54 GNATS OR MOSQUITOES — CHAPTKR III 



the base of the great pigment mass of the eye, the fibre 

 expands into a spindle-shaped body provided with a distinct 

 nucleus. Beyond this the spindle-shaped body contracts 

 into a rod-like structure, the actual ending of which I have 

 not been able to trace, though I think it simply ends in the 

 midst of the transparent contents of the tubular visual 

 elements to be presently described. A further examination 

 of radial sections shows that the pigment-covered sensory 

 cones each consist of a deep portion enclosing the spindle- 

 shaped bodies, which may or may not be radial in direction, 

 and may even be curved, and an outer portion containing 

 the rod-like nerve end, which is always truly radial. If w^e 

 now turn to the examination of tangential sections, we find 

 that the superficial layer of the general pigmented mass 

 consists of a number of cj'lindrical prolongations of the 

 transparent superficial layer of the eye, each enclosed in a 

 layer of pigment, w^iich is very thick at the surface and 

 becomes gradually thinner as the deeper layer containing 

 the spindle-shaped bodies is approached. 



In all sections that are sufficiently truly tangential to 

 afford a clear image, the contained terminal rod can clearly 

 be seen as a well-stained dot exactly in the middle of the 

 transparent contents of the tube of pigment ; but whether it 

 extends through the entire length of the tube, or ends some- 

 where during its course, is more than I can say ; in any case, 

 however, it extends along it for some considerable portion of 

 its length. From what has been said it is clear that the 

 terminal rods can receive only rays the direction of which 

 is parallel to the radius of the sphere of the eye coinciding 

 with the transparent axis of the pigment-clothed visual 

 element, and that such rays will reach this end rod and 

 no other, so that although there be no dioptric apparatus, 

 such as is found in each vi.sual element of the facetted eye of 

 the adult insect, for the purpose of concentrating on the con- 

 tained rod the pencil of rays entering each transparent 

 cylinder, it is, nevertheless, like such eyes, suited only for 

 mosaic vision, and is for such a purpose only less efficient in 

 so far as in that a smaller portion of the entering pencil of 

 rays will actually reach the end-rod placed in the axis of the 



