lo The Structure and Special Physiology of Insects 



^ 



strongly chitinized lines called veins. These veins are corresponding cutic- 



ular thickenings, in the upper and lower walls of 

 the flattened wing-sac, which protect, while the 

 wing is forming, certain main tracheal trunks that 

 carry air to 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 which serve admirably as a skeleton or 

 framework for the thin membranous wings. It 

 has been found that despite the obvious great 

 variety in the \enation, or number and arrange- 

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

 plan of venation is apparent throughout the insect 

 class. The more important and constant veins have 

 been given names, and their branches numbers 

 (Fig. i8). By the use of the same name or 

 number for the corresponding vein throughout all 

 the insect orders, the homologies or morphological 

 correspondences of the veins as they appear in the 

 variously modified vi'ings of the different insects 

 are made apparent. Many figures scattered through 

 this book show the venation of insects of 

 different orders, and the corresponding 

 lettering and numbering indicate the 

 homologies of the veins. As the wing 

 venation presents differing conditions 

 readily noted and described, much use is 

 made of it in classification. 



The differences in the wings them- 

 selves, that is, in number, relative size 

 of fore and hind wings, and in struc- 

 ture, i.e., whether membranous and 

 Pij, j(^ delicate, or horny and firm, etc., have 



F.C. i6.-Sphinx moth, showing proboscis; '''^''^^ '^*^^" "*«^ *« distinguish the 

 at left the proboscis is shown coiled up larger groups, as orders, of insects, 

 on the under side of the head, the nor- and the first classification, that of 

 mal position when not in use. (Large . 



figure, one-half natural size; small fig- Lmna'US (1750 app.), divides the clasS 



ure, natural size.) jmo orders almost solely on a basis 



of wing characters. The ordinal names expressed, to some degree, the 



differences, as Diptera,* Iwo-winged; Lepidoptera, scale-winged; Coleoptera, 



sheath-winged, and so on. As a matter of fact, there may be much differ- 



* The derivation of the Linna:an ordinal names is given on p. 223. 



