1 64 THE INTEGUMENTAL SKELETON OF THE IMAGO. 



of each varies greatly in different insects. In the generalised 

 Neuroptera and even in iEschna, the only movements 

 which are at all extensive are those of elevation and 

 depression ; rotation is limited, and flexion and extension are 

 still more limited — hence the wing joint is comparatively simple. 

 Rotation is mainly effected by the simultaneous elevation of the 

 pro- and depression of the metapterygium, or vice versa. In the 

 Hymenoptera and Diptera rotation and flexion are provided 

 for by the segmentation of the pterygia, which consist of 

 complex sclerites uniting the rigid nervures with the thoracic 

 wall. These sclerites form a wing root which has some 

 resemblance to the carpus of a vertebrate. The sclerites 

 articulate with each other by complex surfaces, not only in 

 series, formed by the segmentation of the roots of the main 

 nervures ; but laterally, those of the propterygium articulating 

 with those of the mesopterygium, and those of the meso- 

 pterygium with those of the metapterygium. 



In the more highly modified Insecta there is a constant 

 tendency towards the reduction of the wings to a single pair, 

 either by a locking mechanism which unites the posterior 

 border of the anterior with the anterior border of the posterior 

 wing, or by their function as an organ of flight being in abey- 

 ance in one or other pair. A few insects in all orders are 

 apterous. 



In the Diptera the anterior wings only are organs of flight; 

 the posterior pair are greatly reduced in magnitude, and form 

 complex sensory organs, which are known as halteres, or 

 balancers. 



Owing to the almost equal development of both the 

 anterior and posterior wing roots and systems of nervures 

 in the Diptera, the wing often appears to consist of both an 

 anterior and a posterior wing united, its anterior half closely 

 resembling the anterior, and its posterior half the posterior, 

 wing of the Hymenoptera. There can be, however, no 

 question as to their being the homologues of the anterior 

 wings only, nor as to the homology of the posterior wings with 

 the halteres. The only authority who ever questioned this 



