542 W. L. TOWER, 



hind wings 10 : 1. In 0. hrunneum the ratio for the elytra is 3 : 1, 

 hind wings 7:1. In P. punctata, elytra 2: 1, hind wings 5:1. In 

 C. femorata elytra 4: 1, hind wings 3: 1. 



Concerning the cause of this behavior of the h}T;)odermis I have 

 no definite evidence. Superficially it appears to be due to one of 

 two agencies, or to a combination of them. The change observed in 

 the contents of the hypodermal cells may be due to the entrance of 

 the haemolymph into the wing, which expands it and causes neces- 

 sarily a rearrangement of the tissue to accommodate the increasing size 

 of the organ, or the change may be due to some stimulus other than 

 mechanical necessity, which causes the nucleus and cytoplasm of the 

 cells to move to the distal end, or to the outer surface of the organ, 

 thereby causing a great increase in the surface area of each cell and 

 as a result, an increase in the size of the entire organ. To occupy 

 the space produced in the center of the organ by this process the 

 haemolymph is forced in by the contraction of the abdominal seg- 

 ments. Probably the first alternative will be the most commonly ac- 

 cepted one, since the usual opinion is that it is the entrance of the 

 haemolymph under pressure that expands the wings. It should be 

 noted, however, that this shifting of the cell contents begins and 

 reaches a considerable development before any haemolymph enters 

 the wing sinuses. In some pathological cases where the wings failed 

 to develop properly the veins and sinuses were found densely 

 crowded with haemolymph, showing that the pressure had been 

 great, but the contents of the hypodermal cells had not changed in 

 position from that of the early larva. If the entrance of the haemo- 

 lymph and the necessity of mechanical adjustment alone were responsible 

 for this process and expansion of the wings, some trace of it should 

 have occurred in these abnormal cases. It is evident then, that this 

 change in the position of the cell contents of the hypodermal cells, 

 which is so important to wing development, and to insect meta- 

 morphosis in general, is due to a stimulus of whose nature and 

 source we are ignorant, but which undoubtedly belongs to the group 

 of stimuli or causes which bring about the other changes in meta- 

 morphosis. The pressure of the haemolymph and the mechanics de- 

 pendent upon it are, I am certain, of minor importance in wing 

 development, and while they may have some part in the expansion of 

 the wing, the longer part is already well begun before the haemo- 

 lymph finds entrance to the wing, and, in general, I think that the 



