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or less regular rows, with strong bars of chitin projecting in between them. This type 

 of posterior stigmata would, at first sight, appear to be extremely complicated, but I 

 am inclined to think that they merely represent a breaking up of the original long slit 

 into smaller ones to allow of a larger amount of air being taken in. The origin of 

 this type can, however, only be determined by a comparative study of the first-stage 

 larvae of a number of different species. 



Cephalopharynx. This structure in the third-stage larva of the higher Diptera 

 consists of a number of paired sclerites, some of which have fused, articulating with 

 each other to form a strong chitinous skeleton. At the anterior end there is a pair of 



Fig. 1 . — Cephalopharyngeal skeletons of third-stage larvae 



of : (a) Musca nebulo ; (b) Calliphora erythrocephala ; 



(c) Lucilia sericata ; [d) Sarcophaga sp. 



mandibular sclerites or hooks, each with a broad base, which project between the oral 

 lobes and are used by the larva to attach itself to any object, and also in progression. 

 Articulating with the ventral surface of the base of each hook there is a small, 

 irregular sclerite, which Hewitt speaks of as the dental sclerite. Posteriorly the base 

 of each mandible articulates with an irregularly shaped sclerite, which consists of two 

 parts fused ventrally. These, again, articulate with small processes on the anterior 

 end of the large pharyngeal sclerite, which at once suggests the fulcrum of the adult 

 fly, and is shaped like the old-fashioned Spanish stirrup-iron. 



