18 NATURAL HISTORY OF VERTEBRATE ANIMALS 



the mouth by a sudden movement of the tongue (Fig. 9), which is 

 ahnost as sticky as fly paper. An object cravvHng on the ground, 

 such as an earthworm, is approached by jumping, and as it is gulped 

 into the mouth the two front feet may alternately comb the wrig- 

 gling prey away from the head. Frogs in the laboratory may be 

 fed upon crayfish or similar animals. In nature their food is of 

 wide variety. Almost anything that is small enough may be 

 devoured, and sometimes, as in the case of crayfish, the size and 

 number of the objects swallowed is amazing. Insects flying in the 

 air; spiders, insects, snails, and earthworms crawling on the 

 ground; insect larvae, crayfish, and other animals in the water; 

 even other frogs and tadpoles — all are eaten with avidity. 



The writer once observed that some large bullfrogs had devoured 

 a number of English sparrows, which had been inadvertently left 

 in a caged aquarium and had fallen an easy prey with the approach 

 of night. In nature, the bullfrog may devour young chicks and 

 ducklings if they blunder within reach. Generally, only living 

 food is taken by frogs and toads, the animals responding to the 

 moving object, although a frog will sometimes swallow the bodies 

 of dead animals if they are placed in its mouth. For this reason a 

 frog will snap at a fishhook dangling near its head, although such a 

 response usually means a speedy end. Similar responses to objects 

 in motion are typical of many other carnivorous animals. They 

 have doubtless been observed by the reader in fish, which " rise " 

 to the bait that falls across the water with just the right imitation 

 of the natural food; in the lizard, which darts forward only when 

 the fly begins to crawl; or in the stolid barnyard fowl, which eyes 

 suspiciously the insects lying motionless for a second or two after 

 they have been uncovered by scratching, but seizes them as soon 

 as they begin to make off. Motion of this sort is so invariable a 

 sign of something alive, and hence good to eat, that carnivorous 

 animals have come to respond automatically to such stimulation, 

 yet they can learn to make nice distinctions, as with the wily old 

 trout that feeds all day on his natural prey but never takes the most 

 cleverly thrown fly. 



Movements and Locomotion. — In contrast with the salamanders 

 (Fig. 1 B), which are more typical as four-footed vertebrates, the 

 frogs and toads exhibit a specialization of the Hmbs compar- 

 able with that seen in kangaroos, rabbits, the entire class of birds, 

 and in hmnan beings. These animals are not closely related as 



