larder thus stored with several flies[750].
You must not infer that the toils of spiders are in every part of the world formed of such fragile materials as those which we are accustomed to see, or that they are every where contented with small insects for their food. An author in thePhilosophical Transactionsasserts, that the spiders of Bermudas spin webs between trees seven and eight fathoms distant, which are strong enough to ensnare a bird as large as a thrush[751]. And Sir G. Staunton informs us, that in the forests of Java, spiders' webs are met with of so strong a texture as to require a sharp-cutting instrument to make way through them[752].
Nor must you suppose that all the spiders of this country which catch their prey by means of snares, follow the same plan in constructing them as the weavers and geometricians whose operations I have endeavoured to describe. The form of their snares and the situation in which they place them are so various, that it is impossible to enumerate more than a few of the most remarkable.Agelene labyrinthicaextends over the blades of grass a large white horizontal net having at its margin a cylindrical cell, in the bottom of which, secure from birds and defended from the rays of the sun, the spider lies concealed, whence on the slightest movement of her net she rushes out upon her prey.Aranea latens, F. conceals itself under a small net spun upon the upper surface of a leaf, and thence seizes upon any insect that chances to pass over it.Theridium13-guttatumforms under stonesand in slight furrows in the ground a net consisting of threads spun without any regularity in all directions, but so strong as to entrap grasshoppers, which are said to be its principal food; and a similar inartificial snare of simple threads is often spun in windows byTheridium bipunctatumand several other species.Segestria senoculataand its affinities conceal themselves in a long cylindrical straight silken tube, from the mouth of which they stretch out their six anterior feet, whose extremities rest upon as many diverging threads: thus, as soon as an insect walks across any of the threads (which are eight or ten inches long) the insect's toes give it warning of prey being at hand, when it rushes out and seldom fails to secure its victim.