1905.] NATURAL SCIENCES OF PHILADELPHIA. 131 



arrangement and are not, as a rule, without some bend, so these cells 

 could scarcely modify an ommatidial plan of arrangement. The base- 

 ment membrane is considerably smaller in area than the lens chitin, 

 and as a result the room provided for each ommatidium is considerably 

 decreased, so that in contrast with what has been stated, that the 

 ommatidia are not so crowded proximal to the cone, it might be sup- 

 posed that the converse would be true. However, the fact is that in 

 cross-section a larger proportion of space is occupied by outer pigment 

 cells, the interommatidial spaces, near the base of the ommatidia than 

 near the lens; and since, as above stated, these cells are flexible and 

 not crowded, it scarcely seems to follow that this parallel arrangement 

 is due to crowding. 



The hexagonal arrangement is undoubtedly the common plan, at 

 least as far as the lens is concerned, and the tetragonal arrangement 

 may be derived from it as held by Parker, and his arguments for such 

 an origin seem good ; but, on the other hand, the hexagonal arrangement 

 could scarcely give rise to the tetragonal unless preceding the hex- 

 agonal facets the ommatidia were in squares, so that the secondary 

 crowding would bring about the primitive arrangement again. Taking 

 again the case of the honeycomb, no additional crowding could possibly 

 make the cells square, for the more the circular walls (the primitive 

 cells) are crowded the more truly they become hexagonal. However, 

 if the walls were made of four parts, as is the cone, and if they were 

 fastened at their bases in parallel rows, then additional crowding might 

 cause the lens to lose its circular outline and become square, in which 

 case the hexagonal arrangement of the lens would be lost. It seems 

 probable that the cone determines the arrangement rather than the 

 lens-secreting cells, and Parker's figures of Gonodactylus (Parker, 1890, 

 PI. VIII, fig. 93), in which the tetragonal arrangement is found in the 

 large ommatidia and not in the small ones, lend support to this view. 



To sum up, it seems probable that the arrangement of ommatidia, 

 where they are sufficient in number to be said to have any plan at all, 

 is normally the tetragonal plan. If the cones are somewhat com- 

 pressed, as they generally are on account of the way in which a com- 

 pound eye is made up, a hexagonal arrangement of the distal parts of 

 the ommatidia results; but if the pressure is sufficient to cause the 

 cone to lose its circular form then it becomes a square, and the facet 

 plan again becomes tetragonal. 



3. Hair Cells. 



The entire lens of the eye of the bee, especially in the younger 

 individuals, is covered with large hairs, unlike those of the rest of 



