62 



AN INTRODUCTION TO ENTOMOLOGY 



wing overlaps the hind wing, the hind wing is held between the two 

 (Fig. 74). This type of the posterior lobe of the fore wing is termed 

 the jugum or yoke. The structure of the jugum is shown in Figure 7 5 . 

 The fibula. In several groups of insects an organ has been 

 developed that serves to unite the fore and hind wings, but which 

 functions in a way quite different from that of the jugum. Like the 

 jugum it is found at the base of the fore wing; but unlike the jugum 

 it extends back above the base of the hind wing and is clasped over an 

 elevated part of the hind wing; this organ is the fibula or clasp. 

 In some insects, as in the Trichoptera, the fibula consists only of 

 a specialized posterior lobe of the fore wing; in others, as in the 

 genus Corydalus of the order Neuroptera, the proximal part of the 

 fibula is margined by the axillary cord, showing that the axillary 

 membrane enters into the composition of this organ (Fig. 76). 



The hypothetical type of the primitive wing-venation. A careful 

 study of the wings of many insects has shown that the fundamental 

 type of venation is the same in all of the orders of winged "insects. 

 But this fact is evident only when the more primitive or generalized 

 members of different orders are compared with each other. In most 

 of the orders of insects the greater number of species have become so 



modified or specialized as 

 regards the structure of 

 their wings that it is diffi- 

 cult at first to trace out the 

 primitive type. 



This agreement in the 

 important features of the 

 venation of the wings of 

 the generalized members of 

 the different orders of insects 

 is still more evident when 

 the wings of nymphs, naiads, 

 and pupae are studied. It 

 has been demonstrated that 

 in the development of wings 

 of generalized insects the 

 longitudinal wing-veins are 

 formed about preexisting 

 tracheae. In the develop- 

 ment of the wing, these 

 tracheae grow out into the 



Fig. 74. Wings of a hepialid, seen from 

 below; a, accessory vein. 



wing-bud, and later the. wing-veins are formed about them. 



