1 62 THE INTEGUMENTAL SKELETON OF THE IMAGO. 



nymph ; they are afterwards absorbed. When the imago 

 emerges from the pupa, or when the cuticle which covers the 

 nymph is shed, the mesoblast and hypoderm of the wing are 

 both present, so that the wing is still a sac communicating 

 with the thoracic cavity and permeated by a rich plexus of 

 blood sinuses. It is small and thick. 



After the emergence of the imago the wing rapidly expands in 

 length and breadth, at the same time decreasing in thickness. 

 The upper and lower surfaces are drawn together by the 

 contraction and atrophy of the mesoblast, and subsequently of 

 the hypoderm, except at its junction with the thorax and in the 

 course of the largest nervures ; where^ the wing cavity persists, 

 and its mesoblast develops small tracheae, blood sinuses and 

 tendinous cords connected with the smaller wing muscles. 

 The greater number of the nervures become solid chitinous 

 rods, and appear as convex ridges on both surfaces of the wing. 

 Between these the wing membranes come into contact, and all 

 the epiblastic and mesoblastic cells disappear. 



Structure of the Wings. — The wing membranes are supported 

 and extended by branching hollow or solid nervures, which 

 form a reticulated framework. These are chitinized thicken- 

 ings of one or both wing membranes. The number of nervures, 

 their manner of branching, and the extent of the reticulation 

 between their branches, vary greatly, but there is a general 

 agreement in the arrangement of the principal nervures at the 

 attachment of the wing with the thorax in all insects. 



In the Dragon-flies the four wings are alike, and these insects 

 exhibit a very generalised type of wing. In other insects the 

 wings deviate more or less from this type, without, however, 

 altogether departing from it. 



The wings of the Dragon-iiies exhibit an anterior and a 

 posterior margin, a rounded apex, and a truncated base 

 which corresponds to its thoracic attachment. 



The anterior part of the base is folded fanwise, so that it 

 exhibits three ridges and two furrows above, and two ridges 

 below. This portion of the wing is strengthened by five strong 

 nervures, one corresponding to each ridge. 



