BEHAVIOUR 85 



The picture built up in a compound eye of this sort is a mosaic of 

 light and dark spots, each perceived by an individual element. The 

 visual acuity of several Crustacea has been tested by placing them 

 in a round dish, then rotating a pattern of stripes around the dish 

 and finding the finest stripes that the creature responds to, either 

 by swimming in an attempt to keep the same position in relation 

 to a particular group of stripes, or by movements of its eyes show- 

 ing that it is seeing the stripes as they move past. In this way it has 

 been found that the visual acuity is greatest when the proximal 

 pigment is expanded around the retinulae. This is probably effective 



DISTAL PICMENT 

 CELLS 



Fig. 39. Diagrammatic section through the eye of a crayfish. The 



right side is shown in the light adapted state, and is drawn from 



a thicker section than the left side which is in the dark adapted 



condition. 



in reducing stray reflexions from the back of the eye. The distal 

 pigment does not seem to have much effect on visual acuity, but it 

 may be useful in decreasing the sensitivity of the eye in bright 

 light. 



There is some evidence that crabs and lobsters use their eyes 

 when hunting, and can see quite well. One striking example was 

 found during some experiments on hermit crabs. These were being- 

 kept in artificial shells made of glass, so that the soft, succulent- 

 looking abdomen was visible. A starved Portunus puber, a most 

 pugnacious crab, which was in the same aquarium, made violent 



