NO. 6 INSECT ABDOMEN — SNODGRASS 99 



terminal body segment. The ventral muscles of the ninth body seg- 

 ment {C, 4) are attached posteriorly on the intersegmental fold (/) 

 just before the lower part of the basal plate of the appendage, but 

 no muscles are inserted directly on the latter. 



The terminal appendages of the larva of Hydropsy die (fig. 41, 

 F, Pp) represent quite a dififerent type of appendicular organ. Each 

 pygopod here is a large, two-segmented, leglike structure projecting 

 from the ventro-lateral part of the terminal body segment. The 

 proximal segment of the appendage (E, &) is elongate and cylindrical ; 

 the distal segment {d) is a strong, decurved claw with a high, nar- 

 row base articulated by its dorsal end with the dorsal extremity of 

 the basal segment. Ventrally, just before the base of the claw, there 

 is a small triangular sclerite (c) in the lower, membranous wall of 

 the proximal segment. 



The musculature of the terminal appendage of Hydropsyche (fig. 

 41 E) dififers in several respects from that of Platyphylax (D). As 

 in the latter, however, there are no muscles inserted on the base of 

 the organ, but there are an adductor muscle (E, i) inserted on the 

 base of the apical claw {d), and two flexor muscles (^, j) inserted 

 on the small ventral sclerite {c). The adductor (z) arises, not in 

 the basal segment of the appendage as in Platyphylax (D, la, ib) , 

 but in the proximal part of the last body segment. One of the flexor 

 muscles (E, 2) is a broad fan of fibers arising on the dorsal wall of the 

 basal segment, and thus suggests an identity with muscle .? of Platy- 

 phylax (D) arising on the basal plate of the appendage. The other 

 flexor muscle is a long bundle of fibers (E, j) arising on the interseg- 

 mental fold (e) between the eighth and ninth abdominal segments 

 along with the fibers of tiie ventral longitudinal muscles of the ninth 

 segment (4), but separating from the latter posteriorly to enter the 

 appendage. This muscle apparently has no representative in Platy- 

 Phvlax (D). 



The relation between the two types of appendages described above 

 is obscure. The only suggestion that can be made is that the basal 

 plate of the Platyphylax appendage (fig. 41 D, b) corresponds with 

 the basal segment of the Hydropsyche appendage (E, 6) and that the 

 intermediate plate of the former (c) is represented by the small ven- 

 tral sclerite (c) of the latter. The apical claw (d) is evidently the 

 same in ])oth. Still more difficult is it to find possible homologies 

 between either of these two types of trichopterous api)endages and 

 the terminal appendages of the sialid larvae, Chanliodes and Corydalus 

 (^'igs- 35 1^' E, 36 B, E), or the abdominal legs of lei)idopterous larvae 

 (figs. 37, 42 C). The basal plate or basal segment {b) in the tri- 

 7 



