355 



becomes slightly folded in places to prevent further growth 

 within the limited space afforded by the larval cuticle. This 

 folding is very easily confused with the segmentation of the 

 leg, a process which is not completed till some twelve hours 

 later. 



The general features of the formation of the legs have 

 already been described in connection with the transformation 

 of the external characters of the insect. Among the most 

 characteristic of these changes are to be mentioned the develop- 

 ment of the bristles, spines and claws, the deepening of the 

 joints, and especially the curious shrinking of the ectoderm, 

 which transforms the thick ungainly appendages of the late 

 larval and early pupal stages into the slender structures so 

 characteristic of the imago. 



This shrinking of the lege is produced by a shortening 

 and closer packing of the ectodermal cells. In the late larval 

 stage these cells are long and slender, and are loosely arranged, 

 but after pupation, the cells shorten considerably, become 

 rather thicker, and much more closely packed together — they 

 change from a loose columnar to a firm cubical epithelium. 



Segmentation of the leg in the larva, though clearly 

 visible, is nevertheless little more than a series of constrictions 

 due to a slight shortening of cells in this region. In the eight- 

 hour pupa the constriction has become more marked, and even 

 the tarsal joints are now very clearly defined (fig. 16). More 

 marked segmentation is produced by a slight invagination of 

 the cells of the constriction rings, a process which is rapidly 

 followed by the secretion of a chitinous cuticle. In the thirty- 

 six hour pupa this process has advanced considerably. 

 Chitinisation continues for several days, till the whole of the 

 ectodermal cells, with the exception of many of the bristle 

 cells, become converted into the hard shell of the legs. 



The formation of bristles is rendered possible by the 

 shrinking of the legs; it begins about eight hours after 

 pupation and is practically complete thirty hours later. The 

 development of bristles and spines has already been dealt 

 with in connection with the development of the general body 

 integument. Many of the bristles of the legs, however, take 

 on a special tactile function. This is particularly clearly seen 

 on the first tarsal segment of the first leg (fig. 19). On the 

 lower side of this segment is a row of about twenty bristles, 

 the lower ones large, the upper shorter. In suitable prepara- 

 tions the lower ones can be clearly seen to be connected with 

 nerve-fibres, branches of a moderately large nerve which passes 

 down the leg. The bristle cell does not chitinise entirely, but 

 a protoplasmic base is left, below which is a small mesodermal 

 cell which attaches itself by a thin "collar" to the chitinous 



