4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



cidae, Nosopsyllus fasciatus the Dolichopsyllidae, and Hysirichopsylla 

 dippiei the Hystrichopsyllidae. Moreover, these three families to- 

 gether contain over 80 percent of the total number of genera belonging 

 to the order. The three subject species used in this study must be 

 regarded, therefore, as a valid representation for the Siphonaptera 

 as a whole. 



EGG SOURCES 



Ctenocephalides fclis proved to be the most prolific of the three 

 species studied. The eggs of this flea were collected from a cat's 

 sleeping blanket by means of a camel's hair brush. 



Nosopsyllus fasciatus eggs were obtained from a pure culture of 

 this species reared on white mice by the methods described by Leeson 

 (1932). Gravid females were confined in voile-capped vials until they 

 had oviposited. It was found that these imprisoned insects lived 

 longer and deposited more eggs if a piece of cotton was placed in the 

 vial with them. 



The eggs of Hysirichopsylla dippiei were obtained either from the 

 nests of wood rats or from gravid females taken from such nests. 

 Because the eggs of this species are so much larger than those of 

 the several other forms which inhabit Neotonui nests, there was no 

 danger of mistaken identity. 



AGE DETERMINATION 



In order to determine the developmental age of embryos in terms 

 of days, cat flea eggs, newly laid by females imprisoned in vials, were 

 incubated under a constant temperature of 25 ° C. and a relative 

 humidity of 79 percent. Relatively few eggs were incubated in this 

 manner ; these were used as standards and the others studied were 

 interpolated into the series in their proper places. As no attempt was 

 made to obtain Nosopsyllus fasciatus and Hystrichopsylla dippiei eggs 

 of accurately determined incubation age, the developmental stages of 

 these species were compared to known ones of Ctenocephalides felis. 



MICROTECHNIQUE 



In the preparation of whole mounts of Ctenocephalides felis em- 

 bryos, dechorionation was facilitated by heating the eggs at 90 C. 

 for a few minutes. The heat served to coagulate the yolk so that it 

 drew away from the chorion, thereby providing a working space 

 between the shell and the vitelline membrane. The resulting solidifica- 

 tion of the vitellus was a further advantage because a volk so hardened 



