36 THE SENSES OF ANIMALS 



sloping gravel at the other. The laboratory is either too hot 

 or too cold and there is much too much movement going on 

 to suit a nervous creature. 



It is perfectly possible to keep Common Frogs in captivity 

 and to watch them feed if a minimum of trouble is taken to 

 provide the correct surroundings. The most important thing 

 is to keep no more than two frogs; the tank must measure 

 not less than twenty-four inches — preferably more — in length 

 and it should be about fourteen inches high and at least the 

 same in width. Rough turf and moss from a pond side should 

 cover two-thirds of the tank area, and as far as the provision 

 of water is concerned, this is better done by having a flattish 

 shallow dish which will take up the remaining third of the 

 tank. This is more satisfactory than just putting some water 

 in and tilting the tank a little so as to keep it at one end. If 

 this is done, it so often happens that the water becomes foul 

 and to clean it out means disturbing the frogs which is the 

 last thing you want to do. The shallow dish can be removed 

 daily and the water renewed. 



In addition to the turf and moss, it is a good idea to have 

 a clump of growing tall grasses planted. This will mean that 

 some of the insects you put in for food will climb up the 

 stems and so give you an opportunity to see how accurately 

 a frog will jump and seize an insect which is out of its normal 

 feeding range. This demonstrates the way in which a frog 

 must adjust the focus of its eyes very quickly. 



Most laboratory workers use worms for feeding the frogs, 

 but in my experience earthworms do not form any substantial 

 part of a Common Frog's diet. Blow-flies, immature grass- 

 hoppers, moths and crane-flies are more natural prey ; while 

 small snails and the grey and white slugs from cabbages and 

 other garden produce will be taken — at ground level. A 

 great variety of insects can be collected with a sweep-net, 

 the catch being transferred to a jam-jar until a considerable 

 quantity has been collected. A large jarful of varied insects 

 will keep two frogs going for two or three days, and blow- 

 flies can be used as a stand-by. 



To observe the manner in which a frog spots and captures 

 its food you must tip a supply of insects into the tank (which 

 must have a cover or lid of perforated zinc) after which a 



