130 



PROCEEDINGS OF THE ACADEMY OF 



[Feb., 



membrane and the retinular ganglion tracheae with spiral markings 

 occur in all specimens examined (see text fig. 3), but the statement of 



Hickson holds good for all other 

 parts of the optic tract as far 

 as has been observed. Kenyon 

 also mentions the presence of 

 tracheae in this region. 



2. Arrangement of Ommatidia. 



ret.gar>5.a 



Fig. 3. — Section below basement mem- 

 brane, showing retinular ganglion cells 

 and nerve fibrils from ommatidia. 



The facets of the lens are 

 arranged in hexagons, as is true 

 for so many insect eyes, but this 

 is probably not a primitive con- 

 dition. Hexagonal arrangement 

 is what is produced whenever 

 any circular objects are closely 

 pressed together, just as the cells of the honeycomb are hexagonal, 

 and this undoubtedly explains the shape and arrangement of the 

 facets. Parker (for Crustacea) looks upon unfaceted eyes as primi- 

 tive, and probably this is true for insects also. We have, however, 

 in the proximal portion of the eye a different arrangement which 

 is perhaps more primitive than the hexagonal method. At any 

 level proximal to the cone cells the ommatidia are arranged in parallel 

 rows, and the nearer we come to the base of the ommatidia the 

 clearer is this arrangement, until on a section at the level of the base- 

 ment membrane (fig. 18) we see this parallel arrangement very 

 marked. Since here we get a condition in which the ommatidia are 

 not pressed together and therefore are not modified mechanically, 

 it probably represents a more primitive condition than that found 

 in the lens region. In the pupa, even the facets do not have as marked 

 a hexagonal arrangement as they have later, and in the larva we get 

 an arrangement identical with that of the bases of the adult ommatidia. 

 The numerical plan and shape of the parts of the ommatidium may 

 have something to do with the arrangement. The retinular cells are 

 eight in number, but four of these are wider than those which alternate 

 with them, and as a result a cross-section of the retina is roughly a 

 square. The outer pigment cells are twelve in number when their 

 arrangement is unmodified by hair cells, and this number readily ar- 

 ranges itself into a square with three on a side, or into a hexagon with 

 two on a side. Since the outer pigment cells are simply strands of 

 cytoplasm they readily accommodate themselves to an}^ change of 



