EXTERNAL ANATOMY OF INSECTS. 705 
firmer substance, as in Scorpio, Thelyphonus, and Phrynus, 
it is very conspicuous, and in the latter genus exhibits 
many longitudinal folds, as it does likewise in Gryllo- 
talpa, which must permit a vast extension of the abdo- 
men. Inthis membrane, in some cases, as in Dynastes, 
Melolontha, &c., the two or three first spiracles are 
fixed*. In the Hymenoptera and many other insects 
the dorsal segments do not unite by their margin with 
the ventral, but the end of each dorsal laps over that of 
the corresponding ventral. 
Dorsal segments». I shall next notice the segments 
seriatim, in the order of their occurrence, beginning 
with the dorsal ones. The most remarkable circumstance 
with respect to these that occurs to my recollection takes 
place in the Cancroid spiders (Carkinodes cancriformis, 
aculeata, &c.), in which the back of the abdomen is form- 
ed by a plate, in some extended in a transverse direc- 
tion (C. cancriformis), in others in a longitudinal one (C. 
aculeata), of a much harder substance than the under 
side and quite flat, set with strong sharp spines, in the 
former species apparently moveable, and terminating be- 
hind in a piece resembling in some measure the scutellum 
of the Stratyomide and similarly armed with a pair of 
spines ©: in C. aculeata the sides of the abdomen, un- 
der the plate, have a number of longitudinal folds like 
those of Phrynus. In Cryptocerus, a genus of ants pecu- 
liar to South America, the jist segment, not reckoning 
the pedicle, forms almost the whole back of the abdomen, 
and the three last are so minute as scarcely to be distin- 
* Prate VIII. Fie. 9. A", B’. > Ibid, Fic. 5. A’. 
© Ibid. XV. Fre. 10. 
VOL. III. nz 
