334 PARASITISM 



The planidium undergoes no further development until it meets 

 within this host one of its parasites, i.e., the Ichneumon Limnerium 

 validum or the Tachinid Varichceta aldrichi. It then enters the 

 larva of one or the other, but remains there in a quiescent state 

 until either host pupates. Once the latter phase is assumed, the 

 planidium gnaws its way out and becomes an ectoparasite and 

 subsequently transforms into an inert, maggot-like larva. 



In the related family of the Eucharidse, whose members 

 parasitise certain ants, the first stage larva is likewise a planidium. 

 It was in the species Orasema viridis, which parasitises the Texan 

 ant Pheidole instabilis, that Wheeler first discovered this peculiar 

 larval type in 1905. The most complete observations are those 

 of Clausen (1923) on Schizaspidia tenuicornis, whose host is the 

 Japanese carpenter ant Camponotus japonicus. It appears that 

 the Schizaspidia lays large numbers of minute eggs within the 

 buds of certain trees, and the planidia, upon emergence, await 

 opportunity for attaching themselves to the Camponotus workers 

 which roam about the trees attending aphides. On meeting an 

 ant the planidium attaches itself to the hairs of the tarsi and thus 

 gains entry into the nest. It then becomes an ectoparasite of 

 an ant larva, upon whose back it settles down and perforates the 

 cuticle behind the head, on the thorax. Two instars follow, during 

 which the planidium becomes a vermiform grub of the usual 

 Hymenopterous type and, after the host has pupated, the parasite 

 regains its hold following the ecdysis. It reattaches itself on the 

 metathorax beneath the wings or legs and finally pupates within 

 the host's cocoon. When the parasite becomes an imago it leaves 

 the nest and flies away. The effect of the extraction of the body- 

 juices of the host by the parasite results in its inability to become 

 an adult, although death may not supervene until some time 

 afterwards. 



{h) In the Dipterous family Tachinidae there are certain 

 species whose primary larvae emerge from the egg as active 

 migratory organisms somewhat resembling planidia. In 

 Digonochceta setipennis the eggs are laid in the immediate vicinity 

 of resting earwigs, which serve as the hosts. They hatch almost 

 immediately into deeply pigmented larvae, 0-6 mm. long, armed 



