Vol. 1.] ^Yoodlvo)•t]^. — ^V'i}|g Veins of Insects. 57 



Thus far we have designated the corrugations of the surface 

 of the wingpads as wrinkles or folds. This should be explained 

 before we begin to consider the production of the cuticle, for 

 this structure more properly than the hypodermis might be 

 spoken of in these terms. To avoid confusion, therefore, we 

 must point out that when speaking of the cell layers reference 

 is had only to the condition of the outer surface of the cells. 

 The deep surface may or may not follow the course of the 

 outer surface, and at liest does so only approximately. 



The so-called foldings of the cellular layers is rarely or 

 never a folding in the true sense of the term, since it does not 

 involve the lower halves of the cell, but is brought al)Out l)y a 

 rearrangement of the outer ends of the cells such that the 

 outer surface presents a series of ridges regularly alternating 

 with furrows. It occurs immediately before or at the time of 

 the beginning of chitin production. The chitin is a layer which 

 is so molded over ridge and groove as to be thrown into folds. 

 The regularity of the cell grooves would at least suggest that 

 they passively result from the real folding of a plastic layer of 

 some kind, its folds being caused by lateral pressure. The 

 cuticle itself when first formed, or rather the outer portion of it, 

 might possibly serve as such a layer. 



This hypothetical layer might be supposed to meet with too 

 much resistance along the veins to allow the production of 

 longitudinal wrinkles in addition to the transverse ones. The 

 latter may be supposed to arise first, because a short wrinkle 

 across the wing is more easily formed than one lengthwise. 

 By this assumption may be explained the manner in which 

 such a complicated wrinkling as occurs in the membrane eould 

 result in the production of a flat membrane after it is expanded. 



The possible history of this hypothetical layer is that it is 

 produced as a uniform homogeneous layer over the surface of 

 the wingpad cells before they begin to show any wrinkling 

 whatever, and that it then swells by the absorption of water 

 or some other material originating in the cells beneath. 

 Thus arises the lateral pressure that throws the whole layer 

 into folds. The cells separate along lines where the folds press 

 upon them, thus bringing about the grooving of the cellular 

 layer. The importance of the connection between the cells of 

 the two walls of the pad in maintaining uniform conditions has 

 already been dwelt upon. If the connection is intimate, the 



