AQUATIC INSECTS IN NEW YORK STATE 309 



The following year Scliinidt-Sohwedti 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; 

 Ijut he rather inclined to the view that they are closed, and 

 that we have to do with a special structure for aerating the 



nSchmidt-Schwedt, Dr E. Berl. Ent. Zeit. 33:280-308. 



