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The (true) Thorax and its Appendages. 



The true thorax of the imago is represented by the third, 

 fourth, and fifth larval segments, which are all fairly equal 

 to one another in size. Running vertically down each seg- 

 ment, on either side, and close behind the spiracle, is a narrow 

 streak of integument differing from that which covers the 

 remainder of the segment. These six narrow streaks con- 

 stitute the imaginal discs of the thorax (fig. 2). 



Tliey are clearly visible through the cuticle of the 

 advanced larva, and connected with each are the imaginal 

 discs of the thoracic appendages (figs. 2, 5, 6); the first pair 

 bear only the first legs; from the second (mesothoracic) seg- 

 ment develop the first wings above, and the second legs below; 

 the third (metathoracic) disc bears the second wing disc above, 

 and the rudiments of the third legs below. 



The wing discs are rather elongated and can be seen to be 

 enveloped in a distinct sac. The leg discs are much shorter 

 than those of the wings; and the sacs in which they are 

 carried are very distinct, each bearing a small opening on to 

 the surface of the integument, below the larval cuticle. 

 Neither in the wing, nor in the legs, could I detect any 

 indication of a biramous structure. 



During the resting stage of the larva the imaginal discs 

 begin to grow rapidly; the integumentary discs spread out in 

 all directions, the first two, especially the second, rapidly; 

 the last very slowly. The discs of the appendages soon grow 

 out of their sacs ; already in the larva at the time it def aecates 

 the legs have protruded so far that they begin to bend upon 

 themselves beneath the larval cuticle, and we see the earliest 

 indication of segmentation. The wing discs, on the other 

 hand, grow downwards as two large sacs, and do not bend 

 (fig. 3). 



About ten hours after defaecation the discs have, to a 

 large extent, assumed their imaginal shape and size; not till 

 about the time of pupation, however, as will be seen later 

 when we examine the histological structure of the developing 

 discs, is the process of encroaching quite complete. The first 

 thoracic disc is now seen to have projected forwards to form 

 a hood over the upper part of the head ; the second disc 

 has far outstripped the other two, and, growing right under 

 the cuticle of the first thoracic segment of the larva soon 

 assumes its imaginal dimensions; from it about three-quarters 

 of the thorax develops; the metathoracic disc scarcely 

 lengthens at all, and persists as a small ring behind the great 

 mesothoracic segment. 



Tlie legs and wings have meantime been extending, and 

 already in the larva a few hours after if has defaecated the 



