THE EMBRYOLOGY OF FLEAS 



By EDWARD L. KESSEL 



University of San Francisco 



(With 12 Plates) 



INTRODUCTION 



Aside from its general scientific aspects, the study of flea embry- 

 ology is of interest from two particular standpoints. In the first place, 

 such an investigation has been needed to extend our knowledge of the 

 pulicine life cycle. Hitherto the embryological phase of this cycle has 

 been neglected in spite of the fact that its other aspects have received 

 considerable attention. A careful search of the literature reveals only 

 five contributions to the subject of flea embryology, and they are all 

 brief and fragmentary. In some cases the claims made therein have 

 been found to be erroneous in the light of the present study. These 

 discrepancies are doubtless due to the difficulties involved in the tech- 

 nique of preparing the eggs for successful observation. 



Weismann (1863) was the first to undertake an investigation of the 

 development of the flea in the egg stage, but he dismisses the subject 

 with a single page of his lengthy paper which is otherwise concerned 

 with the embryology of Chironomus and Musca. He chose the dog 

 flea, Ctenocephalides canis (Curtis), as the species for his considera- 

 tion and reports that in the egg of this form "although the chorion 

 is not exactly opaque it does not allow observation of the finer details, 

 which, to be ascertained accurately, require a well related series of 

 observations. Yet it mocks at every attempt to remove it without 

 injury to the yolk membrane." Weismann concludes, therefore, that 

 the egg of the flea is not favorable for embryological study. 



The second worker to publish on the subject of flea embryology was 

 Packard (1872). He used the eggs of the cat flea, Ctenocephalides 

 felis (Bouche), although, believing like other workers of his day that 

 cat and dog fleas constituted a single species, he titled his discussion 

 "The Development of Pulex canis." Like Weismann, he was forced to 

 restrict his studies to superficial observations inasmuch as the section 

 method of investigation was not well developed. 



Balbiani (1875) was the third to attempt to work out the early 

 ontogenetic development of fleas. He followed Packard in the use 



Smithsonian Miscellaneous Collections, Vol. 98, No. 3 



