DIVIDED EYES OF CERTAIN INSECTS 465 



thickly covered with slender hairs ' and the remarkable double 

 character of the eyes may be easily overlooked. 



Indeed, it is only upon careful observation that the densely 

 black, small, facetted area is seen at all. If the head of the fly 

 is tilted back by lifting up the proboscis, a hand lens will show 

 the narrow black small facetted area on the extreme ventral 

 surface of the compound eye. This area is scarcely one sixth 

 that of the entire eye and is separated from the large facetted 

 upper surface by a narrow groove or offset. Fig. 11 shows the 

 position of the small facetted part of the eye. Fig. 8, perhaps, 

 shows better the relative extent of the 2 kinds of elements as 

 seen in longitudinal sagittal section. As shown in this Fig. 8, 

 the elements beneath the small facetted region are little more 

 than half the length of those under the large facets. Moreover, 

 the part occupied with small elements is densely pigmented. 

 The rest of the eye has but little pigment. 



The elements of a large ommatidia consist of a thin cuticular 

 hexagonal facet, a pseudocone, a retinula, and iris pigment cells 

 surrounding the pseudocone. The cells of a pseudocone cannot 

 be distinguished from each other in the outer large part of the 

 cone. The lower truncate or slightly rounded apex of the cone 

 is a little denser than its upper part and this denser portion stains 

 more readily. Here the 4 cells making up the cone can be dis- 

 tinguished, each having its nucleus (Figs. 9 and 10, c«, and 

 Fig. 9, B). Cross-sections of the distal ends of 3 neighbor- 

 ing retinula? are shown in Fig. 9, C. Each retinula is made 

 up of 6 cells arranged in a circle around a seventh cell in the 

 center. The inner borders of each of the 6 cells has a rounded 

 deeply stained rhabdomere (as this part of the eye was named 

 by Grenacher, 1879). The rhabdomere of the seventh cell oc- 

 cupies the axis of the retinula. At their distal ends the 6 retin- 

 ular cells overlap entirely the rounded denser apex of the pseu- 

 docone, d (Figs. 9, A, and 10). The seventh cell, together 

 with its rhabdomere and those of the other 6 cells, stop snugly 

 against the inner end of the pseudocone. Near the middle part 



1 Whether these apparent tactile hairs, which cover the eve of Bibio so densely 

 and are found on the eye of Blephorocera less abundantly, are really supplied 

 with tactile sense organs has not been determined bv me. 



