110 THE SENSE ORGANS AND BE FLEX BEHAVIOUR 



instances they appear to be the degenerate vestiges of compound 

 eyes, but for the majority they have no direct relations with the 

 latter organs and, if there is any community of ancestry in the 

 two cases, evidence of such is not forthcoming within the class 

 Insecta. Ocelli may be grouped into two classes, viz., dorsal or 

 primitive ocelli and lateral or adaptive ocelli. 



Dorsal ocelli are characteristic of nymphs and adults and occur 

 in all the chief orders of Pterygote insects, but are not present in 

 larvae. They share the common structural feature of being 

 composed of a greater or smaller number of retinulae grouped 

 together in relation with a single l^ns. In the compound eye it 

 will be recalled that there is a separate lens, or facet, in direct 

 association with each visual element or ommatidium. Dorsal 

 ocelli differ from compound eyes as regards their innervation. 

 Whereas the nerves supplying the compound eyes are in connection 

 with the optic lobes, the ocellar nerves are traceable to ocellar 

 lobes located in the protocerebrum, between the mushroom bodies. 



The typical number of dorsal ocelli is three : they are disposed 

 as a triangle on the head-capsule, with the apex median in position 

 and indicated by the unpaired ocellus of the group. There is a 

 certain amount of evidence in favour of the view that the median 

 ocellus represents the fusian product of original paired organs, 

 and that the primitive number of dorsal ocelli was four. This 

 conclusion is based upon the fact that among Hymenoptera the 

 nerve supply of the median ocellus is frequently double. Further- 

 more, as Patten showed a number of years ago, the median ocellus 

 in the wasp is represented in the prepupa or late larva by a pair 

 of pit-like rudiments which ultimately coalesce. Evidence of 

 paired structure is also present in the median ocellus of Odonata 

 and of Bomhus, while Wheeler (1936) has recorded a high 

 percentage of double anterior ocelli in the ant Atta cephalotes. 



Glaser (1925) records an example of the grasshopper Melanoplus 

 differ eniialis in which the usual median ocellus is replaced by paired 

 organs. These latter, however, are of smaller size and their total 

 superficial area scarcely exceeded that of a normal unpaired ocellus. 

 He also refers to records by Blackman of a similar phenomenon in 

 another species of the same genus, and by Smulyan in a species of 

 saw-fly. That these tetralogical examples may have a phylogenetic 



