— 37 — 



In fig. 11 the ommatidium furthest right is sliowii on the 

 level of the crystalline cells. In this place ommatidium bor- 

 ders with a thick layer of pigment belonging to the chief 

 pigment cells and embracing the crystalline cells. 



In fig. 12 is represented a section through two ommati- 

 dia passing on the level of the distal nuclei (w. ret. 1 — 6) of 

 the rhabdom cells. The rhabdom cells are arranged in the 

 section in the shape of a crown, six of them being about the 

 same size and form, whereas the seventh is very thin and 

 seems to be wedged in between the others. 



Both towards the outside, and towards the axial lumen all 

 the cells, except the seventh, are somewhat convex. The 

 thin seventh cell protrudes only towards the axial lumen, 

 but more than the rest, its exterior surface being concave. 



Figure 13 represents a transverse section passing more 

 proximally through five ommatidia. It is noticeable that in 

 the three upper ommatidia of this section the thin seventh cell 

 is directed downwards, whereas in the two lower ommatidia 

 these cells are arranged contrarywise. Thus, the three upper 

 retinulae present the mirror reflection of the lower. As it 

 was first shown in Diptera by Dietrich, in many insects 

 it is possible to draw a plane of symmetry which divides it 

 into a dorsal and ventral halves. In such cases in each half 

 the retinulae present, with respect to the arrangement of the 

 rhabdom cells, the mirror reflections ^i the retinulae of the 

 other half. In fig. 13 the upper and lower retinulae present 

 this arrangement on both sides of the plane of symmetry 

 just spoken of. The axial lumen attains its maximum in this 

 section. 



The next figure (fig. 14) represents a section through 5 

 ommatidia in the region of the seventh retinal nucleus and 

 presents a somewhat peculiar picture. The narrow seventh 

 rhabdom cell widens considerably in this place, displaces the 

 tracheal space bordering with it, and becomes closely adja- 

 cent to the exterior walls of the neighbouring retinulae. On 

 account of this, the contours of the seventh cell become 

 irregular. The tracheal spaces in this section are very narro- 

 wed and embraced from all sides by the walls of the retinu- 

 lae, thus losing connection with each other. 



The structure of the retinulae of the anterior area of the 

 eye was studied by me in less detail. As it may be seen from 



