AQUATIC INSECTS IN NEW YORK STATDH 309 
‘The following year Schmidt-Schwedt! replied to Dewitz, admit- 
ting that the organs at the base of the spines were true spiracles, 
but probably used only for expelling impure air, while the air 
in the plant cells was taken up by a double row of transverse 
Slits found on the dorsal side of each caudal spine. Where these 
slits occur, they fit like a tongue and groove and are perfectly 
concealed. The air finds its way through the slits into the paired 
dorsal cavities and then by means of openings between the 
paired dorsal cavities and the paired ventral cavities, enters the 
paired ventral cavities along which it follows to the cuticular 
pocket. The rod-like cuticular structure of the cuticular pocket 
was known to Schmidt-Schwedt and though from the study of 
serial sections of this region he could not find any indication of 
an opening leading from the paired ventral cavity into the cuti- 
cular pocket, yet from numerous physical experiments, such as 
heating larva to drive out the air and then trying to determine 
its course, he maintained that the air found entrance from the 
paired ventral cavity through the walls of the cuticular pocket. 
In 1900 E. Dwight Sanderson reviewed the work that had 
been done hitherto and made some original investigations on the 
structure of the caudal spines. He concluded that the ventral 
canal had absolutely no connection with the tracheal system, 
but that “the two upper passages,” that is the paired dorsal 
cavities, “are open above, but can be readily closed by a wedge- 
shaped piece which runs along the top of the appendage,” and 
that “the lateral passages,” that is the paired ventral cavities, 
“open into the tube surrounding the spiracle,” and that there 
is a double series of elliptical openings along the upper part of 
each lateral passage and that from these elliptical openings arise 
several tubes, each of which resembles a coarse miniature gill. 
He was unable to determine whether these tubes are open at the 
tips or not, but, if so, thought that they probably act as a sieve 
through which the air is admitted into the lateral passages; 
but he rather inclined to the view that they are closed, and 
that we have to do with a special structure for aerating the 
1Schmidt-Schwedt, Dr E, Berl. Dnt. Zeit. 33:299-308. 
