668 General and Applied Biology 



of which may be detrimental. Fish may devour worms, crustaceans, in- 

 sects, etc.; frogs may capture worms, insects; snakes may destroy frogs, 

 birds, etc.; owls may kill rabbits, mice, etc.; chicken hawks may kill chick- 

 ens, etc.; cats may destroy rats, mice, birds, etc. Harmful predacious 

 mammals include wolves and cougars which kill sheep, cattle, horses, 

 and big game; dogs and cats may be beneficial predators when they de- 

 stroy harmful animals such as rats, mice, etc. 



VI. INSECTIVOROUS PLANTS 



The so-called insectivorous plants (in sek -tiv' or us) (L. insectus, cut 

 into or insect; vox are, to devour) possess special structures, usually modi- 

 fied leaves or parts of leaves, whereby they are able to trap and devour 

 insects for part of their food (Fig. 330). The specialized structures se- 

 crete enzymes for the digestion of the insects, and the latter are absorbed 



PITCHER PLANT 

 S&rracenift 



SUNDEW 



Drosera 



VENUS'S-rLYTRAF 

 Dionaee 



lUTTXRWORT^ 



Pinguicula 



Fig. 330. — Insectivorous plants. In the pitcher plant the pitcherlike leaves are 

 filled with water in which insects drown and are digested. In the sundew the 

 leaves are supplied with sticky hairs for capturing insects. In the bladderwort 

 (Utricularia) numerous "bladder traps" capture aquatic animals, and one is 

 shown enlarged. In Venus's-flytrap the two halves of the leaves act like a spring- 

 trap to capture insects. The sticky leaves of the perennial butterwort capture 

 insects. (Copyright by General Biological Supply House, Inc., Chicago.) 



by the plant. The so-called "pitcher plants" [Sarracenia) common in 

 bogs possess a pitcherlike device which is filled with water and in which 

 insects drown. Escape is prevented by inwardly directed spines and the 

 digested insects are absorbed. In Venus's flytrap (Dionaea) the special- 

 ized leaves possess a row of "teeth" on the outer margin of each half 

 of the leaf blade. On the upper surface, in the center of each half, are 

 sensitive "hairs" which when stimulated by an insect cause the two halves 

 to spring together to entrap the insect. Digestion by enzymes somewhat 

 resembles that in the pitcher plant. In the common sundew {Drosera) 



