10 MISC. PUBLICATION 5 00, U. S. DEPT, OF AGRICULTURE 



The ninth abdominal segment is of greatest interest both morpho- 

 logically and taxonomically because in the male (fig. 3) it bears what 

 is believed to be a pair of segmental appendages, the clasping ap- 

 paratus. The ninth tergal plate is either fused with the pygidium 

 in both sexes, or, in the male, it may be fused with the dorsal plate 

 of the clasping apparatus. Most of the ninth sternal plate of the 

 male usually consists of a modification and elongation (which may 

 be rather complicated) of the overlapping posterior margin of this 

 plate. The same sternal plate is rudimentary or absent in the female. 



The clasping apparatus of a flea, which is commonly regarded as 

 representing a pair of segmental appendages, consists of the follow- 

 ing parts on each side : The basal body, which is united with the body 

 of the other side by the dorsal plate, an inwardly and forwardly 

 projecting arm known as the manubrium (fig. 3), a dorsal process, 

 and a posterior movable element known as the movable finger. Wag- 

 ner regards the basal body, with its movable finger, as representing 

 a gonopod. 



The tenth abdominal segment usually is regarded as being repre- 

 sented by the pygidial plate. According to Wagner this plate is fused 

 with the tergal plate of the twelfth segment in the female, and in the 

 same sex the sternal plate of the tenth segment is absent. The tenth 

 sternal plate usually is absent in the male, but, according to Wagner, 

 may be present in the form of a subpygidial sclerite. 



The eleventh segment, according to Wagner, has no tergal sclero- 

 tizations, but the sternal plate may be the subanal plate in the male, 

 and the "cerci" of the female may be rudiments of the ventral part 

 of the eleventh segment shifted to a dorsal position. 



The twelfth segment, according to Wagner, is the so-called anal 

 segment, consisting of a simple tergal and sternal plate in the female 

 and a simple tergal plate and two cercuslike sternal sclerites or a single 

 plate in the male. 



It appears to the writers, however, that the anal sclerites of the 

 so-called anal segment are more likely comparable to the epiproct and 

 paraprocts of thysanurans and orthopterous insects, in which case they 

 should represent the sternal and tergal plates of segment XI of the 

 abdomen. 



THE ORIGIN AND RELATIONSHIPS OF THE 

 SIPHONAPTERA 



The origin and the relationships of the Siphonaptera have been 

 moot questions ever since the study of taxonomic entomology began, 

 and during the last century many papers were published dealing with 

 these subjects. Unfortunately the theories advanced were based not 

 only upon wholly insufficient and largely faulty interpretations of the 

 morphology of the fleas themselves but also of the morphology of in- 

 sects in general. Those who are interested in a summary of the theories 

 of these earlier workers are referred to a paper by Packard (61) pub- 

 lished in 1894 under the title On the Systematic Position of the 

 Siphonaptera, with Notes on their Structure. The prevailing ten- 

 dency of these early workers was to regard the fleas as specialized 

 Diptera or specialized Hemiptera. Some of them, however, considered 

 the fleas as a distinct order having relationships with either the Dip- 

 tera, the Hemiptera, the Coleoptera, or the Hymenoptera. 



