POLY EM BRYONY 



353 



owii requirement either hy perforating the integument or by 

 rupturing one of the larger traeheye or an air-sac of its host. It 

 uses its armoured spiraeular extremity to make the necessary 

 perforation in the integument or trachea as the case may be, and 

 the wound thus caused leads to the formation of a sheath similar 

 to the primary sheath already described. Many Tachinids 

 maintain this respiratory adaptation until ready to pupate, while 

 others, such as Centeter cinerea, free them- 

 selves after attaining the third instar, and 

 proceed to devour the viscera of their hosts. 

 In the case of the endoparasitic genus 

 Cryptochwturn, of the family Agromyzid^, 

 Thorpe (1931) has studied the economy of 

 the species C. iceryw in detail, while that of 

 C grandicorne formed the subject of a later 

 communication (Thorpe, 1934). The larvae 

 of these two species occur in the body- 

 cavity of certain Coccidae, the host being 

 Icerya purchasi in the case of the first- 

 mentioned species and Guerinia serratulce in 

 the case of the second species. The larvae 

 are provided with a pair of filamentous 



prolongations of the last abdominal segment Fig. 89. Tachinid larva 

 , . , . . 1 . 1 . . • . attached to a tracheal 



which increase in length in succestive instars 



trunk t by means of a 

 secondary respiratory 

 funnel c. I, Limits of 

 tracheal sheath. s, 

 Spiracle. (From 

 Pantel.) 



in conformity with the needs of the growing 



parasite. These filaments contain blood 



and tracheae and ramify among the organs 



of the host. Experiments of a similar kind 



to those referred to on p. 347 indicate that these caudal filaments 



serve to increase the surface available for respiratory exchange 



between the larva and the blood of the host. When the tracheal 



system becomes in communication with the outside by means of the 



spiracles, the caudal filaments shrivel and are no longer functional. 



Polyembryony and Related Phenomena 



Polyembryony is rare in the whole animal kingdom, and among 

 insects it is almost exclusively confined to certain of the parasitic 



i.A. EXTOMOLOQY. 



