16 



THE CLASS OF INSECTS. 



gradually incurved toward the base (Fig. 18), and the three 

 pairs of rhabdites approach eacli other so closely that the two 

 outer ones completely ensheath the inner, until a complete 

 extensible tube is formed, which is gradually withdrawn entirely 

 within the body. 



The male genital organ is originally composed of three pairs 

 (two pairs, apparently, in ^s- 

 chna, Fig. 19) of tubercles all 

 arising from the ninth abdominal 

 ring, being sternal outgrowths 

 and placed on each side of the 

 r~TT%-~\ mesial line of the body, two be- 

 v_iy_y ing anterior, and very unequal in size, and the 

 Fig. 19. third pair nearer the base of the abdomen. The ex- 

 ternal genital organs are to be considered | 

 as probably homologous with the limbs, as 

 Ganin has shown that they bud out in the 

 same manner from (see p. 704 

 fig. 655) tlie arthromere* 

 '* Tiiis view will apply to the 

 Fig. 21. genital armor of all Insects, so 

 far as we have been able to observe. It is 

 so in the pupa of ^schna (Fig. 21), and 

 the pupa of Agrion (Fig. 22), which com- 

 pletely repeats, in its essential features, the 

 structure of the ovipositor of Bombus. Thus in ^schna and 

 Agrion the ovipositor consists of a pair of closely appressed ensi- 

 form processes which grow out from under the posterior edge of 

 tlie eighth abdominal ring, and are embraced between two pairs 



*This term is proposed as better defining the ideal ring, or primary zoological 

 element of an articulated animal than the terms somite or zodnite, which seem too 

 vague; we also propose the term arthroderm for the outer crust, or body walls, of 

 Articulates, and artliropleura for the j)leural, or limb-bearing region, of the body, 

 bemgthat portion of the arthromere situated between the tergite and sternite. 



Fio. 19. The rudiments of the male intromittent organ of the pupa of iKschna, 

 consisting of two flattened tubercles situated on the ninth ring; the outer pair 

 large and rounded inclosiug the smaller linear oval pair. 



Fig. 20. The same in the Humble-bee, but consisting of three pairs of tubercles, 

 X, II, z ; 8, 9, 10, the last three segments of the abdomen. 



Fig. 21. The rudimentary ovipositor of the pupa of ^^schna, a Dragon-fly. 



Fig. 22. The same in pupa of Agrion, a small Dragon-fly. Here the rudiments 

 of the eleventh abdominal ring are seen, d, the base of one of the abdominal false 

 gills. The ovipositor of Cicada is formed in the same -w ay. — Figs, li-22 original. 



