HOST RELATIONS 347 



This fact, along with the absence of such organs in all 

 ectoparasitic forms, suggest that they exercise a respiratory 

 function. A caudal appendage has been acquired in certain 

 genera among all the great groups of parasitic Hymenoptera. 

 Among Ichneumons it is prevalent in Exochilum (Anomalon), 

 Limnerium, Mesochorus, Campoplex, Mesoleius, Thersilochus and 

 others. In Braconidic (Fig. 83) it is found in Dmocampus and 

 Adelura ; it is exceptionally well developed in Meieorus, but 

 greatly reduced in Praon and in Aphidius. A similar organ is 

 present in several Chalcid genera {vide Parker, 1921) ; it is always 

 prevalent in the primary larvae of the Figitidse, and in some 

 Proctotrypoidea. There appears to be nothing in the structure of 

 this organ to contradict the view that it is respiratory in function, 

 since it is a thin-walled tube lined by a delicate hypodermal layer 

 and filled with blood, and in some cases it contains trachse also. 

 It appears to be a kind of blood gill which serves to increase the 

 general respiratory surface of the body in order to fulfil the 

 breathing requirements of the young growing larva. This 

 exj^lanation, which has been accepted in some quarters, is con- 

 troverted by the researches of Thorpe (1932). By the use of 

 biological indicators it was shown where, over the body-surface of 

 endoparasitic larvae, oxygen absorption was going on while carbon 

 dioxide output was investigated mainly by means ofpH indicators. 

 Parasitic larvae were dissected out from their hosts, washed free 

 from the tissues of the latter and examined under a microscope 

 in salt or Ringer's solution. Use was made of certain Protozoa, of 

 which Polytoma was found most suitable, and advantage taken of 

 the fact that such organisms migrate to a region where the oxygen 

 tension is lower than that in water saturated at atmospheric 

 pressure. When a culture of these organisms is run under the 

 coverslip these creatures at once arrange themselves in a zone 

 around the parasitic larva where oxygen is being absorbed from 

 the fluid. As the oxygen tension falls below the optimum, owing 

 to respiratory activity of the parasite, the Protozoa retreat 

 further away from the region of the parasite concerned. By using 

 luminous bacteria (B. phosphor escens), which were introduced into 

 the solution in the same way, examination was made in complete 



