INSECTA. 



589 



hardened by the deposition of chitine, so as to form two protective 

 cases for the hinder membranous wings. In this condition the 

 anterior wings are known as the "elytra," or "wing-cases." In 

 some of the He7nipte7-a this change only affects the inner portions 

 of the anterior wings, the apices of which remain membranous, and 



Fig. 442. — a, Schematic view of the right anterior wing of a Palaeozoic Cockroach, greatly 

 enlarged. (After Scudder.) ma, " Marginal " vein, which in this case merely thickens the 

 margin of the wing; me, "Mediastinal" vein; sc, "Scapular" vein; ex, " Externomedian " 

 vein; in, " Internomedian " vein ; aft, " Anal " vein. b, Anterior wing of Butterfly {Castnia 

 veraguana), enlarged; co, "Costal" vein; su, "Subcostal" vein; sc, "Scapular" vein; ex, 

 Branches of the "externomedian" vein; in, "Internomedian" vein; vi, "Internal" vein. 

 (After J. O. Westwood.) 



to these the term " hemelytra " is applied. In the Diptera the pos- 

 terior pair of wings are rudimentary, and are converted into two 

 capitate filaments, called "halteres" or "balancers." In the Strep- 

 siptera the anterior pair of wings are rudimentary, and are converted 

 into twisted filaments. 



