AQUATIC INSECTS IN NEW YORK STATE 305 



formed. The prevalence of histolytic clianges taking place 

 among insects might make this a way out of the difficulty were 

 it not that in all the sections I have examined, I have never 

 found an undoubted leucocyte within this cavity, while in most 

 cases they were abundant in the body cavity adjacent to the 

 hypodermal cells, and, in addition to the above, there was no 

 indication of any histolysis taking place in any other part of 

 the body. No hypothesis concerning this cavity is tenable 

 other than that it was originally completely filled with the 

 hypodermal cells that surround it and that its present condi- 

 tion is an acquired one. If sections are examined at about 

 the level l-iii of figure 8, plate 28, of which figure 13 is such a 

 representation and which is the cephalic limit of this cavity> 

 w^e shall find that some of the hypodermal cells, as at vh 

 are filled with vacuoles, in the sections caudad of the one 

 represented here, the vacuoles in places so completely fill the 

 cells that there are left only delicate threads extending be- 

 tween the cuticle and basement membrane, while the cavities 

 of the vacuoles are either empty or filled with a homogenous, 

 nonstainable substance. Though it seems almost impossible to 

 conceive how such a large cavity could have been formed by 

 the vacuolization of the inner ends of the hypodermal cells, yet 

 I have been unable to find any other explanation that would 

 comply with all the conditions existing here. By this hypoth- 

 esis we are able to explain the origin of the cuticle, which is 

 the most difiicult condition to explain and was unijuestionably 

 formed before the origin of the cavity. There undoubtedly 

 exists some relation between this cavity and the dorso-ventral 

 motion of the caudal spines. In all probability this motion, 

 after the cells were modified into threadlike extensions, rup- 

 tured these threads, and in this way was secondary to the 

 vacuolization of the cells in the formation of the cavity. I hope 

 later to investigate this problem further and determine defi- 

 nitely if possible the origin of the cavity. 



As was pointed out above, the larvae of D o n a c i a j) a 1- 

 m a t a live at a depth of from 3 to 4 feet under water and 



