(69^ 



STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 509 



anteriorly and posteriorly, the latter moiety being the larger ; 

 the discarded larval tracheal system lies compressed against 

 the interior of the pupal case (l.tr.). 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.). Each of these communicates with a knob-like spira- 

 cular process (fig. 10, n.sp.) attached to the future pro- 

 thoracic spiracle of the fly. The proctodseal and stomodaDal 

 portions of the alimentary tract are also withdrawn, and with 

 the latter the cephalo-pharyngeal skeleton, which lies 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 antennae. The pro- 

 boscis is enclosed in a large flat sheath which at this period 

 appears to be distinctly divided into labral (Ibr.) and labial 

 (Ib.) portions. In a 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. 



