ON STRUCTURE

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.

The wing neuration is always rather troublesome, as various authors use different names for the veins and cells. To begin with the anterior wing (e), there are four nerves which start from the base and run horizontally; the first of these, which forms the anterior margin of the wing, is called the costal nervure (1); immediately below this, and running almost parallel to it with scarcely any space between them, is the post-costal nervure (2); these end in the stigma (s), a dark in-crassation towards the apex of the wing; from the stigma a nerve, curving first downwards and then up to the anterior margin of the wing, encloses the marginal cell (A). Below the post-costal nervure, and situated about the centre of the wing, is the third longitudinal nervure called the median nervure (3); behind this again runs the posterior nervure (4), and behind that the actual margin of the wing which is not provided with a protecting nervure, but is only folded back so as to receive the hooks of the posterior wing. Across the wing at, roughly, about a third of its length from the body runs the basal nervure (5); this extends in a somewhat zigzag line from the post-costal to the posterior nervure crossing the median, and

thereby enclosing two cells, the upper basal cell (B) and the lower basal cell (C). From the centre of the apical nerve of each of these cells extends a longitudinal nervure; the upper of these runs out nearly to the apical margin of the wing and is called the cubital nervure (6); this is united to the nervure of the marginal cell by one, two, or three cross nervures, enclosing thereby one, two, or three cells called the first (D), second (E), and third (F) submarginal cells. The nervure from the lower basal cell is a short one, as it is met by a cross nervure called the first recurrent nervure (10), which runs from the cubital to the posterior, thereby enclosing two cells, the first (G) and second (H) discoidal. The second recurrent (11) leaves the cubital nearer the apex of the wing than the first, meeting a nervure which, springing from the outer posterior angle of the second discoidal, closes the third discoidal (I), and, curving slightly upwards, nearly reaches the apical margin of the wing. Beyond the second recurrent, and behind this last nervure which we have been talking about, are two spaces not actually enclosed, but called the first (J) and second (K) apical cells.

The posterior wings have very few cells.

Like the anterior pair they have three longitudinal nervures; the anterior (7), which runs close and parallel to the anterior nerveless margin, and often touches it at about half the length of the wing; the median (8) and posterior (9) run in diverging lines from the base towards the exterior margin of the wing, the anterior and median nervures being almost always joined by a cross nervure, and the median usually united to the posterior by a cross or curved nervure. The actual base of the anterior wing is covered by a little convex somewhat shell-like cap, called the tegula (T). The abdomen is composed of a series of segments in linear arrangement (c¹ c², etc.). These call for no special remark, beyond what has been said in the chapter on males and females, but those who wish to investigate the very interesting questions connected with the terminal segments of these creatures should consult some more technical work.[^[3]] The arrangements of the mouth parts and of the apical segments of the Hymenoptera afford perhaps the most important structural

characters of the order, but they involve an amount of dissection and study which can only be undertaken by those who are inclined to give themselves up to this subject as a speciality.