SOME HUMAN PARASITES 277 



eaten, but, being provided with mouth parts adapted for 

 biting, the insect lives on any decaying organic refuse it 

 can find ; in fact, larvae have been raised on ordinary room 

 sweepings. When fully developed, a matter of a few days 

 at most, the larva spins a small cocoon within which it 

 pupates. The duration of the pupal stage depends largely 

 on the temperature, but it is never prolonged over more than 

 a few days ; then the adult appears and at once proceeds to 

 seek a meal of blood, though fleas, like all parasitic insects, 

 can exist for an extraordinarily long time without feeding. 



The cat flea has so many points in common with the 

 human flea, that it is unnecessary to describe its life-history 

 and habits in detail ; one or two facts are, however, worthy 

 of mention. Like the human flea, this insect is practically 

 cosmopolitan, but whenever it is encountered it may be 

 distinguished from its relative by the fact that it is pro- 

 vided with six to nine spines on its head and fourteen to 

 eighteen on the hind border of its pronotum. The adults 

 adhere much more closely to their hosts than do human fleas, 

 and another striking and important difference between the 

 two species is, that whereas the human flea never oviposits 

 on its host, the cat flea always does so, and its eggs adhere 

 lightly to its host's fur, but they are easily shaken off and 

 so distributed. The very small, oval, white eggs give rise 

 to white, footless maggots in about a fortnight. Like the 

 larvae of the human flea, they live on decaying animal and 

 vegetable matter, and in rather less than a fortnight they 

 attain their full growth, spin a small cocoon and pupate. 

 The pupae, although inactive, closely resemble the mature 

 fleas, and are peculiar in that their legs are free. In ten 

 days or so the adult fleas issue from the cocoons. There 

 may be several broods each season, and the winter is 

 passed either in the mature or larval stage. 



A peculiar feeding habit of fleas is worthy of notice, 

 because of its probable bearing on the transmission of 

 disease, as described in our notes on plague. The flea, like 



