276 INSECTS AND MAN 



the special shape of the fleas enables them to pass more 

 easily between the hairs of their host, just as the other 

 form of flattening enables the cockroaches to squeeze into 

 narrow crevices. Many fleas are eyeless, and this is notably 

 the case in the bat fleas ; whether provided with eyes or not, 

 these insects appear to rely mainly on their antennae for 

 finding their way about, and that these useful organs may 

 not hinder the insects' progress over the body of a hirsute 

 host or be damaged in any way, they fit into grooves at 

 the side of the head and are only protruded when actually 

 in use. Many fleas are provided with peculiar horny 

 appendages known as combs, which may occur on the 

 head, thorax, or abdomen. These combs are not peculiar 

 to the fleas, but are found on other insects of parasitic 

 habit, and they probably serve as aids to locomotion by 

 holding on to the host's hairs. The enormous development 

 of the flea's hind legs, which enables the insect to leap 

 prodigious distances, is of course a striking feature in its 

 anatomy. Although the adult flea is all too common and 

 well known, it is very rarely that the housewife has any 

 idea of the appearance of the eggs, larvae, and pupae, for 

 the insect undergoes complete metamorphosis. All the 

 year round, unless subjected to severe cold, the female flea 

 deposits her eggs to the number of one to five at a time. 

 In a few days the white, worm-like, legless larvae emerge 

 from the eggs, and they do so in a very curious and 

 interesting manner. On the head of each newly hatched 

 larva a thin knife-edged plate may be seen, it is an egg- 

 breaker. When ready to emerge, the larva works this 

 sharp-edged plate against the interior of the shell till a 

 crack is made in it ; more vigorous action causes the crack 

 to open sufficiently to allow the larva to escape. Like the 

 larvae of other insects, this one moults several times as it 

 grows, and with the first moult the egg-breaker, or hatch- 

 ing-plate as it is sometimes called, is lost the insect has 

 no further use for it. At this stage, of course, no blood is 



