10 The Structure and Special ^8T ^ Insects 



1 





lines catted veins. These veins are corresponding v 



uUr thickening^ in die upper and lower watts of 

 the fattened wing-sac, which protect while the 

 wing is forming, certain main tracheal trunks that 

 carry air |o the wing-tissue. After the wing is 

 expanded and dry. the trachea? mostly die out, and 

 the veins are left as firm thick-walled branching 

 tubes whkh serve admirably as a skeleton cr 

 framework tor the thin membranous wings. It 

 has been found that despite the obvious great 

 variety it the venation, or number and arrange- 

 ment of these veins of the wing, a general type- 

 plan of venation is apparent throughout the insect 

 dass. The more important and constant veins have 

 been given names, and their branches numbers 

 (F|j -.s\ :>\ AM -.-.so o: AN sam MOM or 

 number for die corresponding vein throughout all 

 die insect orders, the homologies or morphological 

 correspondences of die veins as they appear in the 

 variously modified wings of die different insects 

 are made apparent Many figures scattered through 

 this book show the venation of insects of 

 different orders, and die corresponding 

 lettering and numbering indicate the 

 homologies of die veins, As the wing 

 .Ation presents differing conditions 

 readily noted and described, much use is 

 made of it in classification. 



The differences in die wings them- 

 selves, that is, in number, relative sue 

 of fore and hind wings, and in struc- 

 ture, i.e., whether membranous and 

 delicate, or horny and firm, etc., have 

 always been used to distinguish the 

 larger groups, as orders, of i: 

 and the first classification, that of 

 Linnaeus (1750 app.). divides die dass 



-- . v^v.crs .-.'"v< -O'.V-'.N , .-. :\>-.s 



of wing characters The ordinal names expressed, to some degree, die 

 differences as Dipteral twvvwinged; Leptdoptera, scale-winged; Cokoptera, 

 and so on. As a matter of fact, there mar be much differ- 



* The dotation of the LJoaaean ot^inii names is giw on p. 



