How disgusting to the eye, how offensive to the smell, would be the whole face of nature, were the vast quantity of excrement daily 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 dry and 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 orders Coleoptera and Diptera. A large proportion of the genera formed, by different authors, from Scarabæus of Linné, viz. Scarabæus, Copris, Ateuchus, Sisyphus, Onitis, Onthophagus, Aphodius, and Psammodius; also Hister, Sphæridium; and amongst the Brachyptera, the majority of the Staphylinidæ, many Aleocharæ, especially of Gravenhorst's third family, many Oxyteli and some Omalia, Tachini and Tachypori, 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; but Ateuchus pilularius[458], a species called in America the Tumble-dung, whose singular manœuvres I shall subsequently have to advert to, Copris lunaris, Geotrupes stercorarius and 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 order Diptera, 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 the Coleoptera—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—the dead carcases of animals. When these begin to grow putrid, every one knows what dreadful miasmata exhale from them, and taint 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, (as Sarcophaga 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 of S. carnaria will 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 under M. vomitoria, that three of these flies will devour a dead horse as quickly as would a lion.
As soon as the various tribes of Muscidæ have opened the way, and devoured the softer parts, a whole host of beetles, Necrophori, Silphæ, Dermestes, Cholevæ, and Staphylinidæ, actively second their labours. Wasps and hornets also come in for their portion of the spoil; and even 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 little Corynetes cæruleus and ruficollis, (which last is so interesting, as having been the means of saving the life of Latreille[460],) and several Nitidulæ[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 the vegetable kingdom. 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 other Coleoptera, 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 the water 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 thousands of 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 important task 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.