242 



INVERTEBRATE ZOOLOGY 



while the tips are membranous. In Orthoptera, the entire front 

 wings are somewhat thickened and are designated as tegmina. 

 Structure and Origin of the "Wings. — Typically, the wings of 

 insects are two pairs of membranous appendages which develop 

 as saclike folds of the body wall. In fully developed wings, this 

 saclike nature is obscured because the two walls of the sac 

 become so closely applied that they appear as a single membrane, 

 and a very delicate one at that. A framework upon which this 



Cosia Subcosh I 



Subcosh 2 



Cubitus 

 Medici 4' 



ki Anal 



■Medial 



■Medial 

 '■Media 3 

 -Medial 

 . Cubi-tus I 

 CubifusS 



Fig. 119. 



Fig. 120. 



Figs. 118-120. — Insect wing venation. 118, hypothetical primitive type of 

 wing venation with cross- veins added; 119, forewing of Hyptia showing great 

 reduction in number of veins; 120, forewing of a May fly showing great increase in 

 number of veins. {Redrawn from Corns took with permission). 



membrane is supported is composed of hollow tubes or veins 

 which originate as modified tracheae of the respiratory system. 

 In the early stages of development, the tracheae are accompanied 

 by nerves and blood-filled outgrowths of the body cavity. The 

 pattern which this framework assumes is spoken of as the "wing 

 venation." In most groups, this pattern shows remarkable con- 

 stancy among the members of the same species. Between genera 

 and the larger groups there is usually considerable variation in the 

 details of arrangement of the veins. It seems, however, that the 

 wings of all insects are directly homologous and that their princi- 

 pal individual veins are likewise homologous. Comparisons of 

 numerous wings and the study of the arrangement of the tracheae 



