VII] THE SENSE ORGANS 141 



would very closely resemble fig. 61 B. The lateral ocellus clearly 

 has its dioptric apparatus distorted for the purpose of lateral 

 vision in fact, it is set at a permanent and very effective "squint." 

 The corneal lens is for the most part quite colourless, but the 

 innermost part of the central "bulb" is coloured ruby-red. 



The Compound Eye (figs. 58, 62-64). 



We have already discussed (p. 11) the size and shape of these 

 eyes in relation to the head. We have now to consider the 

 structure of the eye itself. Each of the hexagonal facets already 

 noticed on the outer surface of the eye corresponds with a single 

 element of the compound eye, known as an ommatidium. An 

 ommatidium has been compared with an ocellus; but we shall 

 see that there is sufficient difference in structure to make the 

 comparison inadvisable. Moreover, it was until recently quite 

 generally held that the compound eye arose as an aggregation of 

 simple eyes similar to ocelli, and that the hexagonal form of the 

 facets was produced by mutual pressure between numerous 

 originally rounded elements crowding closely together. Though we 

 must admit that crowding would in any case produce hexagonal 

 facets, we must at the same time negative the previous assertion 

 on palaeontological grounds, if on no other. For the compound 

 eye is an exceedingly archaic structure, found (in the Trilobites) 

 well developed right back to the Lower Cambrian. Moreover, 

 in the Dragonflies at any rate, the compound eyes develop in 

 the embryo, whereas the ocelli do not begin to develop until 

 near or at the close of larval life. As these insects are amongst 

 the most ancient of the class, this fact is very significant. 



In the compound eye of the Dragonfly, the number of facets, 

 and hence the number of ommatidia, ranges from about 10,000 

 in the smaller Zygoptera to 28,000 or more in the largest Aeschninae. 



Structure of an Ommatidium. 



Fig. 62 shews a longitudinal section through four ommatidia 

 from the compound eye of Austrolestes leda. The hexagonal 

 facet (/) is the external surface of the corneal lens (cnl). The 

 latter is deposited by the agency of the corneagen cells (cng), which 



