io The Structure and Special Physiology of Insects 
strongly chitinized lines called veins. These veins are corresponding artic¬ 
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 tracheae 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 venation, 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. 18). 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 wings 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 
F i6 delicate, or horny and firm, etc., have 
„ , „ . . . . , always been used to distinguish the 
tig. io.—Sphinx moth, showing proboscis; J ° 
at left the proboscis is shown coiled up larger groups, as orders, of insects, 
on the under side of the head, the nor- an d the fi rst classification, that of 
mal position when not in use. (Large _ . . . .. . . . 
figure, one-half natural size; small fig- Linnaeus (l 75 ° a PP*)> divides the class 
ure, natural size.) into orders almost solely on a basis 
of wing characters. The ordinal names expressed, to some degree, the 
differences, as Diptera,* two-winged; Lepidoptera, scale-winged; Coleoptera, 
sheath-winged, and so on. As a matter of fact, there may be much differ- 
* The derivation of the Linnaean ordinal names is given on p. 223. 
