344 



PARASITISM 



derive their nutriment. Respiration, under these circumstances, 

 has much in common with that of aquatic insects, and the necessary 

 oxygen is obtained either from the surrounding medium or by 

 estabhshing a direct or indirect connection with the outer air. 

 With many parasites the demands of growth cannot be fulfilled in 

 this manner throughout life and, as the body-fluids of the host 

 tend to become exhausted, the parasites turn to the more solid 

 tissues as their source of food and become, as it were, internal 

 predators. Their feeding activities entail 

 the laceration of many of the smaller 

 tracheal vessels, which results in the 

 liberation of air in the body of the host. 

 Direct respiration by the parasites now 

 becomes possible, and correlated changes 

 in the respiratory system result. Hymen- 

 opterous parasites develop at this stage 

 a peripneustic tracheal system, and 

 Dipterous parasites may no longer need 

 the special adaptations previously 

 acquired as a means of breathing the 

 outside air. Finally, when larval de- 

 velopment is completed, the parasites 

 either pupate within their host or gnaw 

 their way through its integument and 

 transform outside its body. This, in a 

 few words, is the course of life followed 

 by most entomophagous parasites, but 

 the behaviour of different groups exhibits 

 so great a diversity of phases that few generalisations are possible. 

 It will be convenient, therefore, to discuss Hymenopterous and 

 Dipterous parasites separately. 



Hymenoptera. In its first instar a Hymenopterous parasite 

 is characterised by the absence of spiracles, the tracheal system is 

 rudimentary or wanting, and the body integument is a delicate, 

 flexible membrane. At this stage its diet consists solely of the 

 blood or serous fluids of its hosts ; the mouth-parts are represented 

 by a pair of sharp, piercing mandibles, while the remaining trophi 



Fig. 82. Metapneustic first 

 instar larva L of Blasto- 

 thrix attached to chorion 

 of egg whose pedicel P 

 protrudes through the 

 integument I of the host. 

 S, Spiracles. 



