The Stylopidæ, for which Kirby, [122] in 1811, instituted a distinct Order, which he called Strepsiptera, in allusion to the contortion of the elytra, and to which Latreille [123] subsequently applied the name of Rhipiptera, are, perhaps, the most anomalous of all insects. Great diversity of opinion has existed respecting their affinities; but modern systematists, with but few exceptions, concur in referring them to the Order Coleoptera, and locating them in proximity to Meloë. In the larva state, all the known species of the family inhabit the bodies of hymenopterous insects of the genera Andrena, Polistes, &c., in this particular resembling the dipterous genus Conops, which inhabits the body of humble bees, [124] and apparently in no way inconveniencing their victims; a fact which has been accounted for on the supposition that their existence in the larva state is but short, and that their attacks being directed against the abdomen, and not the thorax, the seat of life in insects, their presence does not affect the activity of the victim. The larva has a soft fusiform body, surmounted by a somewhat globose head. While feeding, the head is towards the base of the abdomen; but on changing to a pupa, this position is reversed, and the head—at first of light brown, but which after a short time becomes black—thrust out between the plates of the abdomen.
The imagos, which are of small size, namely, about the eighth of an inch long, are found during May and June. They have four wings, but the anterior pair, of hard texture, somewhat resembling elytra, but hardly answering to them in structure, are very poorly developed, and curled round the front pair of legs, hence the name bestowed, by Kirby, from [Greek: strepssis], a twisting, and [Greek: pteron], a wing; the posterior wings are fully developed, and fold up like a fan, whence the Order received the name of Rhipiptera from Latreille. The eyes, the facettes of which are few in number, are placed on a footstalk, whence the name of the genus Stylops. The parts of the mouth connect the Strepsiptera with the mandibulated insects, although by some supposed to bear analogy by their functions to those parts in the Diptera. The male only is winged; the female is very like an apodal larva, the larva being an active hexapod.
The family Stylopidæ is divided into four genera, of which two only, Xenos and Stylops, were described by Kirby in the essay referred to above. First, Xenos, from [Greek: xenos], a guest, the most prolific in species, of which Xenos Rossii, sometimes called vesparum, may be taken as the type. Secondly, Elenchus, of which Elenchus Walkeri is the type. Thirdly, Stylops ([Fig. 543]), parasitical on various species of Andrenæ, of which Stylops Melittæ, having a fleshy abdomen and the wings longer than the body, may be considered typical: and lastly, Halictophagus, of which only one species, infesting Halictus æratus[125] named Halictophagus Curtisii, is known to exist, and which makes its appearance in the month of August.