GRASS THRIPS. 105 



the first abdominal somite. They lie in the abdomen without 

 any definite arrangement, occupying the spaces between the other 

 organs and are richly supplied with tracheae 



Each tubule is composed of large cells with prominent nuclei, 

 that are so placed that they give it a spiral appearance. A trans- 

 verse section shows from five to seven cells around the lumen of 

 the tubule. 



' MUSCULAR SYSTEM. 



Fig. 8. 



The muscular system corresponds in its general arrangement 

 to the segmented structure of the body ; that is, most of the 

 muscles are arranged inter-segmentally. The typical arrange- 

 ment is shown in any of the anterior abdominal somites. When 

 a transverse section is taken across one of these somites, fig. 13, 

 four longitudinal rows, each composed of five muscles, are seen. 

 The four rows are placed so that there are two on the dorsal 

 and two on the ventral side. 



The ends of these muscles are attached by a sort of tendon to 

 the infolding of the integument between each two somites. In 

 the last abdominal somite the posterior ends of the longitudinal 

 muscles are attached to ridges in the integument. These muscles 

 are bellied so that they are thicker in the middle than at the ends, 

 and each is nearly square in cross section. The longitudinal 

 muscles of the abdomen are all arranged on this plan. By this 

 arrangement one end of the muscles of two somites being 

 attached at practically the same point, it is possible to bring the 

 fulcrum for any movement of the abdomen to the joint between 

 any two somites, the combined action of the muscles of the 

 somites anterior to this point keeping the anterior part of the 

 abdomen rigid. The abdomen may be bent in practically any 

 direction by means of the longitudinal bands of muscles, which 

 may be contracted individually or in combinations, throughout 

 the length of the abdomen, or in any part of it. 



Near the middle of each abdominal somite there is, on either 

 side, a pair of muscles which run from the dorsal to the ventral 

 surface, fig. 8, stm. These provide for a dorso-ventral con- 

 traction of the abdomen, which is of service in respiration. 



In the sixth and seventh abdominal somites there is, in addi- 

 tion to the typical muscles already mentioned, a set of muscles 



