Imaginal Folds of Head 129 



about by any infolding of the epidermis of the dorsal 

 surface of the prothorax, for the folds, though they lie 

 deep in the prothorax, belong to the larval head exclu- 

 sively. Weismann has shown that in Corethra the 

 integument of the head of the fly is formed from the 

 epidermis of the larval head, and the same thing is true 

 of Chironomus, though here the cephalic invaginations 

 are deeper and more complicated. Their backward pro- 

 longation is facilitated by a transverse fold which runs 

 back from the junction of the larval head and prothorax, 

 and is overarched by the uninterrupted epidermis of the 

 latter. But for this transverse fold, it would not have 

 been easy for the longitudinal folds to extend into the 

 prothorax without implicating the prothoracic epidermis. 

 The transverse fold is derived from the epidermis at the 

 junction of the head with the thorax, and forms a sort 

 of pocket, crescentic in transverse section, and tapering 

 behind. The enclosed space is very inconsiderable, and 

 appears in sections like a thin slit (fig. 98). The pro- 

 thoracic prolongations of the longitudinal folds, whicli 

 give rise to the compound eyes and antennae of the fl3^ 

 open into the floor of the transverse fold. 



As the longitudinal folds gradually deepen, the antennae 

 of the fly, still enclosed within the pupal skin, grow at the 

 same rate. Their basal parts recede further and further 

 into the thorax, remaining all the time attached to the wall 

 of the longitudinal invaginations already formed. The tip 

 of the imaginal antenna is never withdrawn from the short 

 larval antenna, which it is destined to replace. If we 

 suppose a cloth to be spread out between two rails, then 

 a hand grasping the cloth at one place may be made to push 

 downwards and backwards until both hand and arm 

 become buried in a deep fold. The fist will correspond 

 to the bulb of the antenna, the arm to its shaft, and the 

 fold in the cloth to the longitudinal invagination. This 



