24 ELEMENTARY ENTOMOLOGY 



mouth-parts, form the most important basis for classification. Thus 

 most of the orders are distinguished by differences in the wings, as 

 indicated by their names, which usually end in -ptera (from pteron, 

 a wing), and many insects may be classified to the genus or even to 

 the species by the wings alone, this being particularly true of fossil 

 forms, in which the wings are often the only parts well preserved. 

 Most adult insects possess two pairs of wings, borne by the meso- 

 thorax and metathorax, but in some parasitic orders the wings have 

 been lost, and one order, the Thysanura, represents the primitive 

 insect without wings. In the flies (Diptera) only the mesothoracic 

 wings are developed, and the metathoracic wings are represented by 

 clublike appendages, called halteres, or balancers. The relative shape 



and size of the two pairs vary 

 greatly, and frequently the 

 two wings of each side over- 

 lap or are held together by 

 various structures, so that 

 they act as a single organ. 

 The wings are strengthened 

 FIG. 27. Wing of house-fly (Musca domesti- b Y numerous thickenings, 



ca), showing specialization of wing venation called veins, whose number 

 through reduction of veins and position form the basis Q f 



c, costa; r, radius;^, media; cubitus ; a, ^ classification of families, 

 anal. (After Comstock) 



genera, and species. It has 



been shown by Professors Comstock and Needham that the prin- 

 cipal veins are homologous in all the orders of insects, and that 

 they have been derived from one original type, either by the disap- 

 pearance of certain veins, by their growing together, or by the 

 addition of supplementary veins. The typical longitudinal veins, as 

 shown in Fig. 26, are the costa, subcosta, radius, media, cubitus, 

 and anals. The costa (c) is unbranched and strengthens the 

 anterior margin of the wing. The subcosta (sc) is typically two- 

 branched, though often single, and, where the costa is small or 

 wanting, appears to be the first, or anterior, vein. The radius (r) is 

 typically five-branched, the base of the second principal branch, 

 from which the four posterior branches divide, being known as the 

 radial sector. The media (m)-is typically four-branched, though 

 often but two or three branches are present. Cubitus (cu) has 



