638 



MR. G. S. COTTERELL ON THE LIFE-HISTORY 



Tlie chitinised mouth -parts are slender. Commencing pos- 

 teriorly there are a pair of U-shaped sclerites (text-fig. 6, a) with 

 the arms directed backwards. The anterior end of each is ex- 

 tended (text-fig. 6, b) for articulation with a large median sclerite 

 (text-fig. 6, c). The posterior sclerites, which correspond to the 

 lateral pharyngeal sclerites of the full-grown larva, are united 

 dorso-laterally by a chitinised band (text-fig. 6, g). A pair of 

 sclerites are also found embedded in the first segment (text- 

 fig. 6, d). These probably correspond to the buccal sclerites or 

 hooks of the full-grown larva. They articulate at their bases 

 with the median sclerite and give rigidity to the oral lobes. 



The hooks of the second instar appear behind those of the first 

 a few hours before ecdysis, and become functional as locomotory 

 organs (text-fig. 7). 



Text-figure 7. 



Larva of S. stercoraria. 1st and 2nd instars. 



The anterior and posterior spiracles and the remainder of the 

 mouth-parts of the second instar do not appear until just before 

 the moult. An entirely new set of mouth-parts can be seen in a 

 preparation made an hour or so before the moult. 



Ecdysis takes place on the first or second day after hatching, 

 the instar lasting from one to two days. 



During this instar the larva is very active. In the jars used in 

 the laboratory, occasionally the larvae hatched out into too liquid 

 dung to allow of a sufficient supply of air below the surface. 

 They were observed to be hanging on to the surface-film with 

 their posterior spiracles exposed, disappearing on being disturbed, 

 reappearing later in the same place. On one occasion the dung 

 remained sufficiently moist to necessitate this during the first two 

 instars. It was only when the third instar was reached that a 

 sufficient air-supply was obtainable below the surface. Probably 

 under natural conditions, where the dung is exposed to wind and 



