AQUATIC INSECTS IX NEW YORK STATE 301 



section of the alimentary canal of the three apical abdominal 

 segments, where the position of the rectum is shown at irc and 

 the position of the apodeme at da. 



Projecting from the caudal margin of each peritreme is a long 

 pointed spine which extends from the eighth abdominal segment 

 to or near the apex of the abdomen [pl.27, figs.lS-lDc.s]. These 

 are the caudal spines. On plate 28, figure 2, a lateral view of 

 one of these spines is shown enlarged, together with its con- 

 nection with the peritreme, pe, the cuticular pocket into which 

 the spiracle of the eighth abdominal segment opens, cp, and 

 the outer end of the dorsal apodeme, da. Figure 3 represents 

 the apex of a spine still more enlarged, so as to show the teeth 

 on its ventral side. If a spine is examined in cross section near 

 the middle of its length [pl--^^ fig--f^-<?]j it will be found that 

 it is ovate in outline [pl.28, fig. 4], the dorsal surface evenly 

 convex, the sides somewhat flattened, and the ventral surface 

 produced into a prominent angle. The cuticle of the dorsal and 

 lateral surfaces is thick and strongly chitinized, while on the 

 ventral surface the cuticle is thin, soft, and hardly chitinized 

 at all. This is more marked at the base of the spines and is 

 evidently to allow for a freer dorso-ventral motion. The interior 

 of the spine at this point contains 10 cavities, which may be 

 divided into two classes, those containing hypodermal cells and 

 those that are empty. Those of the first class are the largest 

 and most important and will be considered first. Beginning at 

 the ventral side, the angular portion is filled by a large ovate 

 shaped cavity w^hich has a tongue-like projection extending four 

 fifths the length of the section [ fig.li'c] ; on either side of this 

 tongue-like projection is a large cavity, which is ovate in out- 

 line, its mesal sides straight, and with a large tooth-like projec- 

 tion extending into its dorsolateral portion [fig.-ipvc]. Dorsad 

 of each of the cavities just described there is another much 

 smaller cavity, which is almost round in outline [fig.lpdc] and 

 dorsad and mesad of each of these, there is a single minute 

 cavity which is completely filled with cells. The three ventral 

 hypodermal cavities [fig. 4rc and 4pvc] are continuous with the 

 cavity at the base of the spines, the three dorsal hj^podermal 

 cavities [fig. Mc and 4/)(7c] are cross sections of pockets that 

 have probably been formed from infoldings from the dorsal 

 tongue-like portion of the ventral cavity. The cavities of the 

 second class are minute, four in number [fig.lec], a ])air laterad 

 and another pair dorsad of the dorsal pair of hypodermal cav-- 

 ities. 



If another section is examined near the body at the point f-g 

 indicated on plate 28, figure 2, it will be found that the three 

 ventral cavities are much larger [pl.28, fig.5], that the three 



