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. 15, ¢.s.). 
These sclerites he 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 (J.p.) 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 ; the 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. Hecdysis begins anteriorly, and the larva not 
only loses its skin but also the cephalopharyngeal sclerites 
which are attached to the stomodeal portion of the ecdysed 
chitinous integument; the chitinous lining of the proctodezal 
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 
sht-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 
