22 THE SENSES OF ANIMALS 



images to help in getting food or in avoiding obstacles and 

 enemies. 



It is probable that some readers of this book will have less 

 knowledge than others; and, therefore, it is reasonable to 

 include some fairly elementary matter which, it is hoped, 

 will not put off the better-informed student. 



Light is one of the stimuli which brings about a reaction — 

 either positive or negative ; and we speak of animals being 

 positively or negatively phototactic. Very simple experiments 

 will demonstrate this kind of behaviour ; and one such experi- 

 ment can be carried out with water-fleas and earthworms 

 with no more elaborate apparatus than a jar of water con- 

 taining some of the "fleas" and a roughly made cylinder of 

 black paper with a hole the size of a halfpenny cut in it. 

 In the case of earthworms one requires a box of earth and 

 some worms. A source of strong light will be necessary for 

 each experiment. 



Having got the jar of water-fleas ready, observe how these 

 tiny crustaceans swim about in a very haphazard way. Then 

 place the cylinder over the jar, and after a few minutes direct 

 a beam of light on to the place where the hole has been cut. 

 In a very short time it will be seen that the spot where the 

 light penetrates will be crowded with the water-fleas. They 

 have been affected positively by the light and have gathered 

 in numbers where the light is strongest. 



Where earthworms are concerned, if some of them are 

 placed on top of some loose earth in a normally lighted room 

 there will be immediate reaction by the worms, they will 

 start to burrow. If the strong light source is switched on so 

 that it falls directly on the worms their burrowing move- 

 ments will be speeded up and it will not be long before all 

 of them have disappeared ; they have been negatively affected 

 by light and have moved to get away from it as quickly as 

 possible. 



The larvae of some insects will behave in the same manner 

 as earthworms ; and if some maggots are put in a box, half 

 of which is in darkness and half well lighted, the maggots will 

 wriggle their way to the dark portion to avoid the light. 

 Certain caterpillars of moths and butterflies will show that 

 some tolerate light while others do not. The larva of the 



