THE LIFE-HISTORY OF THE BLOW- FLY. 3 



When the larva ceases to feed its crop is greatly distended, 

 but during the resting period it is gradually emptied with the 

 rest of the alimentary canal ; the head is withdrawn within the 

 second annulus, and the whole worm is contracted, and assumes 

 an ovoid form. If disturbed, the head is protruded, and the 

 larva crawls about rapidly, seeking to bury itself again ; after a 

 variable period, however, the contraction of the body becomes 

 permanent, all power of movement is lost owing to the inner 

 layer of the integument and the muscles of the larva having 

 detached themselves from the external cuticle : this is the com- 

 mencement of the pupa state. 



It has long been known that a number of curious cellular 

 bodies exist connected with the nerves and nerve centres of the 

 larva. These were formerly regarded as ganglia, but Dr. 

 Weismann [2] discovered their true nature. They are the 

 rudiments of the fly ; he named them imaginal discs. It has 

 also been suggested that the largest of these imaginal discs 

 is the invaginated portion of the head of the embryo. Quite 

 recently I was fortunate enough to make longitudinal sections 

 of a newly-hatched larva, which not only demonstrate the fact, 

 but also show the exact nature of the imagination (see 

 Chap. IV., Sec. 3, Fig. 7). 



During the resting stage two processes are going on simul- 

 taneously, the various larval organs are undergoing rapid dis- 

 integration, and the imaginal discs are unfolding and increas- 

 ing in size and complexity. 



The Pupa Stage. In warm weather at the end of two, or at 

 most three days, and in cold weather often after the lapse of 

 several weeks or months, the integument of the contracted larva 

 undergoes a change of colour and texture. At first it turns 

 yellow, then red, and finally black, and with this last change it 

 also becomes hard and brittle ; the insect is now a pupa. 



The four anterior rings of the pupa-case are separated from 

 those behind by a seam or raphc, and are readily detached as a 

 cap in the fully-formed pupa. It is by this means the imago, 

 or perfect insect, escapes from the pupa-case. 



If the pupa-case be opened just before it becomes black, 



i 2 



