SIGHT FIELDWORK AND EXPERIMENTS 39 



marked degree of nocturnal vision. Two simple experiments 

 will show what I mean. 



The first is to introduce into the cage or tank a decent- 

 sized moth — in daylight. If the moth comes to rest as soon 

 as it is put in, the tree-frog takes no notice of it. A slight 

 shaking of the foliage will usually make the moth take flight, 

 and one has to be well on the alert to see the actual capture, 

 so quick is the frog to detect the slightest movement. The 

 second experiment is more dramatic. The moth should be 

 put into the cage when the room is in darkness and this must 

 be done quickly and gently in order to ensure as far as 

 possible that the moth does not fly at once. A torch must be 

 at hand, and as soon as the cover is secure you must remain 

 still and in darkness. Listen carefully, and before long you 

 will hear a kind of "plop" as the frog moves to seize the 

 moth which will probably have taken wing. As soon as you 

 hear this noise switch on your torch, when you will see the 

 frog making its last gulp — sometimes having quite a job to 

 get the wings of the moth into its mouth. To do this, the frog 

 will use its front legs as hands to tuck the protruding wings 

 into its mouth. 



You will note that the capture has been made in the dark, 

 yet more often than not the tree-frog will secure the moth as 

 easily as it did in daylight. 



While all the instances I have given show the sharp sight 

 of frogs when their prey is moving, it is diflacult to be precise 

 as to the length of sight possessed by frogs as a whole, since 

 the fact that they themselves move from their original posi- 

 tions, and by leaping enable the insect to be caught, makes 

 it harder to be precise as to the actual length of sight which 

 frogs enjoy. 



Certain other frogs, those which are aquatic rather than 

 terrestrial, will give further evidence of their visual powers 

 under quite different conditions. In Britain we have two 

 such frogs — both introduced from Europe. These are the 

 Edible Frog and the Marsh Frog. 



The former are not numerous though there are still a few 

 colonies to be found in Kent, Surrey, Middlesex and Essex. 

 The Marsh Frog is less erratic and is confined to the Romney 

 Marsh in Kent where it inhabits the dykes and the canals 



