INSECTA 



513 



and the labrum-epipharynx. The thoracic segments are free and there 

 are never any signs of wings. Though the eggs are laid on the host 

 they soon fall off and are subsequently found in little-disturbed parts 

 of the haunts of the host. Thus in houses they come to lie in dusty 

 carpets and unswept corners of rooms. In a few days the larvae hatch 

 and feed on organic debris. The legless and eyeless larva possesses 

 a well-developed head and a body of thirteen segments. At the end 





Fig. 351. The life history of the flea, Ctenocephalus cams. From Imms, after 

 Howard, a, egg; b, larva in cocoon; c, pupa; d, imago; e, larva of flea, 

 Ceratophyllus fascia tus ; f, antenna of imago. 



of the third larval instar a cocoon is spun and the creature turns to 

 an exarate pupa from which the adult emerges, the whole life cycle 

 occupying about a month in the case of Pulex irritans. 



Pulex irritans is the common flea of European dwellings, but by 

 far the most important economically is the oriental rat flea, Xeno- 

 psylla cheopis, which transmits Bacillus pestis, the bacillus of plague 

 from the rat to man. It appears that this bacillus lies in the gut of 



