Fig. 331.—Bombus lucorum. A, Adult larva; B, pupa; C, imago, female. Britain.
In the Order Hymenoptera—especially in the higher forms—the males and females are often different in appearance and structure. In the ants, one of the social groups, the workers, or imperfect females, are quite wingless. There are numerous other groups in which species, not social, are found, having the females wingless while the males have wings. In a few species there is an apterous condition of the male, perhaps usually only as a dimorphic form. In the parasitic division there are species that are apterous in both sexes. The structure of the outer skeleton, or external part of the body, exhibits some peculiarities, the chief of which is the detachment of the side-pieces of the prothorax and their great development. Not less remarkable is the abstraction of a segment from the abdomen to become, as it were, part of the thorax; while between the first and second true segments of the abdomen there exists a joint, or articulation, of the utmost mechanical perfection, enabling the operations of stinging and piercing to be executed with an accuracy that cannot be surpassed.
As a result of the detachment of the thoracic side-pieces, the front legs and the structures connected with them are disjoined from the notum, as shown in Fig. 332, and act in connexion with the head, while the dorsal portion of the segment remains attached to the great thoracic mass. The head is quite free from the thorax and very mobile; the upper organs of the mouth—the labrum and the mandibles—are not subject to modifications equal to those exhibited by the maxillae and lower lip, which parts in the bees are prolonged to form a suctorial apparatus that may exceed in length the whole body of the Insect. The mandibles remain cutting or crushing implements even when the maxillae and lower lip are modified to form a suctorial apparatus of the kind we have mentioned; so that in the higher forms—ants, bees, and wasps—the mouth-pieces are completely differentiated for two sets of functions, one industrial, the other nutritive.
Fig. 332.—Tenthredo, with head fully extended: a, pleuron; b, pronotum; c, membrane; d, mesonotum.
Behind the head there is a large consolidated mass representing the thorax of other Insects, but made up, as we have already indicated, in an unusual manner, and which therefore may be called by a special name, the alitrunk (Fig. 333). The pronotum forms the anterior part of the alitrunk, with which it is usually very closely connected, being indeed frequently immovably soldered thereto. It exhibits, however, considerable variety, and is seen in its simplest and least soldered state in Cephus. In the higher bees the pronotum takes on a form not seen in any other Insects, being one of the most beautiful sclerites to be found in the class (Fig. 334, pronotum of Xylocopa). We have already remarked that in Hymenoptera the lower portions of the prothoracic segment are detached from the upper, so that the pronotum is not supported beneath by a sternum as usual. In the bees in question the pronotum makes up for the removal of the corresponding side-pieces and sternum, by becoming itself a complete ring, its sides being prolonged and meeting in the middle line of the under surface of the body. At the same time a large lobe is developed laterally on each side, overlying and protecting the first breathing orifice. The intermediate stages of this remarkable modification may be observed by dissecting a small series of genera of bees.
Fig. 333.—Alitrunk of Sphex chrysis. A, Dorsal aspect: a, pronotum; b, mesonotum; c, tegula; d, base of anterior, e, of posterior, wing; f, g, divisions of metanotum; h, median (true first abdominal) segment; i, its spiracle; k, second abdominal segment, usually called the petiole or first abdominal segment. B, Posterior aspect of the median segment: a, upper part; b, superior, c, inferior abdominal foramen; d, ventral plate of median segment; e, coxa.
Fig. 334.—Pronotum of a carpenter bee, Xylocopa sp. East India.