"the cricket chirrup in the hearth,"
"the cricket chirrup in the hearth,"
so constant a din every evening must very much interrupt comfort and conversation. These garrulous animals, which live in a kind of artificial torrid zone, are very thirsty souls, and are frequently found drowned in pans of water, milk, broth, and the like. Whatever is moist, even stockings or linen hung out to dry, is to them abonne bouche; they will eat the scummings of pots, yeast, crumbs of bread, and even salt, or any thing within their reach. Sometimes they are so abundant in houses as to become absolute pests, flying into the candles and into people's faces.
At Cuddapa, in the ceded districts to the northward of Mysore, Captain Green was much annoyed by a jumping insect, which from his description I should take for the larva of a species of cricket. They were of a dun colour, and from half to three-fourths of an inch in length. They abounded at night, and were very injurious to papers and books, which they both discoloured and devoured; leather also was eaten by them. Such was their boldness and avidity, that they attacked the exposed parts of the body when you were asleep, nibbling the ends of the fingers, particularly the skin under the nails, which was only discoverable by a slight sorenessthat succeeded. So great was their agility that they could seldom be caught or crushed. They were a mute insect, but probably the imago would make noise enough.
But thewhite ants, wherever they prevail, are a still worse plague than either of these insects—they are the great calamity, as Linné terms them, of both the Indies. When they find their way into houses or warehouses, nothing less hard than metal or glass escapes their ravages. Their favourite food, however, is wood of all kinds, except the teak (Tectona grandis) and iron-wood (Sideroxylon), which are the only sorts known that they will not touch[449]; and so infinite are the multitudes of the assailants, and such is the excellence of their tools, that all the timber-work of a spacious apartment is often destroyed by them in a few nights. Exteriorly, however, every thing appears as if untouched; for these wary depredators, and this is what constitutes the greatest singularity of their history, carry on all their operations by sap and mine, destroying first the inside of solid substances, and scarcely ever attacking their outside, until first they have concealed it and their operations with a coat of clay. A general similarity runs through the proceedings of the whole tribe; but the large African species,(called by SmeathmanTermes bellicosus,)T. fatalisis the most formidable. These insects live in large clay nests, from whence they excavate tunnels all round, often to the extent of several hundred feet; from these they will descend a considerable depth below the foundation of a house, and rise again through the floors; or, boring through the posts and supports of the building, enter the roof, and construct there their galleries in various directions. If a post be a convenient path to the roof, or has any weight to support, which how they discover is not easily conjectured, they will fill it with their mortar, leaving only a trackway for themselves; and thus, as it were, convert it from wood into stone as hard as many kinds of free-stone. In this manner they soon destroy houses, and sometimes even whole villages when deserted by their inhabitants, so that in two or three years not a vestige of them will remain.
These insidious insects are not less expeditious in destroying the wainscoting, shelves, and other fixtures of a house than the house itself. With the most consummate art and skill they eat away all the inside of what they attack, except a few fibres here and there which exactly suffice to keep the two sides, or top and bottom, connected, so as to retain the appearance of solidity after the reality is gone; and all the while they carefully avoid perforating the surface, unless a book or any other thing that tempts them should be standing upon it. Kæmpfer, speaking of the white ants of Japan, gives a remarkable instance of the rapidity with which these miners proceed. Upon rising one morning he observed that one of their galleries of the thickness of his little finger had been formed across his table; and, upon a furtherexamination, he found that they had bored a passage of that thickness up one foot of the table, formed a gallery across it, and then pierced down another foot into the floor: all this was done in the few hours that intervened between his retiring to rest and his rising[450]. They make their way also with the greatest ease into trunks and boxes, even though made of mahogany, and destroy papers and every thing they contain, constructing their galleries and sometimes taking up their abode in them. Hence, as Humboldt informs us, throughout all the warmer parts of equinoctial America, where these and other destructive insects abound, it is infinitely rare to find papers which go fifty or sixty years back[451]. In one night they will devour all the boots and shoes that are left in their way; cloth, linen, or books are equally to their taste; but they will not eat cotton, as Captain Green informs me. I myself have to deplore that they entirely consumed a collection of insects made for me by a friend in India, more especially as it sickened him of the employment. In a word, scarcely any thing, as I said before, but metal or stone comes amiss to them. Mr. Smeathman relates, that a party of them once took a fancy to a pipe of fine old Madeira, not for the sake of the wine, almost the whole of which they let out, but of the staves, which however I suppose were strongly imbued with it, and perhaps on that account were not less to the taste of our epicure Termites. Having left a compound microscope in a warehouse at Tobago for a few months, on his return he found that a colony of a small species of white ant had established themselves init, and had devoured most of the wood-work, leaving little besides the metal and glasses[452]. A shorter period sufficed for their demolition of some of Mr. Forbes's furniture. On surveying a room which had been locked up during an absence of a few weeks, he observed a number of advanced works in various directions towards some prints and drawings in English frames; the glasses appeared to be uncommonly dull, and the frames covered with dust. "On attempting," says he, "to wipe it off, I was astonished to find the glasses fixed to the wall, not suspended in frames as I left them, but completely surrounded by an incrustation cemented by the white ants, who had actually eaten up the deal frames and back-boards, and the greater part of the paper, and left the glasses upheld by the incrustation, or covered way, which they had formed during their depredation[453]." It is even asserted that the superb residence of the Governor-General at Calcutta, which cost the East India Company such immense sums, is now rapidly going to decay in consequence of the attacks of these insects[454].—But not content with the dominions they have acquired, and the cities they have laid low on Terra Firma, encouraged by success the white ants have also aimed at the sovereignty of the ocean, and once had the hardihood to attack even a British ship of the line; and, in spite of the efforts of her commander and his valiant crew, having boarded they got possession of her, and handled her so roughly, that when brought into port,being no longer fit for service, she was obliged to be broken up[455].
And here, I think, I see you throw aside my papers, and hear you exclaim—"Will this enumeration of scourges, plagues, and torments never be finished? Was the whole insect race created merely with punitive views, and to mar the fair face of universal nature? Are they all, as our Saviour said figuratively of one genus, the scorpion, the powerful agents and instruments of the great enemy of mankind[456]?" If you view the subject in another light, you will soon, my friend, be convinced that, instead of this, insects generally answer the most beneficial ends, and promote in various ways, and in an extraordinary degree, the welfare of man and animals; and that the series of evils I have been engaged in enumerating mostly occur partially, and where they exceed their natural limits; God permitting this occasionally to take place, not merely with punitive views, but also to show us what mighty effects he can produce by instruments seemingly the most insignificant: thus calling upon us to glorify his power, wisdom, and goodness, so evidently manifested whether he relaxes or draws tight the reins by which he guides insects in their course, and regulates their progress; and more particularly to acknowledgehis overruling Providence so conspicuously exhibited by his measuring them, as it were, and weighing them, and telling them out, so that, their numbers, forces and powers being annually proportioned to the work he has prescribed to them, they may neither exceed his purpose nor fall short of it.
From the picture I have drawn, and I assure you it is not overcharged, you will be disposed to admit, however, the empire of insects over the works of creation, and to own that our prosperity, comfort and happiness are intimately connected with them; and consequently that the knowledge and study of them may be extremely useful and necessary to promote these desirable ends, since the knowledge of the cause of any evil is always a principal, if not an indispensable, step towards a remedy.
I shall now bid adieu to this unpromising subject, which has so long occupied my pen, and I fear wearied your attention, and in my next bring before you a more agreeable scene, in which you will behold thebenefitswe receive by the ministry of insects.
I am, &c.
My last letters contained, I must own, a most melancholy though not an overcharged picture of the injuries and devastation which man, in various ways, experiences through the instrumentality of the insect world. In this and the following I hope to place before you a more agreeable scene, since in them I shall endeavour to point out in what respects these minute animals are made to benefit us, and what advantages we reap from their extensive agency.
God, in all the evil which he permits to take place, whether spiritual, moral, or natural, has the ultimate good of his creatures in view. The evil that we suffer is often a countercheck which restrains us from greater evil, or a spur to stimulate us to good: we should therefore consider every thing, not according to the present sensations of pain, or the present loss or injury that it occasions, but according to its more general, remote, and permanent effects and bearings;—whether by it we are not impelled to the practice of many virtues which otherwise might lie dormant in us—whether our moral habits are not improved—whether we are not rendered by it more prudent, cautious, and wary, more watchful to prevent evil, more ingenious and skilful to remedy it—and whetherour higher faculties are not brought more into play, and our mental powers more invigorated, by the meditation and experiments necessary to secure ourselves. Viewed in these lights, what was at first regarded as wholly made up of evil, may be discovered to contain a considerable proportion of good.
This reasoning is here particularly applicable: and if the ultimate benefit to man seems in any case problematical, it is merely because to discover it requires more extended and remote views than we are enabled by our limited faculties to take, and a knowledge of distant or concealed results which we are incompetent to calculate or discover. The common good of this terraqueous globe requires that all things endowed with vegetable or animal life should bear certain proportions to each other; and if any individual species exceeds that proportion, from beneficial it becomes noxious, and interferes with the general welfare. It was requisite therefore for the benefit of the whole system that certain means should be provided, by which this hurtful luxuriance might be checked, and all things taught to keep within their proper limits: hence it became necessary that some should prey upon others, and a part be sacrificed for the good of the whole.
Of the counterchecks thus provided, none act a more important part than insects, particularly in the vegetable kingdom, every plant having its insect enemies. Man, when he takes any plant from its natural state and makes it an object of cultivation, must expect that these agents will follow it into the artificial state in which he has placed it, and still prey upon it; and it is his business to exert his faculties in inventing means to guard against their attacks. It is a wise provision that there should exist a race of beings empowered to remove all her superfluous productionsfrom the face of nature; and in effecting this, whatever individual injury may arise, insects must be deemed general benefactors. Even the locusts which lay waste whole countries clear the way for the renovation of their vegetable productions, which were in danger of being destroyed by the exuberance of some individual species, and thus are fulfilling the great law of the Creator, that of all which he has made nothing should be lost. A region, Sparrman tells us, which had been choked up by shrubs, perennial plants, and hard half-withered and unpalatable grasses, after being made bare by these scourges, soon appears in a far more beautiful dress, clothed with new herbs, superb lilies, and fresh annual grasses, and young and juicy shoots of the perennial kinds, affording delicious herbage for the wild cattle and game[457]. And though the interest of individual man is often sacrificed to the general good, in many cases the insect pests which he most execrates will be found to be positively beneficial to him, unless when suffered to increase beyond their due bounds. Thus the insects that attack the roots of the grasses, and, as has been before observed, so materially injure our herbage, the wire-worm, the larvæ ofMelolontha vulgaris,Tipula oleracea, &c., in ordinary seasons only devour so much as is necessary to make room for fresh shoots, and the production of new herbage; in this manner maintaining a constant succession of young plants, and causing an annual though partial renovation of our meadows and pastures. In the rich fields near Rye in Sussex I particularly observed this effect; and I have since at home remarked, that at certain times of the year dead plants may be every where observed, pulled up by the cattle as they feed, whose place is supplied by new offsets.So that, when in moderate numbers, these insects do no more harm to the grass than would the sharp-toothed harrows which it has been sometimes advised to apply to hide-bound pastures, and the beneficial operation of which in loosening the sub-soil these insect-borers closely imitate.
Nor would it be difficult to show that the ordinary good effects of some of those insects, which torment ourselves and our cattle, preponderate over their evil ones. Mr. Clark is inclined to think that the gentle irritation ofGasterophilus Equiis advantageous to the stomach of the horse rather than the contrary. On the same principle it is not improbable that the Tabani often act as useful phlebotomists to our full-fed animals; and that the constant motion in which they are kept in summer by the attacks of the Stomoxys and other flies, may prevent diseases that would be brought on by indolence and repletion. And in the case of man himself, if I do not go so far as with Linné to give the louse the credit of preserving full-fed boys from coughs, epilepsy, &c., we may safely regard as no small good, the stimulus which these, and others of the insect assailants of the persons of the dirty and the vicious, afford to personal cleanliness and purity.
I might enlarge greatly upon the foregoing view of the subject: but this is unnecessary, as numerous facts will occur in subsequent letters which you will readily perceive have an intimate bearing upon it; and I shall therefore proceed to point out the more evident benefits which we derive from insects, arranging them under the two great heads ofdirectbenefits, and those which areindirect; beginning with the latter.
The insects which areindirectlybeneficial to us, may be considered under three points of view: First, as removing various nuisances and deformities from the face of nature: Secondly, as destroying other insects, that but for their agency would multiply so as greatly to injure and annoy us: and Thirdly, as supplying food to useful animals, particularly to fish and birds.
To advert in thefirstplace to the former. All substances must be regarded as nuisances and deformities, when considered with relation to the whole, which are deprived of the principle of animation. In this relation stand a dead carcase, a dead tree, or a mass of excrement, which are clearly incumbrances that it is desirable to have removed; and the office of effecting this removal is chiefly assigned to insects, which have been justly called the great scavengers of nature. Let us consider their little but effective operations in each of their vocations.
How disgusting to the eye, how offensive to the smell, would be the whole face of nature, were the vast quantity ofexcrementdaily falling to the earth from the various animals which inhabit it, suffered to remain until gradually dissolved by the rain or decomposed by the elements! That it does not thus offend us, we are indebted to an inconceivable host of insects which attack it the moment it falls; some immediately beginning to devour it, others depositing in it eggs from which are soon hatched larvæ that concur in the same office with tenfold voracity: and thus every particle of dung, at least of the most offensive kinds, speedily swarms with inhabitants which consume all the liquid and noisome particles, leaving nothing but the undigested remains, that soon dryand are scattered by the winds, while the grass upon which it rested, no longer smothered by an impenetrable mass, springs up with increased vigour.
Numerous are the tribes of insects to which this office is assigned, though chiefly if not entirely selected from the two ordersColeopteraandDiptera. A large proportion of the genera formed, by different authors, from Scarabæus of Linné, viz.Scarabæus,Copris,Ateuchus,Sisyphus,Onitis,Onthophagus,Aphodius, andPsammodius; alsoHister,Sphæridium; and amongst theBrachyptera, the majority of theStaphylinidæ, manyAleocharæ, especially of Gravenhorst's third family, manyOxyteliand someOmalia,TachiniandTachypori, of that author, including in the whole many hundred species of beetles—unite their labours to effect this useful purpose: and what is remarkable, though they all work their way in these filthy masses, and at first can have no paths, yet their bodies are never soiled by the ordure they inhabit. Many of these insects content themselves with burrowing in the dung alone; butAteuchus pilularius[458], a species called in America theTumble-dung, whose singular manœuvres I shall subsequently have to advert to,Coprislunaris,Geotrupes stercorariusand many other lamellicorn beetles, make large cylindrical holes, often of great depth, under the heap, and there deposit their eggs surrounded by a mass of dung in which they have previously enveloped them; thus not only dispersing the dung, but actually burying it at the roots of the adjoining plants, and by these means contributing considerably to the fertility of our pastures, supplying the constant waste by an annual conveyance of fresh dung laid at the very root; by these canals, also, affording a convenient passage for a portion of it when dissolved to be carried thither by the rain.
The coleopterous insects found in dung inhabit it in their perfect as well as imperfect states: but this is not the case with those of the orderDiptera, whose larvæ alone find their nutriment in it; the imago, which would be suffocated did it attempt to burrow into a material so soft, only laying its eggs in the mass. These also are more select in their choice than theColeoptera—not indeed as to delicacy,—but they do not indiscriminately oviposit in all kinds, some preferring horse-dung, others swine's-dung, others cow-dung, which seems the most favourite pabulum of all the dung-loving insects, and others that of birds. The most disgusting of all is the rat-tailed larva that inhabits our privies, which changes to a fly (Eristalis tenax) somewhat resembling a bee.
Still more would our olfactory nerves be offended, and our health liable to fatal injuries, if the wisdom and goodness of Providence had not provided for the removal of another nuisance from our globe—thedead carcasesof animals. When these begin to grow putrid, every one knows what dreadful miasmata exhale from them, andtaint the air we breathe. But no sooner does life depart from the body of any creature, at least of any which from its size is likely to become a nuisance, than myriads of different sorts of insects attack it, and in various ways. First come the Histers and pierce the skin. Next follow the flesh-flies, some, that no time may be lost, (asSarcophaga carnaria, &c.) depositing upon it their young already hatched[459]; others (Musca Cæsar, &c.) covering it with millions of eggs, whence in a day or two proceed innumerable devourers. An idea of the dispatch made by these gourmands may be gained from the combined consideration of their numbers, voracity and rapid development. One female ofS. carnariawill give birth to 20,000 young; and the larvæ of many flesh-flies, as Redi ascertained, will in twenty-four hours devour so much food, and grow so quickly, as to increase their weight two hundred fold! In five days after being hatched they arrive at their full growth and size, which is a remarkable instance of the care of Providence in fitting them for the part they are destined to act: for if a longer time was required for their growth, their food would not be a fit aliment for them, or they would be too long in removing the nuisance it is given in charge to them to dissipate. Thus we see there was some ground for Linné's assertion underM. vomitoria, that three of these flies will devour a dead horse as quickly as would a lion.
As soon as the various tribes ofMuscidæhave opened the way, and devoured the softer parts, a whole host of beetles,Necrophori,Silphæ,Dermestes,Cholevæ, andStaphylinidæ, actively second their labours. Wasps and hornets also come in for their portion of the spoil; andeven ants, which prowl every where, rival their giant competitors in the quantity consumed by them; so that in no very long time, especially in warm climates, the muscular covering is removed from the skeleton, which is then cleansed from all remains of it by the littleCorynetes cæruleusandruficollis, (which last is so interesting, as having been the means of saving the life of Latreille[460],) and severalNitidulæ[461]. Even the horns of animals have an appropriate genus (Trox) which inhabits them, and feeds upon their contents. And not only are large animals thus disposed of, even the smallest are not suffered long to annoy us. The burying beetle (Necrophorus Vespillo) inters the bodies of small animals, such as mice, several assisting each other in the work; and those to which they commit their eggs afford an ample supply of food to their larvæ[462]. Ants also in some degree emulate these burying insects, at least they will carry off the carcases of insects into their nests; and I once saw some of the horse-ants dragging away a half-dead snake of about the size of a goose-quill[463]. Some insects will even attack living animals and make them their prey, thus contributing to keep them within due limits. The common earth-worm is attacked and devoured by a centipede(Geophilus electricus). Mr. Sheppard saw one attack a worm ten times its own size, round which it twisted itself like a serpent, and which it finally mastered and devoured.
But insects are not only useful in removing and dissipating dead animal matter; they are also intrusted with a similar office with respect to thevegetablekingdom. The interior of rotten trees is inhabited by the larvæ of a particular kind of crane-fly with pectinated antennæ (Ctenophora[464]), and other insects, which there find an appropriate nutriment; and a similar diet is furnished to the grubs of the rose-beetle (Cetonia aurata) by the dead leaves and stalks usually to be found in an ant's nest.Staphylinidæ,Sphæridia, and otherColeoptera, are always found under heaps of putrescent vegetables; and an infinite number are to be met with in decomposing fungi, which seem to be a kind of substance intermediate between animal and vegetable. The Boleti in particular have a genus of coleopterous insects appropriated to them[465], and the Lycoperdons another.—Stagnant waters, which would otherwise exhale putrid miasmata and be often the cause of fatal disorders, are purified by the innumerable larvæ of gnats, Ephemeræ, and other insects which live in them and abstract from them all the unwholesome part of their contents. This, Linné says, will easily appear if any one will make the experiment by filling two vessels with putrid water, leaving the larvæ in one and taking them out of the other. For then he will soon find thewater that is full of larvæ pure and without any stench, while that which is deprived of them will continue stinking[466].
Benefits equally great are rendered by the wood-destroying insects. We indeed, in this country, who find use for ten times more timber than we produce, could dispense with their services; but to estimate them at their proper value, as affecting the great system of nature, we should transport ourselves to tropical climes, or to those under the temperate zones, where millions of acres are covered by one interminable forest. How is it that these untrodden regions, where thousands of their giant inhabitants fall victims to the slow ravages of time, or the more sudden operations of lightning and hurricanes, should yet exhibit none of those scenes of ruin and desolation that might have been expected, but are always found with the verdant characters of youth and beauty? It is to the insect world that this great charge of keeping the habitations of the Dryads in perpetual freshness has been committed. A century almost would elapse before the removal from the face of nature of the mighty ruins of one of the hard-wooded tropical trees, by the mere influence of the elements. But how speedy its decomposition when their operations are assisted by insects! As soon as a tree is fallen, one tribe attack its bark[467], which is often the most indestructible part of it; and thousandsof orifices into the solid trunk are bored by others. The rain thus insinuates itself into every part, and the action of heat promotes the decomposition. Various fungi now take possession and assist in the process, which is followed up by the incessant attacks of other insects, that feed only upon wood in an incipient state of decay. And thus in a few months a mighty mass, which seemed inferior in hardness only to iron, is mouldered into dust, and its place occupied by younger trees full of life and vigour. The insects to which this duty is intrusted have been already mentioned in a former letter (p.235—); but none of them do their business so expeditiously or effectually as the Termites, which ply themselves in such numbers and so unremittingly, that Mr. Smeathman assures us they will in a few weeks destroy and carry away the trunks of large trees, without leaving a particle behind; and in places where, two or three years before, there has been a populous town, if the inhabitants, as is frequently the case, have chosen to abandon it, there shall be a very thick wood, and not the vestige of a post to be seen.
I observed in a former letter, that the devastations of insects are not the same in every season, their power of mischief being evident only at certain times, when Providence, by permitting an unusual increase of their numbers, gives them a commission to lay waste any particular country or district. The great agents in preventing this increase, and keeping the noxious species within proper limits, are other insects; and to these I shall now call your attention.
Numerous are the tribes upon which this importanttask devolves, and incalculable are the benefits which they are the means of bestowing upon us; for to them we are indebted, or rather to Providence who created them for this purpose, that our crops and grain, our cattle, our fruit- and forest-trees, our pulse and flowers, and even the verdant covering of the earth, are not totally destroyed. Of these insects, so friendly to man, some exercise their destructive agency solely while in the larva state; others in the perfect state only; others in both these states; and lastly, others again in all the three states of larva, pupa, and imago. For order's sake, and to give you a more distinct view of the subject, I shall say something on each separately.
The first, those which are insectivorous only in theirlarvastate, maybe further subdivided intoparasitesandimparasites, meaning by the former term those that feed upon alivinginsect, and only destroy it when they have attained their full growth; and by the latter, those that prey upon insects already dead, or that kill them in the act of devouring them.
Theimparasiticinsect devourers chiefly belong to theHymenopteraorder; and though it is in the larva state that their prowess is exhibited, the task of providing the prey is usually left to the female, of which each species for the most part selects a particular kind of insect. Thus many species ofCercerisand the splendidChrysidæor golden wasps feed upon insects of their own order. One of the latter (Parnopes incarnata) commits her eggs to the progeny ofBembex rostrata: another (Chrysis bidentata) attacks the young ofEpipone spinipes.
BembexandMellinusconfine themselves toDiptera,the former preying uponEristalis tenax,Bombylii, and the like[468]; the latter amongst others ridding us of the troublesomeStomoxys calcitrans. One of these last I have observed stationed on dung watching for flies, which when seized, she carried to her burrow.
Epipone spinipes, belonging to the family of Wasps, feeds upon certain green apod larvæ, of which the female deposits ten or twelve with each egg.Ammophila vulgarisdestroys caterpillars of a larger size, and it is probable that most of the other Vespoid and SphecoidHymenoptera, viz.Trypoxylon,Philanthus,Larra,Crabro, &c. assist in this great work.
Pompilus, to which genus probably several species mentioned by Reaumur as preying on these insects should be referred, has it in charge to keep the number of spiders within due bounds: and some Sand-wasps lend their aid. One of these last, mentioned by Catesby (Pronæus cæruleus), has been known to seize a spider eight times its own weight[469]. Another species of this genus, which is common in the Isle of France, attacks an insect still more difficult, one would think, to turn to its purpose, the all-devouringBlattaor cock-roach, and is therefore one of the great benefactors to mankind. When this insect perceives a Blatta (called there Kakerlac and Cancrelas) it stops immediately: both animals eye each other; but in an instant the sand-wasp darts upon its prey, seizes it by the muzzle with its strong jaws, and bending its abdomen underneath it, pierces it with its fatal sting. Sure of its victim, it now walks or flies away, leaving the poisonto work its effect; but in a short time returns, and, finding it deprived of power to make resistance, seizes it again by the head, and drags it away, walking backwards to deposit it in a hole or chink of a wall[470].
Grasshoppers are the prey of another sand-wasp, supposed to be theSphex pensylvanicaof Linné, a native of North America, each of which in its larva state devours three of a large green species with which its mother has provided it[471].
From none of the imparasitic insectivorous larvæ do we derive more advantage than from those which devour the destructive Aphides, whose ravages, as we have seen above, are more detrimental to us in this island than those of any other insect. A great variety of species, of different orders and genera, are employed to keep them within due limits. There is a beautiful genus of four-winged flies, whose wings resemble the finest lace, and whose eyes are often as brilliant as burnished metals (Hemerobius), the larvæ of which, Reaumur, from their being insatiable devourers of them, has named the lions of the Aphides. The singular pedunculated eggs from which these larvæ proceed I shall describe when we come to treat upon the eggs of insects; the larvæ themselves are furnished with a pair of long crooked mandibles resembling horns, which terminate in a sharp point, and like those of the ant-lion are perforated, serving the insect instead of a mouth; for through this orifice the nutriment passes down into the stomach. When amongst the Aphides, like wolves in a sheep-fold, they make dreadful havoc: half a minute suffices them to suck thelargest; and the individuals of one species clothe themselves, like Hercules, with the spoils of their hapless victims.
Next in importance to these come the aphidivorous flies (many species ofSyrphidæ), whose grubs are armed with a singular mandible, furnished like a trident with three points, with which they transfix their prey. They may often be seen laid at their ease under a leaf or upon a twig, environed by such hosts of Aphides, that they can devour hundreds without changing their station; and their silly helpless prey, who are provided with no means of defence, so far from thinking of escaping, frequently walk over the back of their enemy, and put themselves in his way. When disposed to feed, he fixes himself by his tail, and, being blind, gropes about on every side, as the Cyclops did for Ulysses and his companions, till he touches one, which he immediately transfixes with his trident, elevates into the air, that he may not be disturbed by its struggles, and soon devours. The havoc which these grubs make amongst the Aphides is astonishing. It was but last week that I observed the top of every young shoot of the currant-trees in my garden curled up by myriads of these insects. On examining them this day, not an individual remained; but beneath each leaf are three or four full-fed larvæ of aphidivorous flies, surrounded with heaps of the skins of the slain, the trophies of their successful warfare; and the young shoots, whose progress had been entirely checked by the abstraction of sap, are again expanding vigorously.
But even these serviceable insects must yield the palm to the lady-bird or lady-cow (Coccinella), the favouriteof our childhood, which, as well as most of its congeners, in the larva state feeds entirely on Aphides[472]; and the havoc made amongst them may be conceived from the myriads upon myriads of these little interesting animals, which are often to be seen in years when the plant-louse abounds. In 1807 the shore at Brighton and all the watering-places on the south coast was literally covered with them, to the great surprise and even alarm of the inhabitants, who were ignorant that their little visitors were emigrants from the neighbouring hop-grounds, where in their larva state each had slain his thousands and tens of thousands of the Aphis, which under the name of theFlyso frequently blasts the hopes of the hop-grower. It is fortunate that in most countries the children have taken these friendly Coccinellæ under their protection. In France they regard them as sacred to the Virgin, and call themVaches à Dieu,Bêtes de la Vierge, &c.; and with us, commiseration for the hard fate of a mother, whose "house is on fire and children at home," ensures them kind treatment and liberty. Even the hop-growers are becoming sensible of their services, and, as I am informed, hire boys to prevent birds from destroying them.—If we could but discover a mode of increasing these insects at will, we might not only, as Dr. Darwin has suggested, clear our hot-houses of Aphides by their means, but render our crops of hops much more certain than they now are. Even withoutthis knowledge, nothing is more easy, as I have experienced, than to clear a plant or small tree by placing upon it several larvæ of Coccinellæ or of aphidivorous flies collected from less valuable vegetables.
Lastly, to close this list of imparasitic insectivorous larvæ, I may mention those of Geoffroy's genusVolucellaso remarkable for their radiated anus, which live in the nests of humble-bees, braving the fury of their stings and devouring their young; and the ant-lion (Myrmeleon) and Reaumur's improperly named worm-lion (Leptis), whose singular stratagems will be detailed in a subsequent letter, both of which destroy great numbers of insects that are so unfortunate as to fall into their toils.
Theparasiticlarvæ, an extremely numerous tribe, must next be considered. These, with the exception of a very few individuals, belong to the orderHymenoptera, and were included by Linné under his vast genusIchneumon, so named from the analogy between their services and those of the Egyptian Ichneumons (Viverra Ichneumon), the former as destroyers of insects, being equally important with the latter as devourers of serpents, the eggs of crocodiles, &c.
The habits of the whole of this tribe[473], which properly includes a great number of distinct genera, are similar. They all oviposit in living insects, chiefly while in the larva state, sometimes while pupæ (Misocampus Puparum);and even while in the egg state (Ich. Ovulorum, L[474].); but not, as far as is known, in perfect insects. The eggs thus deposited soon hatch into grubs, which immediately attack their victim, and in the end ensure its destruction. The number of eggs committed to each individual varies according to its size, and that of the grubs which are to spring from them; being in most cases one only, but in others amounting to some hundreds.
From the observations hitherto made by entomologists, the great body of the Ichneumon tribe is principally employed in keeping within their proper limits the infinite host oflepidopterouslarvæ, destroying, however, many insects of other orders; and perhaps if the larvæ of these last fell equally under our observation with those of the former, we might discover that few exist uninfested by their appropriate parasite. Such is the activity and address of the Ichneumonidans, and theirminuteallies (Pupivora, Latr.), that scarcely any concealment, except perhaps the waters, can secure their prey from them; and neither bulk, courage, nor ferocity avail to terrify them from effecting their purpose. They attack the ruthless spider in his toils; they discover the retreat of the little bee, that for safety bores deep into timber; and though its enemy Ichneumon cannot enter its cell, by means of her long ovipositor[475]she reaches the helpless grub, which its parent vainly thought secured from every foe, and deposits in it an egg, which produces a larvathat destroys it[476]. In vain does the destructiveCecidomyiaof the wheat conceal its larvæ within the glumes that so closely cover the grain; three species of these minute benefactors of our race, sent in mercy by Heaven, know how to introduce their eggs into them, thus preventing the mischief they would otherwise occasion, and saving mankind from the horrors of famine[477]. In vain also theCynipsby its magic touch produces the curious excrescences on various trees and plants, called galls, for the nutriment and defence of its progeny: the parasite species attached to it discovers its secret chamber, pierces its wall however thick, and commits the destroying egg to its offspring. Even the clover-weevil is not secure within the legumen of that plant; nor the wire-worm in the earth, from their ichneumonidan foes. I have received from the late Mr. Markwick that of the former, and Mr. Paul has shown me the destroyer of the latter, which belongs to Latreille's genusProctotrupes. Others are not more secured by the repulsive nature of the substance they inhabit; for two species at least of Ichneumon[478]know how to oviposit it in stercorarious larvæ without soiling their wings or bodies.
The ichneumonidan parasites are either external or internal. Thus the species above alluded to, which attacks spiders, does not live within their bodies, but remains on the outside[479]; and the larva ofOphion luteum, which adheres by one end to the shell of the bulbiferous egg that produced it, does not enter the caterpillar ofEuprepia villica, the moth upon which it feeds[480]. Butthe great majority of these animals oviposit within the body of the insect to which they are assigned, from whence, after having consumed the interior and become pupæ, they emerge in their perfect state. An idea of the services rendered to us by those Ichneumons which prey upon noxious larvæ may be formed from the fact, that out of thirty individuals of the common cabbage caterpillar (the larvæ ofPontia Brassicæ) which Reaumur put into a glass to feed, twenty-five were fatally pierced by an Ichneumon (Microgaster globatus[481]). And if we compare the myriads of caterpillars that often attack our cabbages and broccoli with the small number of butterflies of this species which usually appear, we may conjecture that they are commonly destroyed in some such proportion—a circumstance that will lead us thankfully to acknowledge the goodness of Providence, which by providing such a check has prevented the utter destruction of the Brassica genus, including some of our most esteemed and useful vegetables.
The parasites are not wholly confined to the orderHymenoptera: some insects of other orders, though comparatively very few, destroy our little enemies in the same way.Tachina Larvarum, and another like it described by De Geer, lay their eggs in caterpillars and other larvæ[482]; and Reaumur describes several other flies of similar habits[483]. The order also ofStrepsiptera, lately established[484], appears to be altogether parasitic; but with this difference from thePupivora, that these extraordinary animals are found only uponHymenopterain their perfect state, and do not appear to destroy the insects upon which they prey, but probably prevent theirbreeding. The species at present known are formed into two genera,XenosandStylops, which are confined toMelitta[485]andVespa[486].
The next description of insect destroyers are those which devour them in theirfirstandlaststates.—No beetles are more common after the summer is confirmed, than the species of the genusTelephorus. Preysler informs us that the grub ofT. fuscusdestroys a great many other larvæ[487], and I have observed the imago devour these and alsoDiptera.—Linné has with justice denominated theCicindelæthe tigers of insects. Though decorated with brilliant colours, they prey upon the whole insect race; their formidable jaws which cross each other are armed with fearful fangs, showing to what use they are applicable; and the extreme velocity with which they can either run or fly, renders hopeless any attempt to elude their pursuit. Their larvæ are also equally tremendous with the imago, having eight eyes, four on each side, seated on a lateral elevation of the head, two above and two very minute below, which look like those of spiders, and besides their threatening jaws armed with a strong internal tooth, being furnished with a pair of spines resembling somewhat the sting of a scorpion, which stand erect upon the back of the abdomen, and give them a most ferocious aspect[488]. This last apparatus, according to Clairville, serves the purpose of an anchor for retaining them at any height in their deep cells[489]. Most of the aquatic beetles, at least the Gyrini and Dytisci, prey upon other insects both in their firstand final state. The larvæ of the latter have long been observed and described under the name ofSquillæ, and are remarkable for having their mandibles adapted for suction like those of Hemerobius and Myrmeleon; but they are not like them deprived of a mouth, being able to devour by mastication as well as by suction.—Another tribe of this order which abounds in species, those predaceous beetles which form Linné's great genusCarabus(Eutrechina[490]), is universally insectivorous. One of the most destructive is the grub of a very beautiful species, an English specimen of which would be a great acquisition to your cabinet, it being one of our rarest insects[491], I meanCalosoma Sycophanta. This animal takes up its station in the nests ofLasiocampa processioneaand other moths, and sometimes fills itself so full with these caterpillars, which we cannot handle or even approach without injury, as to be rendered incapable of motion and appear ready to burst. Another beautiful insect of this tribe,Carabus auratus, known in France by the name ofVinaigrier, is supposed to destroy more cockchafers than all their other enemies, attacking and killing the females at the moment of oviposition, and thus preventing the birth of thousands of young grubs[492]. Lastly come theBrachyptera, many of which prey upon insects as well as on putrescent substances. Mr. Lehmann tells us thatsome of them are very useful in destroying a weevil,Apion flavifemoratum[493], the great enemy of our crops of clover seed.
Amongst the devourers of insects in theirperfectstate only, must be ranked a few of the social tribes, ants, wasps, and hornets. The first-mentioned indefatigable and industrious creatures kill and carry off great numbers of insects of every description to their nests, and prodigious are their efforts in this work. I have seen an ant dragging a wild bee many times bigger than itself; and there was brought to me this very morning while writing this letter, anElaterquite alive and active, which three or four ants in spite of its struggles were carrying off. An observing friend of mine[494], who was some time in Antigua, informed me that in that island, a kind of ant which construct their nests in the roofs of houses, when they meet with any animal larger than they can carry off alive, such as a cock-roach, &c., will hold it by the legs so that it cannot move, till some of them get upon it and dispatch it, and then with incredible labour carry it up to their nest. Madam Merian, in her account of the periodical ants mentioned to you before[495], and which is confirmed by Azara[496], notices their clearing the houses of cock-roaches and similar animals; andMyrmica omnivorais very useful in Ceylon in destroying the former insect, the larger ant, and the white ant[497].
You are not perhaps accustomed to regard wasps and hornets as of any use to us; but they certainly destroy an infinite number of flies and other annoying insects. The year 1811 was remarkable for the small number of wasps,though many females appeared in the spring, scarcely any neuters being to be seen in the autumn[498]; and probably in consequence of this circumstance, flies in many places were so extremely numerous as to be quite a nuisance. Reaumur has observed that in France the butchers are very glad to have wasps attend their stalls, for the sake of their services in driving away the flesh-fly; and if we may believe the author of Hector St. John'sAmerican Letters, the farmers in some parts of the United States are so well aware of their utility in this respect, as to suspend in their sitting-rooms a hornet's nest, the occupants of which prey upon the flies without molesting the family.
There are other devourers of insects in their perfect state, the manners and food of whose larvæ we are unacquainted with. St. Pierre speaks of a lady-bird, but it probably belonged to some other genus, of a fine violet colour, with a head like a ruby, which he saw carry off a butterfly[499]. Linné informs us thatClerus formicariusdevoursAnobium pertinax. A fly related to thePanorpa communisappears created to instill terror into the pitiless hearts of the tyrants of our lakes and pools,—the all-devouringLibellulina[500]. TheAsilialso, which are always upon the chase, seize insects with their anterior legs and suck them with their haustellum. The cognate genusDioctria, particularlyD. œlandica, prey uponHymenoptera, by some unknown means instantaneously killing the insect they seize. Many species alsoofEmpis, whose haustellum resembles the beak of a bird, carry off in itTipulariæand other smallDiptera; and what is remarkable, you can seldom take these insects in coitu, but the female has a gnat, some fly, or sometimes beetle, in her mouth. Can this be to deposit her eggs in, as soon as they are impregnated by the male? or is it designed for the nuptial feast? EvenScatophaga stercorariaandscybalaria, and probably many others of the same tribe, feed upon small flies, though their proboscis does not seem so well adapted for animal as for vegetable food.
The most unrelenting devourers of insects appear to be those belonging to my fourth division, which attack them undereveryform. These begin the work of destruction when they are larvæ, and continue it during the whole of their existence.—The earwig that haunts every close place in our gardens, and defiles whatever it enters, probably in some degree makes up for its ravages by diminishing the number of other insects. The cowardly and cruelMantis, which runs away from an ant, will destroy in abundance helpless flies, using its anterior tibiæ, which with the thigh form a kind of forceps, to seize its prey. The water-scorpions (Nepa,Ranatra, andNaucoris), whose fore legs are made like those of theMantis, the water-boatman (Notonecta), which always swims upon its back, andSigara, all live by rapine, and prey upon aquatic insects. Some of this tribe are so savage that they seem to love destruction for its own sake. One (Nepa cinerea) which was put into a basin of water with several young tadpoles, killed them all without attempting to eat one.
Those remarkable genera of the tribe of water-bugs(HydrocorisæLatr.), which glide over the surface of every pool with such rapidity, being gifted with the faculty of walking upon the water,Hydrometra,Velia, andGerris, subsist also upon aquatic insects. A large number of the land-bugs (GeocorisæLatr.) plunge their rostrum into the larvæ ofLepidoptera, and suck the contents of their bodies; andReduvius personatus, which ought on that account to be encouraged, is particularly fond of the bed-bug.
But of all the insects that are locomotive and pursue their prey in every state, none are greater enemies of their fellow tribes than theLibellulina, and none are provided with more powerful and singular instruments of assault. In the larva and pupa states, during which they live in the water and prey upon aquatic insects, they are furnished with two pair of strong jaws, covered by a kind of mask armed with a pair of forceps or claws, which the animal has the power of pushing from it to catch any thing at a distance[501]. When an aquatic insect passes within its reach, it suddenly darts forth the mask, opens the forceps, seizes the unfortunate victim, and brings it within the action of its jaws.
When they assume the imago state, their habits do not, like those of the white ants, become more mild and gentle, but on the contrary are more sanguinary and rapacious than ever; so that the name given to them in England, "Dragon-flies," seems much more applicable than "Demoiselles," by which the French distinguish them. Their motions it is true are light and airy; their dress is silky, brilliant and variegated, and trimmed with the finest lace:—so far the resemblance holds; but theirpurpose, except at the time of love, is always destruction, in which surely they have no resemblance to the ladies. I have been much amused by observing the proceedings of a species not uncommon here,Anax Imperatorof Dr. Leach. It keeps wheeling round and round, and backwards and forwards, over a considerable portion of the pool it frequents. If one of the same species comes in its way, a battle ensues; if other species ofLibellulinapresume to approach, it drives them away, and it is continually engaged in catching case-worm flies and other insects (for the species of this tribe all catch their prey when on the wing, and their large eyes seem given them to enable them the more readily to do this,) that fly over the water, pulling off their wings with great adroitness and devouring in an instant the contents of the body. From the number of insects of this tribe which are every where to be observed, we may conjecture how useful they must be in preventing too great a multiplication of the other species of the class to which they belong.
Lastly, under this head, not to dwell upon some other apterous genera, devourers of insects, as the scorpion and centipede,PhalangiumandGaleodes, must be enumerated the whole world of Spiders, extremely numerous both in species and individuals, which subsist entirely upon insects, spreading with infinite art and skill their nets and webs to arrest the flight of the heedless and unwary summer tribes that fill the air, which are hourly caught by thousands in their toils; one of them (Theridium13-guttatumRossi), we are told, even attacking the redoubted Scorpion[502].
So much for the insect benefactors to whom it is givenin charge to keep the animals of their own class within their proper limits; and I cannot doubt that you will recognise the goodness of the Great Parent in providing such an army of counterchecks to the natural tendency of almost all insects to incalculable increase. But before I quit this subject I must call your attention to what may be denominatedcannibal insects, since in spite of those declaimers who would persuade us that man is the only animal that preys upon his own species[503], a large number of insects are guilty of the same offence. Reaumur tells us, that having put into a glass vessel twenty caterpillars of the same species, which he was careful to supply with their appropriate food, they nevertheless devoured each other until one only survived[504]; and De Geer relates several similar instances[505]. The younger larvæ ofCalosoma Sycophantaoften take advantage of the helpless inactivity into which the gluttony of their maturer comrades has thrown them, and from mere wantonness it should seem, when in no need of other food, pierce and devour them. A ferocity not less savage exists amongst theMantes. These insects have their fore legs of a construction not unlike that of a sabre; and they can as dexterously cleave their antagonist in two, or cut off his head at a stroke, as the most expert hussar. In this way they often treat each other, even the sexes fighting with the most savage animosity. Rösel endeavoured to rear several specimens ofM. religiosa, but always failed, the stronger constantly devouringthe weaker[506]. This ferocious propensity the Chinese children have, according to Mr. Barrow, employed as a source of barbarous amusement, selling to their comrades bamboo cages containing each aMantis, which are put together to fight. You will think it singular that both in Europe and Africa these cruel insects have obtained a character for gentleness of disposition, and even sanctity. This has arisen from the upright or sitting position, with the fore legs bent, assumed in watching for their prey, which the vulgar have supposed to be a praying posture, and hence adopted the belief that a child or traveller that had lost his road would be guided by taking one of these pious insects in his hands and observing what way it pointed.Mantis fausta, though not as some suppose worshiped by the Hottentots, is yet greatly esteemed by them, and they regard the person upon whom it alights as highly fortunate[507]. A similar unnatural ferocity is exhibited byGryllus campestris, of which having put the sexes into a box, I found on examining them that the female had begun to make her meal off her companion.—The malign aspect of the scorpion leads us to expect from it unnatural cruelty, and its manners fulfill this expectation. Maupertuis put a hundred scorpions together, and a general and murderous battle immediately began. Almost all were massacred in the space of a few days without distinction of age or sex, and devoured by the survivors. He informs us also that they often devour their own offspring as soon as they are born[508]. Spiders are equally ferocious in their habits, fighting sanguinary battles, which sometimes end in thedeath of both combatants; and the females do not yield to the Mantes in their unnatural cruelty to their mates. Woe be to the male spider that after an union does not with all speed make his escape from the fangs of his partner! Nay, De Geer saw one that, in the midst of his preparatory caresses, was seized by the object of his attentions, enveloped by her in a web, and then devoured—a sight which, he observes, filled him with horror and indignation[509].
Such are the benefits which we derive from the insects that keep each other in check. Here they are thedestroyersto which we are chiefly indebted: but we are in another point of view under nearly equal obligations to thedestroyed; for they are insects, either wholly or in part, that form the food of some of our most esteemed fishes, and of birds that are not more valuable to us as articles for the table, than as the songsters that enliven our groves. But before proceeding to the details which this view of the subject involves, I ought not to omit pointing out to you that many quadrupeds, which though not all of direct utility to us are doubtless of importance in the scale of being, derive a considerable part of their subsistence from insects.
The harmless hedgehog and the mole, to begin at the lower end of the series, are both said to be insectivorous[510]; the latter devouring large quantities of the wire-worms. The greedy swine will root up whole acres in search of the grubs of cockchafers, of which they are very fond; and perhaps the good they do is greater than the harm, if their attack be confined to grass that having been underminedby these grubs would soon die: they also dig up the larvæ of the destructiveCicada septendecim, called the American locust[511], on which, when in their perfect state, the squirrels are said to grow fat[512]. The badger, Lesser informs us, will eat beetles: and its kinsman the bear has the character of being very fond of ants and of honey; which last is also said to be a favourite article with the fox, who has sometimes the audacity to overturn bee-hives, and even to attack wasps' nests in search of it. He will also eat beetles.
Sparrman has given an amusing account of the honey-ratel, (Viverra mellivora,) which has a particular instinct enabling it to discover bees, and attack them in their entrenchments. Near sun-set the ratel will sit and hold one of his paws before his eyes, in order to get a distinct view of the object of his pursuit; and when, in consequence of his peering about in this manner, he sees any bees flying, he knows that at this time of the day they are making for their habitations, whither he follows them, and so attains his end[513]. Another species of Viverra (V. prehensilis) is also reputed to be an eager insect-hunter. The young armadillos feed on a species of locust; but no quadruped can with more propriety be called insectivorous than the ant-eaters (Myrmecophaga), which, as their name imports, live upon ants. The great ant-eater, when he comes to an ant-hill, scratches it up with his long claws, and then unfolds his slender worm-like tongue, (which is more than two feet long, and wet with saliva,) and when covered with ants draws it back into his mouth and swallows thousands of them alive,renewing the operation till no more are to be found. He also climbs trees in search of wood-lice and wild-honey. Bats, as every one knows, are always flitting about in summer evenings, hawking for insects: and the Lemur and monkeys will also eat them.
Insects likewise afford a favourite kind of food to many reptiles: the tortoise; frogs and toads; and lizards too of different kinds. St. Pierre mentions a small and very handsome species in the island of Mauritius, that pursues them into the houses, climbs up the walls, and even walks over glass, watching with great patience for an opportunity of catching them[514]. The common snake also is said to receive part of its nutriment from them.
But to revert to insects as indirectly advantageous to us, by furnishing food to fishes and birds, beginning with the former.
Our rivers abound withfishof various kinds, which at particular seasons derive a principal part of their food from insects, as the numerous species of the salmon and carp genus. These chiefly prey upon the various kinds ofTrichoptera, in their larva state called case- or caddis-worms, and in their imago may-flies (though this last denomination properly belongs only to theSialis lutaria, which generally appears in that month,) andEphemeræ. Besides these, the waters swarm with insects of every order, as numerous in proportion to the space they inhabit, as those that fill the air, which form the sole nutriment of multitudes of our fish, and the partial support of almost all.
Reaumur has given us a very entertaining account of the infinite hosts of Ephemeræ that by myriads ofmillions emerge at a certain season of the year from some of the rivers in France, which, as it is well worth your attention, I shall abridge for you.
These insects in their first and intermediate state are aquatic: they either live in holes in the banks of rivers or brooks below the water, so that it enters into their habitations, which they seldom quit; or they swim about and walk upon the bed of the stream, or conceal themselves under stones or upon pieces of stick. Though their life, when they assume the perfect state, is usually extremely short, some being disclosed after sun-set, laying their eggs and dying before sun-rise; and many not living more than three hours; yet in their preparatory state their existence is much longer, in some one, in others two, in others even three years.