254 ZOOLOGY FOR MEDICAL STUDENTS chap. 



In the genus Oestrus the egg is retained within the body of the mother 

 until it has developed into a young larva. This is deposited in the 

 neighbourhood of the nostrils and finds its way into the frontal sinuses — 

 irregular cavities in the skull communicating with the nose — where it 

 passes its larval existence. 



In all these Oestridae the larva makes its way out of the body before 

 it assumes the pupal condition. 



The last subdivision of the Diptera calling for special mention is that 

 of the HiPPOBOSCiDAE, the adults of which are blood-sucking. They 

 show a tendency to pass their time creeping about among the feathers 

 or hair of their host instead of alighting on it merely for short periods 

 when feeding. Correlated with this we find in different members of the 

 group less or more marked reduction of the wings, culminating in such 

 forms as the Sheep-ked (Melophagus), in which they have entirely dis- 

 appeared, the creature spending its whole life among the wool of the 

 host. Such cases are of interest as illustrating how members of a group 

 of animals characterized by great activity and freedom of movement 

 may become transformed in the course of evolution into highly specialized 

 parasites whose life is confined entirely to the body of their host. 



Having dealt with the eight main orders of insects we now come to 

 three less conspicuous groups, the members of which are entirely parasitic 

 in habit, and in correlation with this are wingless and in other ways 

 modified in structure. 



The Fleas (Aphaniptera or Siphonaptera) are especially characterized 

 by the shape of the body, being greatly compressed from side to side, 

 instead of being depressed dorsiventrally as is the case with other 

 flattened insects. The mandibles are long piercing styles, and between 

 them is an unpaired piercer which may be hypopharynx or labrum. 



There are many species of flea, each having its favourite host but 

 being often quite ready to bite animals of other species. In the case of 

 the ordinary human flea (Pulex initans) the small whitish worm-like 

 larva lives in dust, especially under carpets. Where large numbers of 

 eggs have been deposited and where the resulting fleas have had no 

 opportunity of being carried away by human beings, as in deserted huts 

 or camping grounds, they may accumulate in myriads. . Persons camp- 

 ing in such spots may develop a high temperature from the bites of 

 hundreds of fleas. After a time, as in the case of other biting insects, 

 a considerable degree of immunity is developed to the irritation caused 

 by the bite of the flea. For keeping away fleas the most effective safe- 

 guard is the frequent washing of floors as the insects in question cannot 



