The first, those which are insectivorous only in their larva state, maybe further subdivided into parasites and imparasites, meaning by the former term those that feed upon a living insect, 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.
The imparasitic insect devourers chiefly belong to the Hymenoptera order; 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 of Cerceris and the splendid Chrysidæ or golden wasps feed upon insects of their own order. One of the latter (Parnopes incarnata) commits her eggs to the progeny of Bembex rostrata: another (Chrysis bidentata) attacks the young of Epipone spinipes.
Bembex and Mellinus confine themselves to Diptera, the former preying upon Eristalis tenax, Bombylii, and the like[468]; the latter amongst others ridding us of the troublesome Stomoxys 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 vulgaris destroys caterpillars of a larger size, and it is probable that most of the other Vespoid and Sphecoid Hymenoptera, 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-devouring Blatta or 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 poison to 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 the Sphex pensylvanica of 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 the largest; 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 of Syrphidæ), 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 favourite of 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 the Fly so 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 them Vaches 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 without this 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 genus Volucella so 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.