58 



University of California Publications. [Entomology 



two wrinkling membranes would find the points of resistance 

 practically identical on the two sides, and so might produce 

 sets of foldings sufficiently the counterpart of each other to 

 accomplish the coordination of the two membrane areas (Fig. 

 22, B). 



The amount of expansion necessary in our hypothetical 

 layer Avould not be as great as is usually estimated. From a 



number of measurements of 

 wingpads and adult wings, it 

 appears that an increase of 

 from three to four diameters 

 would be sufficient. Even this 

 is very large as compared 

 with the power of most sub- 

 stances to swell, but certainly 

 not impossible. Whatever the 

 cause of wrinkling may be, it 

 is without doubt the same all 

 over the body of the insect. 

 The folding of the hypoder- 

 mis in the wingpads is differ- 

 ent in degree, but not in 

 kind, from that which accompanies increase in size elsewhere. 

 Just before each molt the hypodermis secretes two substances 

 of peculiar nature. One is watery, or slightly mucilaginous, 

 and serves first to separate the old cuticle from the cells, pre- 

 liminary to the production of the new cuticle, and acts as a 

 lubricant during the process of molting. The other is chiti- 

 nous, but of a peculiar kind, clearly distinguishable from the 

 remainder of the cuticle. The nature of neither of these 

 substances is understood, and one of them may represent the 

 hypothetical layer. 



The hypothesis thus offered as an explanation of the grooving 

 of the developing veins and wing membrane is, in brief: 

 (1) That there is secreted by the hypodermal cells shortly 

 previous to each molt a substance that by swelling throws 

 itself into folds, and where the hypodermal cells are in the 

 right condition insinuates itself between these cells; (2) That 

 there is produced at once a new cuticle, which follows down 

 into each of the grooves thus formed, but straightens out as soon 

 as the insect emerges. 



FIG 22. Diagrams illustrating folding 

 of win<.'s at time of development of cuti- 

 cle between two veins of T yphlonjba conies . 

 A, longitudinal section of vein; B, of 

 membrane: C, tangential section of mem- 

 brane. 



