AQUATIC INSECTS IN NEW YORK STATE . 305 
formed. The prevalence of histolytic changes 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-m of figure 8, plate 28, of which figure 13 is such a 
representation and which is the cephalic limit of this cavity, 
we 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 difficult condition to explain and was unquestionably 
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 Donacia pal- 
mata live at a depth of from 3 to 4 feet under water and 
