92 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



throughout the orders, unless one or the other or both are absent. 

 In addition to these fundamental, musculated processes, however, 

 there are innumerable other secondary genital processes having no 

 necessary homology in the different orders, which may be developed on 

 the ninth segment, on the aedeagus, or on the segments preceding 

 and following the ninth. These structures, except in rare cases, have 

 no muscles of their own, and are not independently movable, though 

 some of them may be moved incidentally by the usual segmental mus- 

 cles attached at their bases. The movable claspers derived from the 

 styli, however, are sometimes divided, and each may be separated into 

 two quite distinct parts provided individually with groups of muscle 

 fibers. In such cases there will appear to be, as in some of the Hy- 

 menoptera, a pair of movable lobes on each side of the genital 

 apparatus, 



A more detailed analysis of the structure of the organs of ovi- 

 position and copulation, as shown in the principal orders of insects, 

 will form the subject matter of Part II of this paper, wherein will be 

 presented also a larger body of evidence in support of some of the 

 statements that seem arbitrary in the brief discussion given above. 



THE CERCI (UROPODS) 



The prevalence of cerci in so many orders of insects, and the almost 

 universal occurrence of the organs in the more generalized groups 

 leave little doubt that the cereal appendages are primitive structures, 

 and that, in some form, they must have been characteristic features 

 of the early insect ancestors. The anatomy and structural variations 

 of the cerci are well known ; functionally the appendages are in most 

 cases sensory organs, though they are frequently modified in form 

 to serve mechanical purposes ; morphologically they are subjects of 

 diverse opinion among speculative entomologists. The essential facts 

 known concerning the cerci can be briefly stated. 



In the Thysanura the cerci evidently belong to the eleventh ab- 

 dominal segment. The last typical segment of the body in such forms 

 as Nesomachilis (fig, 7 A) is the tenth (X), which is a complete 

 annulus. From within the posterior margin of this segment there 

 project the three terminal filaments, of which the lateral pair are 

 the cerci (Cer). If the group of filaments is pulled out of the tenth 

 segment, it is seen that the three of them arise from a common basal 

 ring (B, XI), which has all the aspects of a reduced segment, in this 

 case the eleventh, normally concealed within the tenth. The eleventh 

 annulus presents a wide dorsal region (C, XIT) prolonged into the 



