STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 509 
anteriorly and posteriorly, the latter moiety being the larger ; 
the discarded larval tracheal system les compressed against 
the interior of the pupal case (J.ér.). Communication with 
the external air is formed for the nymphal! developing 
tracheal system by means of a pair of temporary pupal 
spiracles, which appear as minute spine-like lateral projec- 
tions between the fifth and sixth segments of the pupal case 
(n.sp.). Hach of these communicates with a knob-like spira- 
cular process (fig. 10, .sp.) attached to the future pro- 
thoracic spiracle of the fly. The proctodeal and stomodeal 
portions of the alimentary tract are also withdrawn, and with 
the latter the cephalo-pharyngeal skeleton, which hes on its 
side on the ventral side of the anterior end of the pupal case. 
The histogenesis of the nymph is extremely rapid, so that 
at the end of about thirty hours, in the rapidly developing 
specimens, it has reached the stage of development shown in 
fig. 10, in which most of the parts of the future fly can be 
distinguished although they are ensheathed in a protecting 
nymphal membrane. The head, which with the thorax has 
been formed by the eversion of the cephalic and thoracic 
imaginal discs from their sacs, is relatively large: two small 
tubercles (an.) mark the bases of the antenne. The pro- 
boscis is enclosed in a large flat sheath which at this period 
appears to be distinctly divided into labral (/br.) and labial 
(/b.) portions. Ina short time the parts of the proboscis are 
seen to develop in these sheaths (fig. 11). The femoral and 
tibial segments of the legs are closely adpressed and lie 
within a single sheath. ‘The wings (w) appear as sac-like 
appendages, and, as the nymphal sheath of the wing does not 
grow beyond a certain size, the wing develops in a slightly 
convoluted fashion by means of a fold which appears in the 
costal margin a short distance from the apex of the wing. 
With a constant temperature of about 35°C., or even less, 
the exclusion of the imago may take place between the third 
? 
1 The word “nymph” is used here to designate that stage in the develop- 
ment which begins with the appearance of the form of the future fly, and 
ends when the exclusion of the imago takes place. 
