634 MR. G. S. COTXERELL ON THE LIFE-HISTORY 



(text-fig. 2,i). Each set consists of five teeth, the middle one 

 being the most prominent. They are placed on each side of the 

 oral aperture between the two lobes of the oral disc. They pro- 

 bably correspond to the teeth in the House-fly, forming the lateral 

 edges of the gutters or continuations of the common trunks of 

 the united pseud otrachefe described by Graham-Smith. There 

 are three rows on each side of the oral pit in the House-fly, but 

 only one pair on each in Scatophaga. They function to a very 

 small extent for rasping in the House-fly, and in Scatophaga they 

 exist essentially for rasping. If this point is correct, the mouth- 

 parts are identical with those of the sucking Muscids, except for 

 certain modifications of the chitinised structures, particularly of 

 the paraphyses and teeth, in accordance with its predaceous habits. 



The method of sucking is similar to that of the Blow-fly or 

 House-fly. The liquid food is sucked up through the pseudo- 

 tra.chea3 into the oral pit and so into the mouth. 



By the alternate upward and downward movement of the 

 paraphyses the oral disc is worked in a backward and forward 

 direction, and, consequently, a rasping is caused by the teeth on 

 any surface they are in contact with. In this manner the 

 thinly-chitinised parts of other Diptera are easily punctured. 

 The internal tissues are broken down by further rasping, and 

 apparently partially dissolved by the large amount of saliva 

 secreted. The liquid food is then sucked up through the pseudo- 

 trachese. 



The theca serves not only as an external skeleton to the 

 haustellum, but also protects it from whatever damage it might 

 incur from coming in contact with the jagged edges of chitin 

 when inserted in a hole in its victim. 



Breeding Media. 



The breeding media consist entirely of excrement providing a 

 sufficient consistency for the larva to complete its life without it 

 becoming dry. This includes human excrement, poultry, sheep 

 and cattle excrement, and horse excrement. The last-mentioned 

 is very rarely used for oviposition. Sheep and cattle excrement 

 is preferred chiefly on account of its viscosity and the amount 

 obtainable in pastures. Cattle excrement was used in the 

 breeding-jars in the laboratory. 



(At the Royal Naval Cordite Factory at Holton Heath a large 

 mass of sludge accumulated from septic sewage tanks : this sludge 

 was in the open, and was about the consistency of fresh cow-dung; 

 it contained an enormous number of this fly in all stages of 

 development, and formed a very suitable breeding medium. 

 I believed that the marked absence of House-flies and Blow-flies 

 at Holton Heath was in part due to this, but I had no direct 

 evidence, — H. M. L.) 



