_Elytra or Wing-cases, Membranous Wings--their shape and uses._--The wings of insects consist either of one or two pairs. When two pairs are present, they are divided into an anterior or upper pair, and a posterior or under pair. In some instances the anterior pair are greatly modified, and present a corneous condition. When so modified, they cover the under wings when the insect is reposing, and have from this circumstance been named elytra, from the Greek ἔλυτρον, a sheath. The anterior wings are dense, rigid, and opaque in the beetles (fig. 89, _r_); solid in one part and membranaceous in another in the water-bugs (fig. 90, _r_); more or less membranous throughout in the grasshoppers; and completely membranous in the dragon-flies (fig. 91, _e e_, p. 172). The superior or upper wings are inclined at a certain angle when extended, and are indirectly connected with flight in the beetles, water-bugs, and grasshoppers. They are actively engaged in this function in the dragon-flies and butterflies. The elytra or anterior wings are frequently employed as _sustainers_ or _gliders_ in flight,[81] the posterior wings acting more particularly as _elevators_ and _propellers_. In such cases the elytra are twisted upon themselves after the manner of wings. [81] That the elytra take part in flight is proved by this, that when they are removed, flight is in many cases destroyed. [Illustration: FIG. 89. FIG. 90. FIG. 89.--The Centaur Beetle (_Augusoma centaurus_), seen from above. Shows elytra (_r_) and membranous wings (_e_) in the extended state. The nervures are arranged and jointed in such a manner that the membranous wings can be folded (_e_) transversely across the back beneath the elytra during repose. When so folded, the anterior or thick margins of the membranous wings are directed outwards and slightly downwards, the posterior or thin margins inwards and slightly upwards. During extension the positions of the margins are reversed by the wings twisting and rotating upon their long axes, the anterior margins, as in bats and birds, being directed upwards and forwards, and making a very decided angle with the horizon. The wings in the beetles are insignificantly small when compared with the area of the body. They are, moreover, finely twisted upon themselves, and possess great power as propellers and elevators.--_Original._ FIG. 90.--The Water-Bug (_Genus belostoma_). In this insect the superior wings (elytra or wing covers _r_) are semi-membranous. They are geared to the membranous or under wings (_a_) by a hook, the two acting together in flight. When so geared the upper and under wings are delicately curved and twisted. They moreover taper from within outwards, and from before backwards.--_Original._] [Illustration: