Wings four, membranous, with few nervnres; sometimes absent. Mouth always provided with biting-jaws or mandibles; the maxillcae and labium generally converted into a suctorial organ. Females having the extremity of the abdomen furnished with an ovipositor (terebra oraculeus), consisting generally of five or six pieces, of which the two outer form a protective sheath. Besides the compound eyes, there are usually three ocelli placed on the top of the head. The antennae are generally filiform or setaceous. The metamorphosis is complete, but the various parts of the pupa are visible through the delicate enclosing membrane. The larvae are sometimes provided with feet, and live on vegetable food (as in the Ten-thredinidae, fig. 191); but they are mostly footless, without a distinct head, and fed by the adult.
Fig. 190. - Goat-moth (Cossus ligniperda) and Caterpillar.
The Hymenoptera form a very extensive order, comprising the Bees, Wasps, Ants, Ichneumons, Saw-flies, etc. The ovipositor, which is characteristic of the females of this order, is very commonly modified so as to constitute a saw (serra), a boring organ (terebra), or a sting (aculeus).
As regards the principal groups of the Hymenoptera, the Saw-flies (Tetithredinidae and Siricidae) form a very natural section, which is often spoken of as that of the Terebrantia, as the females have the ovipositor converted into a saw or borer. The larvae of the Saw-flies (fig. 191) feed upon vegetable matter, and have pro-legs. Another important group is that of the Gall - flies (Cynipidae), all of which lay their eggs in the soft tissues of plants (generally the leaves). The resulting "galls" are due to the abnormal cell-growth excited locally in the plant by the irritation caused by the puncture of the mother's ovipositor in depositing the eggs. The larvae are footless. In the allied group of the Ichneumons (Ichneumonidae), the larvae are also footless, and the eggs are deposited by the females in the larvae or pupae of other insects, upon whose tissues the young support themselves after hatching All the other Hymenoptera have the ovipositor of the female converted into a sting (not always the case in the Ants), and they may therefore be grouped together under the common title of Aculeata. The principal families included under this name are the Ants (Formicidae), the Wasps (Vespidae), the Hornets (Crabronidae), the Bees (Apidae), and the Bumble-bees (Bombidae).
Amongst the Hymenoptera we find social communities, in many respects resembling those of the Termites, of which a description has already been given. The societies of Bees and Ants are well known, and merit a short description.
The social Bees, of which the common Honey-bee (Apis mellifica), is so familiar an example, form organised communities, consisting of three classes of individuals - the males, females, and neuters. As a rule, each community consists of a single female - "the queen" - and of the neuters, or "workers." The impregnation of the female is effected by the production of males, or "drones," during the summer. After impregnation has been effected, the drones, as being then useless, are destroyed by the workers. The eggs produced by the fecundated queen are mostly intended to give origin to neuters, to which end they are placed in the ordinary cells. The ova which are to give origin to females - the "queens " of future colonies - are placed in cells of a peculiar construction, and the larvae are fed by the workers with a special food. The ova which are to produce males are likewise placed in cells, which are slightly larger than those allotted to the workers. It is asserted, however, that this is not the sole or true cause of the production of the males; but that the ova which are intended to produce drones are not fertilised by the female with the semen which she has stored up in her spermatheca, and are therefore produced by a process of parthenogenesis. That the males are produced parthenogenetically in some, at any rate, of the Hymenoptera, appears to have been placed beyond a reasonable doubt by the researches of Von Siebold. (See Introduction.)
Fig. 191. - Gooseberry Saw-fly ( Tenthredo grossu-lariae), larva, pupa, and imago.
In the Bumble-bees (Bombidae), and in the Wasps (Vespidae), we have societies essentially the same as in the Honey-bee. In a large community of Wasps, or "vespiary," there may be several hundred females, of which few survive the winter, and live to found fresh colonies next spring. The number of males is about equal to that of the females, but, unlike the drones of the Bees, the males work actively and defend the nest. As amongst the Bees, solitary species are not uncommon.
The Ants (Formicidae) likewise form communities, consisting of males, females, and neuters (fig. 192). The males and females, as we have seen in the case of the Termites, are winged, and are produced in great numbers at a particular period of the year. They then quit the nest and pair, after which the males die. The females then lose their wings and fall to the ground, when they become the queens of fresh societies. In some Ants, as in the Termites, the neuters are divided into two classes - the workers and the soldiers - of which the former perform all the duties necessary for the preservation of the society except defending the nest, this being left to the soldiers. In other cases, as many as three distinct orders or "castes" of neuters may be present in the same nest. Amongst the more singular of the habits and instincts of Ants two may be mentioned - the instinct of making slaves, and that of milking, so to speak, the little Plant-lice (Aphides). As regards the first of these, it is found that certain Ants possess the extraordinary instinct of capturing the pupae of other species of Ants, and bringing them up as slaves. The relations between the masters and the slaves vary a good deal in different species. In the case of Polyergus rufescens, for instance, the masters are entirely dependent upon their slaves; the males and females do nothing except reproducing the species, and the neuters perform no other labour except that of capturing fresh slaves. The masters are in this case unable even to feed themselves, and their existence is maintained entirely by the devotion of the slaves. In Formica sanguinea, on the other hand, the number of slaves is much less, and both masters and slaves occupy themselves in performing most of the duties necessary for the community. The masters, however, go alone when on slave-making expeditions; and in case of a migration, the masters carry the slaves in their mouths.
Fig. 192. - a Winged male of Ant; b Wingless worker of Ant; c Pupa of Ant; d Lavra of Ant, enlarged ; e The Great Saw-fly (Sirex gigas).
A second singular fact in the history of Ants is found in the relations which subsist between them and the Aphides, or Plant-lice. The Aphides secrete, or rather excrete, a peculiar viscid and sweet liquid, by means of a gland which is situated towards the extremity of the abdomen, and communicates with the exterior by two tubular filaments. Ants are extremely fond of this excretion, and it is a well-established fact that the Aphides allow themselves to be milked, as it were, by the Ants. For this purpose the Ant touches and caresses the abdomen of the Aphis with its antennae, whereupon the latter voluntarily exudes a drop of the coveted fluid.
The belief that our European Ants stored up grain for winter consumption, though generally asserted by the ancients, has been, until recently, discredited by scientific observers, upon the ground that our Ants are known to be carnivorous in their habits. Mr Moggridge has, however, recently shown that there are exceptions to this rule, and that some of the Ants of the south of Europe (such as some of the species of Aita) not only eat vegetable food, but really execute the feats imputed to them by the old writers. They do, namely, store up a provision of grain for the winter, and they prevent this from germinating by gnawing the radicle.