442 THE INVERTEBRATA 



reminiscent of the Diptera. The labial palps, held together, serve to 

 support the other parts, a function which is performed by the 

 labium in the Diptera. In piercing, the mandibles are most important 

 and the blood is drawn up a channel formed by the two mandibles 

 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 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, 

 Xenopsylla cheopis, which transmits Bacillus pestis, the bacillus of 

 plague. It appears that this bacillus lies in the gut of the flea and 

 the faeces deposited on the skin of the host are rubbed into the wound 

 by the scratching which follows the irritation from the bite. 



Ceratophyllus fasciatus, the European rat flea, also transmits the 

 plague organism as also can Pulex irritans, but since the latter does 

 not live successfully on rats, it is never likely to prove a source of 

 trouble. 



Order STREPSIPTERA 



Small parasitic insects, allied to the Coleoptera, with winged, free- 

 living males and larviform females, which never leave the interior of 

 their host. 



Sty lops causes great modification of its host, the bee, Andrena, 



