138 THE SENSE ORGANS [CH. 



placed on the epicranium that they isolate a small central triangular 

 area called the vertex. This, in many Anisoptera, is raised up into 

 a tubercle or vesicle. The median ocellus lies at the apex of the 

 triangle, which is anterior, while the two lateral ocelli occupy the 

 base angles. The angle of the apex is always obtuse, lying usually 

 between 120 and 170. 



The median ocellus is so placed as to look forward, while the 

 laterals look sideways. This difference is especially marked when 

 the vertex is swollen. The median ocellus then becomes sunk 

 into a pit, with the wall of the vertex rising like a cliff behind it. 

 Thus all lateral and posterior rays of light are cut off. Again, 

 the lateral ocelli become slightly raised up on the sides of the 

 tubercle, so as to face somewhat horizontally outwards. Thus 

 they receive chiefly lateral rays. 



The median and lateral ocelli differ considerably in structure. 

 The former is always the larger, and is either oval or reniform in 

 shape, with its major axis transversely placed. Both by its 

 structure and nerve-supply it shews signs of having been originally 

 double. It is therefore to be regarded as the product of the fusion 

 of two originally separate ocelli in the middle line. In structure 

 it is bilaterally symmetrical. The lateral ocelli, on the other 

 hand, are smaller simple oval bodies, not bilaterally symmetrical. 

 Their parts are much contorted and shifted out of place, probably 

 in order to accommodate the organ to the reception of lateral 

 rays of light only. 



Structure of the Median Ocellus (fig. 61 A, c, D). 



Fig. 61 A gives a section of this organ in Austrolestes leda, 

 passing obliquely through the middle of the lens and through the 

 proximal set of rhabdomes, and including several layers of cells. 

 The ocellar nerves are also shewn, though these are actually not 

 quite in the same plane as the other structures. The ocellus 

 consists of the following parts: 



(i) A thick convex corneal lens (cnl). When examined in thin 

 sections, this lens is seen to consist of a very large number of 

 delicate layers of chitin, all of which are transparent, but not all 

 colourless. The outer portion is absolutely clear, and extends 

 to the border of the organ around its whole circumference, where 



