594 EXTERNAL ANATOMY OF INSECTS. 



These, however, are only exceptions to the rule; but in 

 the Diptera, unless we consider the alula, the representa- 

 tives of the secondary wings a , as a distinct pair, there 

 are never more than two wings, and one instance is 

 known in which an insect of this Order has none b . 

 Certain genera or individuals of the Tetrapterous Orders 

 are also furnished with alulae: besides Dytiscus, Blatta, 

 Phalcena hexaptera, which have been before noticed c , 

 they may be detected in miniature in Ammophila K. and 

 affinities ; these all may be regarded in some slight de- 

 gree as insects with six wings. 



ii. Kinds. Under this head we may consider the or- 

 gans of flight as to their situation and as to their sub- 

 stance. As to their situation, usually the first pair are 

 attached to the mesotlwrax, and the second to the meta- 

 thorax ; but in one instance, as has been before ob- 

 served' 1 , in the Strepsiptera K., the anterior pair belong 

 to the manitrunk, and the posterior to the mesothorax. 

 As to their substance, they take the several denomina- 

 tions of elytra, tegmina, hemelytra, and wings, for the 

 most part according to its variations, as will be seen 

 more at large hereafter. Under this head I shall only 

 further observe, that in many instances the organs of 

 flight appear to be mere abortions or rudiments, which 

 serve to exemplify what has been more than once stated, 

 that the CREATOR has seen it good to approach to new 

 organs gradually as well as to new forms. Thus elytra are 

 mere rudiments that do not serve to protect the wings in 

 Atractocerus ; tegmina in some species of Phasma, Aery- 



a See above, p. 559. b Chionea amneoides Dolm. 



c See above, p. 5G0, and Vol. II, 348, 352—. 

 d See above, p. 59], Note c. 



