H08T RELATIONS 



345 



are reduced to the condition of fleshy lobes forming the boundary 

 of the suctorial mouth. Salivary glands and Malpighian tubes are 

 usually well developed, and the mid- intestine is a capacious sac, 

 but without direct continuity with the proctodaeum. Respiration 

 is cutaneous, and the necessary oxygen is derived from the blood 

 of the host. It is true that little is definitely known with respect 

 to the oxygen-carrying 

 capacity of insect blood, but 

 all the evidence strongly 

 indicates that it is from the 

 latter source that endopara- 

 sites first obtain the oxygen 

 they may require. Species of 

 certain Chalcid genera, in- 

 cluding Blastothrix, Encyrtus, 

 Microterys and Phcenodiscus 

 (Fig. 83), which parasitise 

 Coccidse, are exceptional in 

 being metapneustic in the first 

 instar (Imms, 1918 ; Silvestri, 

 1919 ; Maple, 1937). In these 

 genera the egg is provided with 

 an elongated pedicel, whose 

 apex protrudes through the 

 integument of the host to the 

 exterior. The anal extremity 

 of the larva, in such cases, is 

 closely grasped by the per- 

 sistent chorion of the egg in 

 such a manner that the 

 spiracles are able to inspire the outside air through the pedicel, 

 which serves as a respiratory funnel (Fig. 82). The chorion in 

 this region is specially modified and is freely permeable to air. 

 During their later life these parasites free themselves from all 

 connection with the egg pedicel, and Acquire a peripneustic 

 respiratory system. Special organs, either in the form of a 

 caudal appendage or of an anal vesicle, are present in diverse 



Fig. 83. Two metapneustic larvae of 

 Phcenodiscus emeus attached by their 

 egg pedicels to integument of host 

 {Sphcerolecanium). (After Silvestri.) 



