520 THE WONDER OF LIFE 



temperature the regrowth does not occur, and the decapi- 

 tated earthworms he imprisoned without walls. 



Fly Trap. As a fine example let us take Venus's 

 Fly Trap (Dioncea muscipula), a member of the Sundew 

 family (Droseraceae) in which we find a graded series of 

 adaptations to catching and digesting insects. The trap 

 of Dionsea is the much modified leaf. The blade consists 

 of two nearly semicircular halves, united by a strong midrib ; 

 the surface is studded with reddish glands, and bears on 

 each side three sensitive jointed hairs ; on each margin 

 there are about twenty spikes directed upwards and 

 inwards ; the stalk of the leaf is like the handle of a tea- 

 spoon with a channelled upper surface and a narrow 

 isthmus where it joins the blade. 



When an insect, attracted to the glistening glandular 

 surface, touches one of the upstanding jointed hairs, the 

 halves of the blade begin at once to close in upon one 

 another, the spikes on one side fitting in between those 

 on the other, like the teeth of a rat trap. This happens 

 quickly, but the movement stops before the leaf is com- 

 pletely closed a fact which Darwin explained ingeniously. 

 Insects of small size, hardly worth catching, escape between 

 the crossed teeth, and the leaf soon re-opens. A larger 

 victim, unable to get through the prison bars, touches the 

 sensitive hairs again and induces a second and more vigor- 

 ous contraction, which proves fatal. No more effective 

 adaptation could be imagined. 



Let us follow the story a step further. When the Venus 

 Fly Trap is tricked into closing, it opens again in twenty- 

 four hours. But when it shuts on a big fly it remains with 

 the two lobes pressed against one another for a week or 

 more. The secreting glands are stimulated and pour out 



