Although in the foregoing chapter a little has been said on this subject, there is a great deal more that a student should learn about the general form of these creatures.
They begin life as white or nearly colourless grubs, which, after various changes of skin, assume what is called the nymph or pupa stage, during which a change occurs, believed to be peculiar to the Hymenoptera; the fifth segment of the larval body is transferred to the mass which is called the thorax, so that a portion of what looks like thorax is really the first segment of the abdomen. Continental writers call this portion sometimes the first abdominal segment and sometimes the median segment, but Newman gave it a definite name, the "propodeum", and the most convenient method seems to be to call it so, and treat it as a part of the thorax, calling the first or basal segment of the abdomen
that which immediately follows the regional constriction, which occurs between the propodeum and the abdomen.
Fig. 28.
a Head. a1 Antennæ. a2 Ocelli. a3 Compound eyes.
b1 Prothorax. b2 Scutum of Mesothorax. b3 Scutellum of Mesothorax. b4 Post-Scutellum of Metathorax. b5 Propodeum.
c1 c2, etc., Segments of Abdomen.
Legs. d1 Coxa. d2 Trochanter. d3 Femur. d4 Tibia. d5 Tarsi. d6 Calcaria or Spurs. d7 Unguiculi or claws. d8 Pulvillus.
e Front wing. 1 Costal nervure. 2 Post Costal nervure. 3 Median nervure. 4 Posterior nervure. 5 Basal nervure. 6 Cubital nervure. 10 1st Recurrent nervure. 11 2nd Recurrent nervure.
f. Hind wing. 7 Anterior nervure. 8 Median nervure. 9 Posterior nervure.
Cells. A Marginal. B Upper basal. C Lower basal. D 1st Submarginal. E 2nd Submarginal. F 3rd Submarginal. G 1st Discoidal. H 2nd Discoidal. I 3rd Discoidal. J 1st Apical. K 2nd Apical.
The perfect insect when it emerges has therefore a head, a thorax of four segments, and an abdomen of seven visible dorsal segments in the male, and of six in the female. The ♂ has six ventral segments exposed, and often the apex of the eighth, which is frequently elongate, the seventh being almost always short and hidden; the eighth dorsal segment can be discovered hidden under the seventh, but it is very rarely exposed. The head (a) bears numerous appendages; a pair of antennæ (a1), usually of thirteen joints in the male and of twelve in the female; two compound eyes (a3), composed of many facets; three simple eyes (or ocelli) (a2), which are situated on its vertex; two mandibles; two maxillæ, bearing palpi on each side, of a varying number of joints; and a labium, or tongue, which also bears at its base two four-jointed palpi (cf. fig. 20).
The thorax, as we are considering it, consists of four segments—the prothorax (b1), which bears the two front legs; the mesothorax (b2), which bears the intermediate pair of legs and the anterior pair of wings; and the metathorax (b3), which bears the posterior pair of wings and the hind legs. The
propodeum has no appendages. The mesothorax above has two parts, a larger portion in front called by some the scutum (b2), and a smaller portion behind called the scutellum (b3). These are separated from each other by a transverse impression, and the scutellum is often raised into a sort of little shield; behind this is another little elevation called the post-scutellum (b4); this is really the dorsal apex of the metathorax, and behind this lies the propodeum (b5). Each leg is composed of various parts, and articulates into a cavity of the thorax called the acetabulum. The first two joints of the leg, the coxa (d1) and the trochanter (d2), are very short; then follows the femur or thigh (d3); then the tibia or shin (d4); and finally the tarsi (d5), which compose the foot. At the apex of the tibia are usually two spines called the calcaria (d6). The tarsi are five-jointed, the joints following each other in a linear arrangement, and in the Anthophila the basal joint is more or less dilated; the apical joint bears two claws (unguiculi, d7) which are sometimes toothed, and between them, in some genera, there is what is called a pulvillus (d8) or cushion; this is very large and dilated in some of the fossors.