The organs for mechanical purposes existing at the extremity of the body in Hymenoptera exhibit a great diversity of form; they are saws, borers, piercers, or stings. Notwithstanding their great differences they are all, in their origin, essentially similar, and consist of six parts developed from limb-like prolongations on the penultimate and antepenultimate segments of the larva, as described by Packard and Dewitz.[[412]] These processes have by some been thought to be not essentially different from abdominal legs, and Cholodkovsky has recently advocated this opinion.[[413]]
The legs of bees exhibit modifications for industrial purposes. In the stinging Hymenoptera the trochanters are usually of a single piece, and these Insects are called monotrochous; but in most of the other forms the trochanters are more or less distinctly divided into two parts (Fig. 345, b). The usual number of joints in the tarsus is five, but is subject to diminution in many cases. In the bees and ants the first joint is altered in form; in the bees to act as an instrument for gathering or carrying pollen; in the ants to act, as it were, as a second tibia. Between the claws there is a very perfect pad, already described and figured on p. [106].
The wings are remarkable for the beautiful manner in which the hinder one is united to the anterior, so that the two act in flight as a single organ. The hind wing is furnished with a series of hooks, and the hind margin of the front wing is curled over so that the hooks catch on to it. In some of the parasitic forms the wings are almost destitute of nervures, and have no hooks. The powers of flight in these cases are probably but small, the wings merely serving to float the Insect in the air. In some Hymenoptera, especially in Pompilides and Xylocopa, the wings may be deeply pigmented or even metallic; and in some forms of Tenthredinidae, Ichneumonidae, and Braconidae the pigmentation assumes the form of definite patterns.
Fig. 337.—Wings of a carpenter bee. A, The pair of wings separated; a, position of the hooks: B, the same wings when united by the hooks. C, Portions of the two wings: a, the series of hooks; b, marginal hairs; c, portion of edge of front wing, of which the other part has been broken away in order to show the hooks.
The studies of the internal anatomy of Hymenoptera are at present by no means numerous or extensive. The alimentary canal (Fig. 69) possesses a crop, gizzard, and chylific stomach in addition to the oesophagus and intestine. The social Hymenoptera have the power of disgorging matter from the alimentary canal for the purpose of supplying food for their young.
Fig. 338.—Central nervous system (supra-oesophageal ganglion and ventral chain) of a worker ant, Camponotus ligniperdus. (After Forel.) a, Cerebral hemisphere; b, primordial cerebral lobe or pedunculate body (depressed so as to show other parts); c, olfactory lobe (raised from natural position); d, nerve to labrum; e, antennary nerve; f, scape of antenna; g, eye; h, optic nerve; i, longitudinal commissures connecting the hidden sub-oesophageal ganglion with k, the prothoracic ganglion; l, mesothoracic, m, metathoracic ganglion; s, ganglion of the petiole; n, nerve from petiole to other part of abdomen; r, q, o, 2nd, 3rd, 4th abdominal ganglia; p, terminal nerve to cloaca; t, bases of legs.
The crop—which is situated in the anterior part of the abdomen—is the reservoir that contains this matter. The mode of disgorgement is believed to be pressure exerted on the crop by contraction of the abdomen. Salivary glands are present in remarkable variety. The tracheal system possesses, in the higher winged forms, large saccular dilatations situated at the side of the abdomen. The nervous system is of peculiar interest on account of the high intelligence of many of the members of this Order; and on this point of the anatomy, Brandt[[414]] has made rather extensive investigations, having examined it in the adult of seventy-eight species, and in the larva of twenty-two. In the adult there are two cephalic—the supra- and the sub-oesophageal—two or three thoracic, and from three to seven abdominal ganglia. The bees, wasps, and some other of the Aculeata have only two thoracic ganglia, while some ants have three. The supra-oesophageal ganglion is very large. The most remarkable fact revealed by Brandt's investigations is the great difference that exists between the sexes and the worker caste in the same species. The pedunculate bodies of the supra-oesophageal ganglion are considered to be in their development correlative with that of the intelligence or instinct. In the workers of the social Hymenoptera these bodies are very large, while in the males and females they are small. The workers and females of Bombus have six abdominal ganglia, while the males have only five; and the worker of the honey-bee has five abdominal ganglia, while the male and the queen-bee have but four. In the leaf-cutting bee (Megachile) the male has four abdominal ganglia and the female five, and in the wasps the workers have five, the males and females six. The nervous system in the larvae shows but little difference between the ganglia, which are thirteen in number, eight being abdominal. In the embryo of the bee Kowalewsky has observed seventeen ganglia. The changes that take place from the embryonic to the imago condition are therefore directed to the reduction in number of the ganglia, which is accomplished by the fusion of some of them. In the adult Hymenopterous Insect it would appear that the first abdominal ganglion is always joined with the last thoracic.
Sub-Orders.—The distinction in the form of the abdominal articulation, previously alluded to (p. [492], Fig. 336, A, B), divides the Hymenoptera into two great sub-Orders, the members of which are very different in their habits and life-histories. The Sessiliventres are plant-eaters; their larvae (Fig. 343, A) are provided with legs, and are able to procure their vegetable food for themselves. The larvae of the Petiolata are maggot-like and helpless, and are dependent for food on supplies afforded them by their parents or companions. It is said by Dewitz that although the larvae of the Petiolata appear to be legless, there are thoracic legs within the body. The metamorphosis, so far as it is known, and the early life-history of the Sessiliventres are very similar to those of butterflies and moths, except that the pupa is soft and has no hard external skin. A few of these plant-eating Sessiliventres become carnivorous in the perfect state—a change of habit that is most unusual in Insects, though the reverse occurrence is common. The larvae of the Petiolata exhibit, in the cases that have been examined, the peculiarity that the alimentary canal has not any outlet posteriorly until the termination of the larval stage of existence is approaching. In some cases there is no anal orifice; in others this orifice exists, but there is no communication between the stomach and the posterior intestine.