HOST RELATIONS 351 



entirely as primary parasites of Lecaniine scale insects and mealy 

 bugs. The males, on the other hand, only develop as hyper- 

 parasites of larvae of its own or other species of parasites. It 

 would seem that the stimulus of fertilisation results in a marked 

 change in host-selection by the female Chalcid. Prior to mating 

 these oviposit freely in the body-cavity of a parasitic Hymen- 

 opterous larva, but after mating egg-laying only occurs within a 

 scale insect. This peculiar and apparently unique phenomenon 

 requires further investigation. 



Diptera. Endoj^arasitic Dipterous larvae differ from those of 

 almost all Hymenoptera, which lead a similar mode of life, in two 

 important features : (1) When they issue from the egg they are 

 provided with a metapneustic tracheal system ; and (2) they 

 nearly always establish a respiratory connection with the outside 

 air during some stage of their development. Exceptions to these 

 generalisations are relatively few. In cases where the early 

 parasitic life is passed in the body-cavity of the host or within 

 some specific organ of the body the spiracles, although well 

 developed, appear to be non-functional, or they may sometimes 

 be vestigial or wanting. Also, in a small number of species, 

 chiefly Pipunculidse and Sarcophagidse, the whole larval existence 

 is passed within the bodies of the hosts without acquiring any 

 respiratory connections with the exterior. The. vast proportion of 

 the parasitoid Diptera belong, however, to the family Tachinidse 

 and almost all the species of this extensive group respire free air, 

 either by means of a perforation of the body-wall of the host or 

 by acquiring a secondary connection with its tracheal system. In 

 either event the parasitic larva becomes enclosed in a sheath 

 (" gaine de fixation " of Pantel), which may be of a primary or 

 secondary character. Whichever way it is formed it benefits host 

 and parasite alike ; it maintains the parasite in intimate connection 

 with the necessary oxygen supply, but it is also a protective 

 reaction on the part of the host against irritation and microbic 

 infection induced by the presence of the parasite. 



A primary sheath is always cutaneous in origin, and is formed 

 as an integumentary ingrowth from the margin of the initial 

 perforation made by the larva in entering the host. The perfora- 



