Vol. 1.] 



Woo(lwo)i]i. — Wi))g Veins of Insects. 



FIG. 21. Cross-section of wingpad 

 of nymph of T yphloojba comes, .show- 

 ing vein cavities 



figures of the condition of the cells in the larger insects are 

 given by Mayer ('96). The section shown in Fig. 21 illus- 

 trates the cell arrangement in one of the smaller leaf hoppers. 

 It is always difhcult to make out exactly what the nature of 

 the union of the cells really is, 

 except in the smaller forms, 

 where there is no real connec- 

 tion, but only close contact. 



Usually the correspondence of 

 the two parts of the vein is 

 apparently exact, lint one can 

 easily find abundant examples 

 of a failure to meet, such as is shown in Fig. 17. A more 

 common condition is where there is a considerable differ- 

 ence in the relative size of the parts on the two sides of the 

 wing; the small part, occupying any position from side to 

 side of the larger section, though in this case the veins give 

 one the impression of perfect conformity, except when exam- 

 ined very carefully. The possible range of variation of this 

 sort, before the nonconformity would ordinarily be discovered, 

 is often more than half the diameter of the vein, and cursory 

 examination might even fail to distinguish a displacement 

 that is not distinctly more than the diameter of the vein. 



The difficulty of the problem arises from the fact that the 

 wing is not developed flat, but wrinkled to a remarkable degree, 

 especially in the membrane areas, wdiere, in addition to the 

 transverse wrinkles, found also on the veins, there is still 

 greater folding in a longitudinal direction, providing for lateral 

 expansion. This difference in the character of the folding, 

 when it is developed just before chitinization, is the most evi- 

 dent difference between the vein and membrane regions of the 

 wingpad. Probably the wrinkling was at first irregular; that 

 is, with no definite arrangement, but more or less uniform over 

 the whole wing; with the development of veins, the greater 

 need of room for the thicker cuticle of the vein regions gradu- 

 ally led these regions to be folded in one plane only; the 

 membrane areas, on the contrary, retain the irregular method 

 of wrinkling, and to compensate for want of lateral wrinkling 

 in the vein areas, suffered an increased degree of wrinkling in 

 the lateral direction. With all the changes incident to this 

 readjustment, it is necessary that the cells of the two sides of 



