24 THE SENSES OF ANIMALS 



is of interest in relation to the sight of insects both aquatic 

 and terrestrial. 



The Amphibia — frogs, toads and newts — are very ade- 

 quately endowed with vision; but it is necessary that an 

 object be moving for the eyesight to be fully used. On the 

 whole the Amphibia are short-sighted; and though not 

 enough work has been done on this aspect, it is probable that 

 about three feet is the maximum distance for effective use 

 of the eyes, which are almost exclusively concerned with the 

 catching of prey. A frog, toad or newt may be able to detect 

 a moving object at a slightly greater distance, but the animal 

 will approach nearer before showing by its alert attitude that 

 it recognizes the moving object as something to eat. There 

 is some variation in length of sight as between species, but 

 my own experience in the field and with captive amphibians 

 leads me to think that my views are correct. Nearly all 

 amphibians have good nocturnal vision since so many of 

 them feed at night ; but tree-frogs also feed well during day- 

 light. (See Plate 2.) 



In the Reptiles the role of the eye is subject to some varia- 

 tions and modifications as between tortoises and turtles; 

 crocodiles ; and lizards and snakes. 



Tortoises and turtles have good vision; the latter being 

 able to see well under water, and land tortoises in their 

 terrestrial environment. But it seems likely that, so far as 

 acuity of vision is concerned, the aquatic species use their 

 eyes with greater effect than those species that live entirely 

 on land. Tortoises, with a few exceptions, are vegetarian and 

 their food requires locating not capture ; and as these reptiles 

 have good powers of scent they depend equally upon their 

 powers of sight and smell. However, as is the case with most 

 snakes and lizards, turtles, terrapins and pond tortoises are 

 much stimulated by movement on the part of their prey. 



Snakes and their vision offer some nice problems which 

 ought to provide the naturalist with much scope for work in 

 the field. Many kinds have the vertically elliptical pupil 

 usually associated with nocturnal vision, yet others without 

 this particular feature are also known to hunt their prey at 

 night. Much further study is required on this subject, and 

 there is no reason why the field worker should not provide 



