Those remarkable genera of the tribe of water-bugs (Hydrocorisæ Latr.), which glide over the surface of every pool with such rapidity, being gifted with the faculty of walking upon the water, Hydrometra, Velia, and Gerris, subsist also upon aquatic insects. A large number of the land-bugs (Geocorisæ Latr.) plunge their rostrum into the larvæ of Lepidoptera, and suck the contents of their bodies; and Reduvius 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 the Libellulina, 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 their purpose, 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 Imperator of 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 of Libellulina presume 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, Phalangium and Galeodes, 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 (Theridium 13-guttatum Rossi), we are told, even attacking the redoubted Scorpion[502].
So much for the insect benefactors to whom it is given in 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 denominated cannibal 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 of Calosoma Sycophanta often 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 the Mantes. 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 of M. religiosa, but always failed, the stronger constantly devouring the 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 a Mantis, 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 by Gryllus 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 the death 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 the destroyers to which we are chiefly indebted: but we are in another point of view under nearly equal obligations to the destroyed; 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 undermined by these grubs would soon die: they also dig up the larvæ of the destructive Cicada 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.