NO. 6 INSECT ABDOMEN — SNODGRASS 59 



fundamental plan of the abdominal musculature of pterygote insects. 

 According to Grassi, the musculature of an abdominal segment of 

 Campodea comprises longitudinal dorsal and ventral muscles, oblique 

 dorsal and ventral muscles, and dorsoventral lateral muscles. The 

 longitudinal muscles are clearly the internal dorsals, and internal 

 ventrals. The oblique dorsal muscles are the external dorsals. The 

 external ventrals are represented by a pair of muscles convergent from 

 the posterior margin of the segment to the mid-sternal region below 

 the ganglion. These muscles Grassi terms mnsculi suhganglionares. 

 In addition to these there are also small oblique and transverse lateral 

 ventral muscles. The true lateral muscles include several small tergo- 

 sternal fibers on the sides of each segment. Finally there are the 

 muscles of the styli and eversible vesicles. 



By comparison with Campodea, or with almost any other insect, 

 the body musculature in the Japygidae is extremely intricate, being 

 highly complicated by the presence of numerous muscles that appear 

 to have no relation to muscles in a simple type of musculature. The 

 following account of the abdominal musculature of a member of this 

 group is based on a study of specimens of the Australian Heterojapyx 

 gallardi, for which the writer is indebted to Dr. R. J. Tillyard. 

 Females of this huge japygid reach a length of 40 millimeters, and 

 a dissection of the muscles in well preserved specimens is not a 

 particularly difficult task. 



The entire body musculature of Heterojapyx anterior to the ninth 

 abdominal segment is highly complex, there being in each of the first 

 eight segments of the abdomen at least 40 pairs of muscles, the 

 arrangement of which makes a most intricate pattern against the body 

 wall (fig. 24). In the mesothorax and metathorax the musculature is 

 quite as complex, and in many details quite different from that of the 

 abdomen, and is more diversified by the presence of the leg muscles. 

 In the ninth abdominal segment the musculature is simplified. In the 

 tenth it consists of a single pair of fiber bundles, but these constitute 

 two great lateral muscles, almost completely occupying the segment, 

 which act as adductors of the cereal forceps. 



The 40 muscles in either half of a typical al)dominal segment of 

 Heterojapyx, shown in figure 24 representing segment VI, are com- 

 prised in the following groups : 



I. Dorsal muscles. — A median band of inner longitudinal inter- 

 segmental dorsals (A, la, ib. le) ; two medio-lateral oblique inter- 

 segmental dorsals (A, B, 2, 5) ; two latero-median oblique interseg- 

 mental dorsals (B, 4, 5) ; and an outer longitudinal intersegmental 

 dorsal (B, d). 



