AQUATIC INSECTS IN NEW YORK STATE 301 
section of the alimentary canal of the three apical abdominal 
segments, where the position of the rectum is shown at rec 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 [p1l.27, figs.18-19cs]. 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 aspine is examined in cross section near 
the middle of its length [pl.28, fig.2d-e], 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 which has a tongue-like projection extending four 
fifths the length of the section [ fig.4vc]; 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 dorso-lateral portion [fig.4pve]. Dorsad 
of each of the cavities just described there is another much 
smaller cavity, which is almost round in outline [fig.4pdc] 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. 4ve and 4pve] are continuous with the 
cavity at the base of the spines, the three dorsal hypodermal 
cavities [fig. 4de and 4pdc] 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.4ec], a pair 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-¢ 
indicated on plate 28, figure 2, it will be found that the three 
ventral cavities are much larger [p1.28, fig.5], that the three 
