The parasitic larvæ, an extremely numerous tribe, must next be considered. These, with the exception of a very few individuals, belong to the order Hymenoptera, and were included by Linné under his vast genus Ichneumon, 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 of lepidopterous larvæ, 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 their minute allies (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 larva that destroys it[476]. In vain does the destructive Cecidomyia of 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 the Cynips by 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 genus Proctotrupes. 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 of Ophion luteum, which adheres by one end to the shell of the bulbiferous egg that produced it, does not enter the caterpillar of Euprepia villica, the moth upon which it feeds[480]. But the 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 of Pontia 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 order Hymenoptera: 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 of Strepsiptera, lately established[484], appears to be altogether parasitic; but with this difference from the Pupivora, that these extraordinary animals are found only upon Hymenoptera in their perfect state, and do not appear to destroy the insects upon which they prey, but probably prevent their breeding. The species at present known are formed into two genera, Xenos and Stylops, which are confined to Melitta[485] and Vespa[486].
The next description of insect destroyers are those which devour them in their first and last states.—No beetles are more common after the summer is confirmed, than the species of the genus Telephorus. Preysler informs us that the grub of T. fuscus destroys a great many other larvæ[487], and I have observed the imago devour these and also Diptera.—Linné has with justice denominated the Cicindelæ the 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 first and final state. The larvæ of the latter have long been observed and described under the name of Squillæ, 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 genus Carabus (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 mean Calosoma Sycophanta. This animal takes up its station in the nests of Lasiocampa processionea and 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 of Vinaigrier, 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 the Brachyptera, many of which prey upon insects as well as on putrescent substances. Mr. Lehmann tells us that some 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 their perfect state 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, an Elater quite 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; and Myrmica omnivora is 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's American 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 that Clerus formicarius devours Anobium pertinax. A fly related to the Panorpa communis appears created to instill terror into the pitiless hearts of the tyrants of our lakes and pools,—the all-devouring Libellulina[500]. The Asili also, which are always upon the chase, seize insects with their anterior legs and suck them with their haustellum. The cognate genus Dioctria, particularly D. œlandica, prey upon Hymenoptera, by some unknown means instantaneously killing the insect they seize. Many species also of Empis, whose haustellum resembles the beak of a bird, carry off in it Tipulariæ and other small Diptera; 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? Even Scatophaga stercoraria and scybalaria, 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 under every form. 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 cruel Mantis, 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, and Naucoris), whose fore legs are made like those of the Mantis, the water-boatman (Notonecta), which always swims upon its back, and Sigara, 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.