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 



ourhood of the nostrils and finds its way into the frontal sinuses 



IT cavities in the skull communicating with the nose where it 



its larval existence. 



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

 -limes the pupal condition. 



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

 til" tlif HIPPOBOSCIDAE, the adults of which are blood-sucking. They 

 >lu>\v 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 

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

 <>l 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, 



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. 



re 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 

 imary human flea (Pulex irritans) the small whitish worm-like 

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

 deposited and where the resulting fleas have had no 

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

 :i<K they may accumulate in myriads. Persons camp- 

 's may develop a high temperature from the bites of 

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



of immunity is developed to the irritation caused 

 bite Dt tlu- tlea. For keeping away fleas the most effective safe- 

 lre<|uent washing of floors as the insects in question cannot 



