140 THE SENSE ORGANS [CH. 



generally accepted view that this organ is formed by the fusion 

 of two originally complete and separate ocelli in the median line. 



The visual cells are elongated, with the more basal portion 

 swollen and containing the nucleus. They are arranged in sets 

 of three, closely fused together (fig. 61 c, D). Each set or triad 

 gives rise, along the inner junctions of its component cells, to a 

 very remarkable organ called the rod or rhabdome (rh), which is 

 consequently, in transverse section, seen to be of triradiate form. 

 It is semi-transparent and of a pale yellowish colour. Its function 

 is generally held to be that of a ligkt-transmuter (see p. 148). Each 

 rhabdome is of moderate length, extending practically the whole 

 length of the triad of cells forming it. It is somewhat thicker 

 basally than distally, but its three radii are of about the same 

 length throughout. In fig. 61 A, only the upper portions of the 

 proximal rhabdomes are shewn cut[70]. 



The visual cells of the proximal group appear to be much 

 better developed than those of the distal group. The latter are 

 considerably shorter. In Austrolestes I have not succeeded in 

 detecting any rhabdomes connected with these cells, though such 

 are certainly present in the larger ocelli of Aeschna brevistyla. 

 This suggests that, in some Dragonflies, the ocelli can only focus 

 objects clearly at one fixed distance, while in others the same 

 organs are effective for two separate distances. In both cases, 

 the form of the lens shews that the objects viewed must be brought 

 very close up to become visible. 



The retina is surrounded by a layer of black iris pigment (pg) 

 developed in cells which are continuous with the corneagen cells, 

 and hence with the hypodermis. 



The ocellar nerve (ocri) is, as already explained, of a double 

 nature, the two diverging portions being separated by a considerable 

 number of nerve-cells (we). 



Structure of the Lateral Ocellus (fig. 61 B). 



This organ shews exactly the same parts as the median ocellus. 

 It differs from the latter, however, in being unsymmetrical in all 

 sections, and is obviously a single and not a double organ. We 

 may remark that a section through the median ocellus taken 

 along a vertical plane through the letters cnl and A in fig. 61 A 



