NO. 3 EMBRYOLOGY OF FLEAS KESSEL 59 



cuticula makes its appearance during the sixth or final day of develop- 

 ment. This forms a sclerotized covering, the exoskeleton of the larva. 

 It is secreted by the cells of the epidermis, and, as it is impermeable, 

 its presence becomes manifest in attempts to stain late embryos for 

 whole mounts. Unhatched larvae, after being kept in alcoholic borax 

 carmine for several days, remained absolutely unstained except for a 

 light coloration of the ectodermal regions of the digestive tract. This 

 coloration was apparently due to seepage of the stain through the 

 mouth and anal openings. 



The setae also appear during the final day of embryonic develop- 

 ment. As in other insects, they are produced as elongated hardened 

 processes of certain of the epidermal cells. 



The hatching spine of the flea embryo likewise makes its appearance 

 during the final day of development. It, too, is a secondary epidermal 

 structure and occurs as a sharp ridge located on the middorsal line of 

 the head (pi. 3, fig. 31). 



HATCHING 



Unlike certain other insects such as Leptinotarsa (Wheeler, 1889), 

 Lina (Graber, 1877), the Neuroptera (Smith, 1922), and Tenebrio 

 (Sikes and Wigglesworth, 1931), the flea larva is not invested in a 

 cuticular envelope at the time of hatching. 



The normal hatching process of fleas has been described by Sikes 

 and Wigglesworth (1931) and agrees in most points with the observa- 

 tions made during this study. Late in the last or sixth day of its life 

 within the egg shell, the now fully differentiated larva expands to 

 fill completely the lumen of the egg. This increase in size is due to 

 a distention of the larva brought about by its swallowing the amniotic 

 fluid. Since the withdrawal of the serosa and amnion into the vitellus 

 prior to the dorsal closure, the amniotic fluid has filled the space 

 between the vitelline membrane and the embryo. The ingestion of this 

 fluid may be observed clearly through the relatively transparent 

 chorion of Ctcnocephalides felis. When the larva swallows the amni- 

 otic fluid its spiracles become exposed to the air, and following this 

 exposure the liquid contents of the tracheal system is absorbed and 

 air takes its place. After a time the larva begins to move about within 

 the egg, and it is at this point that the hatching spine which has been 

 described above comes into use. The spine acts as a can-opener and 

 in due time normally pierces both the vitelline and chorionic mem- 

 branes. As it moves about inside the shell, the larva ordinarily travels 

 in a longitudinal direction. Almost always, therefore, the slit made 



