24 THE FLEA [CH. 



enables the animal to glide with greater facility 

 through the hairs of its host. Like other insects, a 

 flea is readily divided into a head, a thorax, and an 

 abdomen. The head is rounded on the top and front 

 and shows no obvious trace of segmentation ; but 

 what is known of the development of other insects 

 leads one to think that it must properly be regarded 

 as a number of segments closely fused together. On 

 the under side of the front part of the head is a 

 beak or proboscis for piercing and sucking, composed 

 of the mouth-parts, whose structure is worthy of 

 minute study. It will be best to examine them in 

 detail in a subsequent chapter. 



Some fleas have eyes, others have none. The 

 common mouse-flea (Leptopsylla musculi) is blind. 

 The bat-fleas are also destitute of eyes. The noc- 

 turnal habits of their hosts would render eyes of 

 little or no use. If eyes are present they are large 

 and placed on either side of the head. Each is a 

 simple eye or ocellus ; the compound eyes, divided 

 into a great number of hexagonal facets, which are 

 characteristic of many insects, are never found in 

 fleas. 



Nothing is known about the flea's powers of 

 vision, but there is no reason to suppose that they 

 are at all acute. The eyes are marked with pigment. 

 Ocelli appear to be primitive types of insect eye 

 which are, perhaps, an inheritance from a wormlike 



