Vol. 1.] Woodwortli . — Wing Veins of Insects. 53 



into consideration the constant features of a developing wing, 

 the hypoderniis of the wingpad, and the cuticle that it secretes. 



The first trace of the venation in a wingpad is a series of 

 slight separations between the two layers of hypoderniis run- 

 ning lengthwise of the pad. Exactly when this arises is diffi- 

 cult to make out with certainty; it may exist from the very 

 first, but the indication is so slight that one can not be sure of 

 its presence until the cavities have attained considerable size. 

 By this time the veins can be seen in surface view as well as 

 in sections. In surface view they appear as slightly paler 

 stripes with no definite boundaries. They are from their first 

 appearance in a position approximating that which they occupy 

 in the adult venation. The paleness of the Ininds is doubtless 

 due to the transparency of the blood in the vein cavity. The 

 vein cavity hardly becomes distinguishable before a difference 

 begins to appear between the cells of the vein region and those 

 pertaining to the memln-ane. The difference is chiefly one of 

 size, perhaps wholly that. 



The early details of the development of the wing vary 

 greatly and in many particulars in different insects. An im- 

 portant difference that distinguishes the lower insects from the 

 higher is the fact that, in the former the wingpad takes part 

 in each ecdysis, whereas in the latter the wings part from an 

 old cuticle onl}'- at the molt by which the insect becomes adult. 

 These two processes of growth are not so fundamentally differ- 

 ent as they at first appear to be, because a complete transition 

 between the two types is afforded by a single order, the beetles, 

 indeed in one case by individuals of the same species. 



In order to take part in the ecdyses, all the hypodermal 

 cells must retain the same power of secreting chitin until the 

 pupal stage is reached. Up to this time the smaller vein cells 

 are neither more nor less active in this respect than the larger 

 cells of the membrane. The differentiation of the vein and 

 membrane cells in the early stages is very incomplete, and 

 they remain thus similar almost without further change until 

 after the penultimate molt. In some cases the veins appear 

 as wrinkles in the cuticle of the wingpad, but in other cases the 

 wingpad remains smooth in all the stages. When the wrinkles 

 are present, we can consider the wing to have progressed in its 

 development to an ontogenetically later condition, but it ap- 

 pears to be in nowise a gainer by this precociousness. 



