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STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 507 



like apertures situated on a small prominence (p.sp.).' There 

 are no anterior spiracular processes in the first larval stage. 

 The oval lobes are relatively large and on the internal ventral 

 surface of each there is a small T-shaped sclerite (fig, 13, t.s.). 

 These sclerites lie lateral to the falciform mandibular sclerite 

 (m.s.). The cephalopharyngeal skeleton of the first larval 

 instar is slender and, in addition to the sclerites already men- 

 tioned, consists of a pair of lateral pharyngeal sclerites or 

 plates (Lp>) deeply incised posteriorly, forming well pro- 

 nounced dorsal and ventral processes. The lateral plates are 

 connected antero-dorsally by a curved dorsal sclerite (d.p.s.). 

 The anterior edges of the lateral plates are produced ventrally 

 into a pair of slender processes (h.s.), the anterior portions of 

 these processes, which represent the hypostomal sclerite, are 

 involute and articulate with the mandibular sclerite. The 

 alimentary canal of the first larval instar is relatively shorter 

 than that of the adult, and consequently it is not so convo- 

 luted ; th ; salivary glands are proportionately large. 



The first larval instar may undergo ecdysis as early as 

 twenty hours after hatching, but it is usually from twenty- 

 four to thirty-six hours that the ecdysis takes place : under 

 unfavourable conditions with regard to the factors governing 

 the development, the first larval instar sometimes lasted three 

 or four days. Ecdysis begins anteriorly, and the larva not 

 only loses its skin but also the cephalopharyngeal sclerites 

 which are attached to the stomodaeal portion of the ecdysed 

 chitinous integument ; the chitinous lining of the proctodasal 

 portion of the alimentary tract is also shed. 



The second larval stage or second instar. This 

 stage is provided with a pair of anterior fan-shaped spira- 

 cular processes similar to those of the mature larva. The 

 posterior spiracular orifices are shown in fig. 12. They are 

 slit-like apertures rather similar to those of the first instar 

 but larger in size. The cephalopharyngeal skeleton is thick- 

 ened and less slender in form than that of the first instar. It 

 resembles the cephalopharyngeal skeleton of the mature larva 

 except that the posterior sinuses of the lateral pharyngeal 



