112 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



Lateral ocelli present no general uniformity of structure. They 

 are the functional visual organs of insect larvae and are antecedent 

 to the future compound eyes of the imago. It has already been 

 explained in an earlier chapter that insect larvae represent phylo- 

 genetically earlier phases of development than either nymphs or 

 imagines. They issue from the egg at a stage before the adult eyes 

 appear, and consequently have acquired provisional visual organs 

 that are functional only so long as the larval period lasts. 



In their very simple 

 form, as seen in larvae 

 of Lu cilia, for example, 

 the ocelli consist merely 

 of groups of bipolar 

 sensory cells, devoid of 

 pigment, while the 

 overlying cuticle has 

 been modified into a 

 lens-like structure for 

 the condensation and 

 transmission of incident 

 light rays (Fig. 51). 

 According to Ellsworth 

 (1933) these larvae are 

 extremely sensitive to 

 light, and by localising 

 the latter into a small 

 " pencil " he was able 

 to show that this 

 sensitivity is confined to the organs just mentioned. In larva* 

 of the Lepidoptera' certain Coleoptera and Neuroptera the 

 lateral ocelli occur, as a rule, in a group on either side of the 

 head and in close association with the site where the imaginal 

 eyes are to be developed. In these orders each ocellus is usually 

 separate from its fellows : it is provided with a cuticular lens 

 with differentiated visual cells and sensory rods beneath. In 

 larvae of the saw-flies there is a single lateral ocellus on either 

 side which exhibits a structure very similar to a typical dorsal 



Fig. 51. Section tlirough a pair of ocelli of a 

 Lucilia larva. x 450. c, collar ; cy, 

 cuticular cylinder ; e, outer cuticle ; e^, 

 inner cuticle ; /, nerve fibrils from retinal 

 cells, r ; ^, hypodermis ; /, cuticular lens ; 

 m, limiting membrane ; n, nerve ; r, 

 retinal cell ; r^, nucleus of retinal cell ; 

 .V, sheath, within which the lens moves 

 outward or inward ; v, prcretinal membrane. 

 (From Ellsworth. 1933.) 



