354 



In the neck there is also a phragma, being a hoop -like forward 

 and backward extension of the prothoracic shield, all round 

 the neck. 



But the most remarkable of all are the great cephalic 

 phragmas, which give attachment to the muscles of the 

 antennae and mouth appendages (fig. 43). Early in pupal 

 life a long tube-like invagination of the integument takes 

 place on either side of the face, half-way between the mouth 

 and the antennae. These invaginations grow upwards and 

 inwards and terminate at the rear of the lower part of the 

 brain. Meanwhile a second pair of tube-like invaginations 

 has been formed at the rear of the head, a little below the 

 neck ; growing inwards they meet the anterior pair of invag- 

 ( inations. Since both pairs have a narrow lumen, we have the 

 curious fact that at this stage a pair of long narrow canals 

 I run right through the head from front to rear, well above and 

 L either side of the mouth ! The secretion of chitin, however, 

 soon takes place and the canals are obliterated, being 

 gradually replaced by an exceedingly powerful rod of chitin. 

 The integument on the three thoracic segments undergoes, 

 during larval and pupal life, a number of remarkable changes 

 which terminate in the formation of the wings and legs. The 

 histogenesis of these structures will be considered first ; that of 

 the other appendages of the insect, wliioh usually resemble it 

 closely, can be considered more briefly. 



The Legs. — The imaginal discs of the legs arc clearly 

 visible in the earliest larvae. Here they occur, a pair in each 

 of the three segments, as rather extensive but sharply defined 

 areas rather thicker than the remainder of the integument, 

 and lying on each side of the nerve cord, on the ventral side 

 of the animal. The cells composing the discs are long and 

 rather narrow (fig. 10). At the end of larval life the cells 

 have increased greatly in number, and, as a result of the 

 growth of the surrounding integumental cells the imaginal 

 discs have become invaginated below the surface (figs. 2, 5, 6). 

 On the base of this invagination the cells lengthen and divide 

 to form a large prominent papilla. In the resting larva this 

 papilla begins to increase in size, and soon grows out of the 

 invagination as a hollow appendage, dragging the mesoderm 

 after it. 



Cell division by mitosis is very rapid, and the developing 

 leg grows downwards, so that in the larva, before hatching, 

 very well defined legs are present, hidden beneath the larval 

 cuticle (fig. 3). The integument of the legs in common with 

 that of the rest of the body develops a cuticle, the completion 

 of which is followed by the moulting of the larva. So rapid 

 has been the growth of the appendage, that its integument 



