Anterior wings longitudinally plicate in repose; the pronotum extending back, so as to form on each side an angle reposing on the tegula; the basal segments of the hind body not bearing nodes or scales; the hind tarsi formed for simple walking. The species either solitary or social in their habits; some existing in three forms, males, females, and workers.
Fig. 26—Upper aspect of pronotum and mesonotum of a wasp, Eumenes coarctata. a, Angle of pronotum; b, tegula; c, base of wing; d, mesonotum.
This division of Hymenoptera includes the true wasps, but not the fossorial wasps. The name applied to it has been suggested by the fact that the front wings become doubled in the long direction when at rest, so as to make them appear narrower than in most other Aculeata (Fig. 27). This character is unimportant in function so far as we know,[[40]] and it is not quite constant in the division, since some of the Masaridae do not exhibit it. The character reappears outside the Diploptera in the genus Leucospis—a member of the Chalcididae in the parasitic series of Hymenoptera—the species of which greatly resemble wasps in coloration. A better character is that furnished by the well-marked angle, formed by the pronotum on the dorsal part (Fig. 26). By a glance at this part a Diplopterous Insect can always be readily distinguished.
Three families are at present distinguished in the Diploptera, viz. Eumenidae, Vespidae and Masaridae. We anticipate that Eumenidae and Vespidae will ultimately be found to constitute but one family.
Fam. 1. Eumenidae—Solitary True Wasps.
Claws of the feet toothed or bifid; middle tibiae with only one spur at tip. Social assemblages are not formed, and there is no worker-caste, the duties of nest-construction, etc., being performed solely by the female.
The Eumenidae, or solitary wasps, are very little noticed by the ordinary observer, but they are nevertheless more numerous than the social Vespidae, about 800 species being known. In Britain we have sixteen species of the solitary, as against seven of the social wasps. The Eumenidae exhibit a considerable diversity in form and structure; some of them have the pedicel at the base of the abdomen very elongate, while in others this is so short as to be imperceptible in the ordinary position of the body. A repetition of similar differences of form occurs in the social wasps, so that notwithstanding the difference in habits there seems to be no satisfactory way of distinguishing the members of the two families except by the structure of the claws and tibial spurs.
Fig. 27.—Eumenes flavopicta ♀. Burma. The wings on the left in the position of repose, to show folding.