270 MESSES. MIALL AND HAMMOND ON THE DEVELOPMENT 



V. Imaginal Folds in the Head of the Larva o/'Chironomus. 



In larva? about half an inch long the epidermis of the top of the head begins to be 

 infolded along two nearly longitudinal lines {If, fig. 17, PI. XXIX), which run forwards 

 from the junction of the head and thorax, diverging a little as they do so. These lines 

 correspond to the margins of the clypeus in the larval head. The epidermis, thus 

 carried into the interior, gives rise to new cuticular organs, first to the pupal cuticle, and 

 subsequently to the various external organs of the head of the fly. The cuticle of the 

 head of the pupa is of less interest, and its formation need not be particularly described. 

 The compound eye and antenna of the fly originate in these epidermic folds, and are 

 therefore developed at a distance from the larval cuticle, though they are from the first 

 external in their morphological position. The outer wall, the bottom, and ultimately 

 the inner wall of each invagination (PI. XXIX. figs. 19, 20, 21, & 22, and PI. XXX. 

 figs. 24 & 25) develop facets, and thus give rise to the compound eye of the fly. In the 

 larva this compound eye looks into the cavity of the invagination, and its concavity as 

 well as its deeply sunk position contrast strongly with the convexity and exposed position 

 of the imaginal eye. The imaginal antenna originates as a secondary duplication of the 

 invagination around the antennal nerve of the larva, which duplication in all stages of 

 growth is continued up to the larval antenna. 



In larvae which are not far from pupation the folds are no longer confined to the 

 region of the head. They extend backwards into the prothorax, and the part which 

 forms the compound eyes comes to lie wholly behind the larval head. This backward 

 extension is not brought 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 exclusively. 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 com- 

 plicated. Their backward prolongation is rendered possible by a transverse fold (tf, tf, 

 PI. XXIX. fig. 17, PL XXX. fig. 24, &c.) 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, the longitudinal folds could not have extended 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 (PI. XXIX. fig. 19, tf) and tapering behind (fig. 17, tf). 

 The enclosed space is very inconsiderable, and appears in section like a thin slit. The 

 prothoracic prolongations of the longitudinal folds, which give rise to the compound eyes 

 and antennae of the fly, open into the floor of the transverse fold (fig. 19). 



As the longitudinal folds gradually deepen, the growing antennas of the fly, still 

 enclosed within the pupal skin, grow with it. Their basal parts recede further and 

 further into the thorax, remaining all the time attached to the wall of the longitudinal 

 invaginations already formed (figs. 21 & 22). The tip of the imaginal antenna is never 

 withdrawn from the short larval antenna, which it is destined to replace. If we suppose 



