384 



be little doubt that the origin of this membrane in Calliphora 

 is identical with what happens in this wasp. 



The Ocelli. — Tlie development of these structures can be 

 followed from the earliest larvae right throughout larval and 

 pupal life to the mature condition of the adult wasp. Tliis 

 is rendered possible by the fact that around those three small 

 areas of the head ectoderm from which the ocelli will later 

 develop the somatopleure of the head is deflected downwards 

 (fig. 66) and becomes continuous with the splanchnopleure 

 covering the brain, these curious structures are doubtless the 

 remnants of what must once have been a very extensive con- 

 nection between the ectoderm and the nerve cord as it sank 

 inwards in the embryo. From this fortunate occurrence, it is 

 possible to trace the development of the complex ocellus from 

 a stage in which it is represented by a single pair of minute 

 cells (fig. 66), a condition in which we see it in the larva of 

 the first instar. 



The ectodermal cells covering the head at this stage are 

 small in number though rather large and irregular; two cells, 

 however, included in each area covered by the conical deflected 

 somatopleure are considerably smaller than these. During 

 larval life these cells undergo division, so that in the larva 

 which is about to defaecate, one sees a conical mass of about 

 a dozen cells, rather slender and elongated, in the place which 

 in the early larva was occupied by only two (fig. 77). 



These cells continue to multiply mitotically, so that in 

 the freshly formed pupa the ocelli are represented each by a 

 rounded thickening of the ectoderm in which the cells are 

 beginning to arrange themselves in concentric layers, at the 

 same time increasing somewhat in length (fig. 67). During 

 the next four hours there is an active proliferation and elonga- 

 tion of these cells, giving the whole structure ati appearance 

 very like that of a mammalian taste bud. The cells are 

 elongated and spindle-shaped, and present each a short pro- 

 cess externally. These cells become the visual cells of the 

 ocellus, and their short processes, which together form a small 

 group at the extremity of the sense organ, project freely 

 from it. 



Meanwhile the head ectoderm surrounding the ocelli pro- 

 liferates and begins to encroach upon the area which has till 

 now been occupied by the ocellar cells. In the twelve-hour 

 pupa (fig. 68) this can be seen to result in a gradual con- 

 striction of the upper end of the ocellus, which at the same 

 time begins to be forced down below the surface. 



In the thirty-six hour pupa this process is complet-e ; the 

 ectoderm has grown right across the ocellus, and in its middle 

 is seen to undergo a distinct lens-like swelling (fig. 69). 



