HYMENOPTERA (Gr. ὑμήν, a membrane, and πτερόν, a wing), a term used in zoological classification for one of the most important orders of the class Hexapoda (q.v.). The order was founded by Linnaeus (Systema Naturae, 1735), and is still recognized by all naturalists in the sense proposed by him, to include the saw-flies, gall-flies, ichneumon-flies and their allies, ants, wasps and bees. The relationship of the Hymenoptera to other orders of insects is discussed in the article [Hexapoda], but it may be mentioned here that in structure the highest members of the order are remarkably specialized, and that in the perfection of their instincts they stand at the head of all insects and indeed of all invertebrate animals. About 30,000 species of Hymenoptera are now known.

After C. L. Marlatt, Bur. Ent. Bull. 3, N.S., U.S. Dept. Agric.
Fig. 1.—A, Front of head of Saw-fly (Pachynematus); a, labrum; b, clypeus; c, vertex; d, d, antennal cavities. C and D, Mandibles. E, First maxilla; a, cardo; b, stipes; c, galea; d, lacinia; e, palp. B, Second maxillae (Labium); a, mentum; b, ligula (between the two galeae); c, c, palps. Magnified.
After C. Janet, Mem. Soc. Zool. France (1898).
Fig. 2.—Jaws of Hive-bee (Apis mellifica). Magnified about 6½ times. a, mandible; b, c, palp and lacinia of first maxilla; d, e, g, h, mentum, palp, fused laciniae (ligula or “tongue”) and galea of 2nd maxillae.Fig. 3.—Median section through mid-body of female Red Ant (Myrmica rubra). H, Head; 1, 2, 3, the thoracic segments; i., ii., the first and second abdominal segments; i., being the propodeum.

Characters.—In all Hymenoptera the mandibles (fig. 1, C, D) are well developed, being adapted, as in the more lowly winged insects, such as the Orthoptera, for biting. The more generalized Hymenoptera have the second maxillae but slightly modified, their inner lobes being fused to form a ligula (fig. 1, B, b). In the higher families this structure becomes elongated (fig. 2, g) so as to form an elaborate sucking-organ or “tongue.” These insects are able, therefore, to bite as well as to suck, whereas most insects which have acquired the power of suction have lost that of biting. Both fore- and hind-wings are usually present, both pairs being membranous, the hind-wings small and not folded when at rest, each provided along the costa with a row of curved hooks which catch on to a fold along the dorsum of the adjacent fore-wing during flight. A large number of Hymenoptera are, however, entirely wingless—at least as regards one sex or form of the species. One of the most remarkable features is the close union of the foremost abdominal segment (fig. 3, i.) with the metathorax, of which it often seems to form a part, the apparent first abdominal segment being, in such case, really the second (fig. 3, ii.). The true first segment, which undergoes a more or less complete fusion with the thorax is known as the “median segment” or propodeum. In female Hymenoptera the typical insectan ovipositor with its three pairs of processes is well developed, and in the higher families this organ becomes functional as a sting (fig. 5),—used for offence and defence. As regards their life history, all Hymenoptera undergo a “complete” metamorphosis. The larva is soft-skinned (eruciform), being either a caterpillar (fig. 6, b) or a legless grub (fig. 7, a), and the pupa is free (fig. 7, c), i.e. with the appendages not fixed to the body, as is the case in the pupa of most moths.

Fig. 4.—Fore-Wings of Hymenoptera.

1. Tenthredinidae (Hylotoma)—1, marginal; 2, appendicular;3, 4, 5, 6, radial or submarginal;7, 8, 9, median ordiscoidal; 10, sub-costal;11, 12, cubital or branchial;and 13, anal or lanceolatecellules; a, b, c, submarginalnervures; d, basal nervures;e, f, recurrent nervures; st,stigma; co, costa.

2. Cynipidae (Cynips).

3. Chalcididae (Perilampus).

4. Proctotrypidae (Codrus).

5. Mymaridae (Mymar).

6. Braconidae (Bracon).

7. Ichneumonidae (Trogus).

8. Chrysididae (Cleptes).

9. Formicidae (Formica).

10. Vespidae (Vespa).

11. Apidae (Apathus).

Structure.—The head of a hymenopterous insect bears three simple eyes (ocelli) on the front and vertex in addition to the large compound eyes. The feelers are generally simple in type, rarely showing serrations or prominent appendages; but one or two basal segments are frequently differentiated to form an elongate “scape,” the remaining segments—carried at an elbowed angle to the scape—making up the “flagellum”; the segments of the flagellum often bear complex sensory organs. The general characters of the jaws have been mentioned above, and in detail there is great variation in these organs among the different families. The sucking tongue of the Hymenoptera has often been compared with the hypopharynx of other insects. According to D. Sharp, however, the hypopharynx is present in all Hymenoptera as a distinct structure at the base of the “tongue,” which must be regarded as representing the fused laciniae of the second maxillae. In the thorax the pronotum and prosternum are closely associated with the mesothorax, but the pleura of the prothorax are usually shifted far forwards, so that the fore-legs are inserted just behind the head. A pair of small plates—the tegulae—are very generally present at the bases of the fore-wings. The union of the first abdominal segment with the metathorax has been already mentioned. The second (so-called “first”) abdominal segment is often very constricted, forming the “waist” so characteristic of wasps and ants for example. The constriction of this segment and its very perfect articulation with the propodeum give great mobility to the abdomen, so that the ovipositor or sting can be used with the greatest possible accuracy and effect.

Mention has already been made of the series of curved hooks along the costa of the hind-wing; by means of this arrangement the two wings of a side are firmly joined together during flight, which thus becomes particularly accurate. The wings in the Hymenoptera show a marked reduction in the number of nervures as compared with more primitive insects. The main median nervure, and usually also the sub-costal become united with the radial, while the branches of radial, median and cubital nervures pursuing a transverse or recurrent course across the wing, divide its area into a number of areolets or “cells,” that are of importance in classification. Among many of the smaller Hymenoptera we find that the wings are almost destitute of nervures. In the hind-wings—on account of their reduced size—the nervures are even more reduced than in the fore-wings.

The legs of Hymenoptera are of the typical insectan form, and the foot is usually composed of five segments. In many families the trochanter appears to be represented by two small segments, there being thus an extra joint in the leg. It is almost certain that the distal of these two segments really belongs to the thigh, but the ordinary nomenclature will be used in the present article, as this character is of great importance in discriminating families, and the two segments in question are referred to the trochanter by most systematic writers.

After C. Janet, Aiguillon de la Myrmica rubra (Paris, 1898).
Fig. 5.—Ovipositor or Sting of Red Ant (Myrmica rubra) Queen. Magnified. The right sheath C (outer process of the ninth abdominal segment—9) is shown in connexion with the guide B formed by the inner processes of the 9th segment. The stylet A (process of the 8th abdominal segment—8) is turned over to show its groove a, which works along the tongue or rail b.