271 MESSES. MIALL AND HAMMOND ON THE DEVELOPMENT 



become bit by bit convex, not by any gradual widening of the fold, but by a steady 

 extension of the convex surface at the expense of the concave fold (PL XXXI. fig. 33). 

 Tbe process is hard to describe, but may be closely imitated by cutting a hollow india- 

 rubber ball into balves, and everting one of the hemispheres with the fingers. During 

 the process of eversion the compound eyes are drawn downwards and backwards, so 

 that they get behind and beneath the bases of the antennae (PI. XXX. fig. 29). The 

 morphologically external surface of the eyes, which was previously turned towards the 

 lumen of the invagination, now looks outwards (PI. XXXI. fig. 33) ; the optic nerve, 

 which was distributed to the (temporarily) convex and outer surface of the eye, still 

 takes its course to the same surface, now become concave and internal ; and the walls 

 of the head now for the first time enclose the brain. The inner walls of the paired 

 invaginations, which were very long and enclosed a narrow median space or sinus 

 (s', PI. XXIX. figs. 18-22), contract more and more, and give rise to the central parts of 

 the head of the fly. 



Sections taken through the pupal antenna indicate the change resulting in the 

 formation of the split in that organ to which reference has been made (page 26S). The 

 portion of the wall destined to form the hollow of the shaft projects in a marked manner, 

 and is much more delicate than the remaining portion (see PI. XXX. fig. 30). 



The muscles of the larval head, as well as of some other parts of the body which become 

 completely transformed, disappear by a process which leaves behind a number of more 

 or less empty sheaths. This process reminds us strongly of the histiolysis which takes 

 place in Muscidae, as well as in many other animals. We have not, however, in spite 

 of many efforts, arrived at such proofs of histiolysis as can readily be obtained in 

 Muscidae. We have never, for example, seen unmistakable sarcolytes within the 

 supposed phagocytes, and cannot therefore appeal to any of our preparations as 

 furnishing a demon stration of the process of histiolysis in Chironomus, which is on 

 general grounds highly probable. 



VII. Comparison of Chironomus with some other Insects. 



In the larva of Culex, as we find from Dr. C. H. Hurst's partly unpublished descrip- 

 tions and preparations, there are no deep invaginations for the compound eyes or antennae 

 of the imago. The compound eye forms beneath the larval eye-spots, and is at first 

 relatively simple and of few facets. The number increases by the gradual formation 

 of j>artial and marginal invaginations, each of which forms a new element. The imaginal 

 antenna grows to a much greater length than that of the larval antenna, and its base is 

 accordingly telescoped into the head while the shaft becomes h*regulaiiy folded*. Culex, 

 though more modified than Chironomus in many respects, e. g. in the mouth-parts, is 

 relatively primitive with respect to the formation of the imaginal head, and shows a 

 mode of development of the eye and antenna which we may suppose to have characterized 

 a remote and comparatively unspecialized progenitor of Chironomus. 



* " The Pupal Stage of Culex " (Studiea from Biol. Lab. Owena Coll. vol. ii. 1890). See also Manch. Micr. 

 Soc. 1890. 



