Westwood says (and most entomologists are aware of the fact): "The species of _Gerris_, _Hydrometra_, and _Velia_ are mostly found perfectly apterous, though _occasionally with full-sized wings_. _Chorosoma miriforme_, _Prostemma guttula_, _Pachymerus brevipennis_, &c., are generally found with very short wing-covers, but sometimes with full-sized wings[49]." In like manner, the _Cimex apterus_, Linn. (one of the _Lygaeidae_) "exhibits, in an eminent degree, the ordinary occurrence of an imperfect perfect-state; whilst individuals are occasionally found _with fully developed organs of flight_[50]". _Lyaeus brevipennis_, Lat., also ordinarily occurs with abbreviated hemelytra; but it has been found with them perfect by Westwood, as well as with metathoracic wings. None of the above examples however would appear to do more than refer to the alary instability of the Insecta, as a matter of fact; but this is all for which we are now contending,--the preceding chapter having been in part devoted to some of the presumptive _causes_ of it. Whether the specimens of _Oncocephalus griseus_, to which Spinola called attention, were insular ones, I cannot say; but he seems to have noted an example in which an _opposite_ phaenomenon to those which Mr. Westwood has cited, was displayed, and moreover to have speculated on the conditions producing it, when he suggests: "L'influence du climat septentrional parait avoir arrete le developpement des organes du vol[51]." And, again, when commenting upon the other tendency in a representative of the _Reduviadae_, he says ('Essai,' p. 96): "Je pense que la presence des ailes et leur developpement dependent du climat." Whilst treating of two British species of the same family, Mr. Westwood observes: "The _Prostemma guttula_, Fab., and _Coranus subapterus_, Curt., are interesting on account of their being generally found in an undeveloped imago state,--the latter being either entirely apterous or with the fore-wings rudimental, although occasionally to be met with having the fore-wings completely developed[52]." The common _Phosphuga atrata_ of our own country has the organs of flight very rudimentary, and much too small for use: yet the late Mr. Holme of Oxford has mentioned[53], that he has several times taken it on the wing, during the hot sunshine. And, concerning the _Olisthopus rotundatus_, he states[54] that every specimen which he captured in the Scilly Islands was subapterous.