318 PARASITISM 



like sting is used for piercing the cell and depositing an egg within. 

 The first instar Sapyga larva devours the egg of the host and 

 subsequently feeds solely upon the vegetarian stores within the 

 cell. 



The foregoing remarks emphasise the diversity and intergrading 

 exhibited by parasitism among insects. Growth of recent 

 knowledge of the subject is too extensive to allow of adequate 

 treatment of all its phases within a restricted compass. For this 

 reason discussion is mainly limited to the parasitoid Hymenoptera 

 and Diptera (Tachinidae). 



Host-selection 



Our knowledge of the fundamental aspects of parasite specificity 

 admits of but few generalisations. Notwithstanding the mass of 

 information which exists relative to the hosts of entomophagous 

 parasites, we are completely in the dark as to the determining 

 causes which lead certain parasites constantly to select certain 

 host-species, while others utilise hosts of different taxonomic 

 relationships. The vast majority of parasites are but seldom 

 restricted to a single host-species, not many are confined to an 

 individual genus, and those so accredited are becoming fewer in 

 number with increasing knowledge. 



Among Hymenopterous parasites examples of more or less 

 restricted host specificity are numerous. Thus, the Proctotrypid 

 Embidobia is only known to develop in the eggs of Embioptera ; 

 the Proctotrypid Polygnotus selects Cecidomyid larvae as its 

 hosts ; the whole family Scelionidse are egg-parasites, while 

 the species of Scelio appear to attack only locust eggs ; the 

 Braconid Dachnusa parasitises Dipterous larvae of the family 

 Agomyzidae ; the Cynipid Ibalia and the Ichneumonid genera 

 Rhyssa and Thalessa are parasites of Siricidae ; Aphidius and its 

 allied Braconids attack Aphididae, while another Braconid genus, 

 e.g., Perilitus, confines itself almost exclusively to adult Coleoptera ; 

 and so on. 



Among Tachinid parasites specificity of host-selection is, as a 

 rule, far less restricted than in Hymenoptera, but the exceptions 

 present certain puzzling problems. Thus the species of the 



