SENSATION AND REACTION 85 



areas or corneal facets) towards the underlying optic ganglion. 

 It is therefore evident that such an eye is adapted for the 

 transmission to the ganglion of a number of nerve impulses 

 due to the impact of light waves from many quarters of the 

 insect's surroundings, these impulses travelling along con- 

 verging paths towards the central system, and giving rise 

 to visual impressions from a wide range of direction. A 

 fly with two such eyes, large and sub-globular, occupying 

 the greater part of the surface of the head, should evidently 

 be able to receive impressions through the eyes from before, 

 above, and beneath as well as from either side, and to some 

 extent from behind. While we recognise thus the great 

 scope of an insect's vision as regards direction, we remember 

 that the corneal area of the compound eyes, consisting of a 

 modified portion of the head-cuticle, cannot be moved, and 

 we realise that insect vision must differ greatly from our own 

 and that of most vertebrate animals, in that neither both 

 eyes nor one can be brought to bear especially on a com- 

 paratively small region of the environment, the close 

 examination of which might be desirable. Even a super- 

 ficial view of an insect's compound eye inclines us therefore 

 to the opinion that the sight of such a creature is by no 

 means like ours, and we may surmise that what the insect 

 gains in range of direction it may lose in perception of 

 distant objects as well as in clearness of definition. Before 

 pursuing this line of discussion it will, however, be advisable 

 to consider in some detail the structure of the elements 

 that compose an insect's compound eye. 



The eye of the well-known Honey Bee has been well 

 described by E. F. Phillips (1905), and his account is 

 summarised in R. E. Snodgrass's works on that insect 

 (1910, 1925). Each corneal facet is the outer area of a 

 thickened transparent section of the cuticle which may be 

 regarded as a lens (Fig. 29, cr). Beneath each lens is a 

 transparent crystalline cone due to the modification of 

 four special cone-cells derived from the skin (Fig. 29, c). 

 Beneath each cone is a clear rod (rhabdom, r) due to the 

 modification and elongation of four other cells. Around 



