PROGRESSION IN OR THROUGH THE AIR. 171 



particularly as elevators and propellers. In such cases the elytra 

 are twisted upon themselves after the manner of wings. 



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 arrang-ed and jointed in such a manner that the membranous wings can 

 be folded (,e) transversely across the back beneath the elytra during repose. 

 When 30 folded, the anterior or thick margins of the membranous wings are 

 directed outwards and slightly downwards, the posterior or thin margins in- 

 wards and shghtly 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 insigniticantly small when compared with the area of the body. They are, 

 moreover, finely twisted upon themselves, and possess great power as pro- 

 pellers and elevators. — Original. 



Fig. 90. — The Water-Bug {Genus bdostomd). 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 back- 

 wards.— Original. 



