Characteristic Pupal Organs 139 



the old cuticle and tlie retracted epidermis. In such 

 cases we have found that the pupa dies within a short 

 time. The complete removal of the larval cuticle from 

 the body is a matter of time, and may occupy several 

 hours. The old cuticle becomes much A\Tinkled, and is 

 ultimatel}^ torn into shreds, being gradually rubbed off 

 by the almost incessant movements of the pupa. Occa- 

 sionally the larval skin is still adherent to the pupa when 

 the fly emerges. 



Sections taken through the pupal head a little after the 

 time of change illustrate the e version of the imaginal 

 head. The compound eyes, which were deeply invaginated, 

 become bit by bit convex, by progressive eversion of the 

 folds. During the process they are drawn downwards 

 and backwards, so that they get behind and beneath the 

 bases of the antennae. The morphologically external 

 surface of the eyes, which was j)reviously turned inwards, 

 now looks outwards ; the optic nerve, which was distri- 

 buted to the temporarily convex surface, still takes its 

 course to the same surface, now concave ; and the walls 

 of the head, for the first time since the first larval moult, 

 enclose the brain. 



It will be understood from the history of its develop- cimracter- 

 ment that the pupa of Chironomus is structurally little organs, 

 more than the fly enclosed in a temporary skin. The 

 compound eyes and antennae, the mouth-parts, legs, and 

 wings of the fly are all there, complete in outward form, 

 and usually exhibiting on microscopic examination all 

 the histological detail of the same organs in their active 

 condition. The only structures peculiar to the pupa 

 seem to be the prothoracic respiratory appendages or 

 tracheal gills, and the fin-like expansions of the abdomen 

 (Plate, figs. 5, 6). 



A pair of conical prominences are borne on the top 

 of the pupal head ; they enclose a pair of imaginal 



