Il8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



on the mesothoracic and metathoracic legs of Machilis closely resem- 

 bling the styli of the abdomen. Without this coincidence, or if 

 Machilis and its coxal spurs had not survived to modern times, it is 

 doubtful if entomologists would ever have thought of regarding the 

 abdominal styli or the cerci as other than direct rudiments of legs. 



Conclusions. — The abdominal appendages of insects are rudimen- 

 tary limbs. Each consists of a basis and usually one or more distal 

 appendicular parts, including a stylus, an eversible or retractile vesicle, 

 and a gonapophysis. From the facts known of the comparative struc- 

 ture of the abdominal appendages, and from theoretical considera- 

 tions we may draw the following tentative conclusions relative to the 

 homologies of the parts of the appendages, but it must be admitted 

 that the evidence at hand is not sufficiently definite to establish any 

 particular view concerning them. 



The limh bases of the abdominal appendages are the lobes or plates 

 of the walls of the body segments that support the appendicular proc- 

 esses. They are usually well developed in larval insects, but in most 

 adults they are partially or wholly united or blended with the sternum, 

 or in the male genital segment fused also with the tergum ; in the 

 eleventh segment they are reduced to small basal rings of the cerci, 

 or entirely obliterated. Generally there is no distinction between coxal 

 and subcoxal regions in the limb bases of the abdomen. In position 

 on the body the abdominal limb bases usually fall in line with the 

 subcoxae or pleural areas of the thorax. But since there is no 

 apparent reason for the development of large subcoxal plates on 

 the abdominal segments, such as those of the pterygote thorax, 

 it is not to be supposed that the limb bases of the abdomen repre- 

 sent the subcoxae alone. It is more probable that their principal parts 

 are derived from the flattened coxae, or that the structures in most 

 cases may represent primitive limb bases undifferentiated into coxae 

 and subcoxae. In the caterpillars and sawfly larvae, however, the 

 principal segment of each abdominal leg appears to be the coxa, which 

 is quite distinct from a subcoxal lobe of the wall of the body segment 

 to which it is attached. The abdominal limb bases are seldom movable 

 on the body, since they commonly lack muscles corresponding with 

 the basal muscles of the gnathal and thoracic appendages arising on the 

 body wall. Exceptions to this rule are found in the male genital seg- 

 ment, where the bases of the gonopods are occasionally provided with 

 muscles and are indej^endently movable ; but in such cases it is to be 

 suspected, at least, that the muscles are secondary developments. 



The styli, or other homologous appendicular processes of the ab- 

 domen, are of wide occurrence in insects, and serve a variety of 



