THE ANIMAL COMMUNITY 67 



South America there is the mussarama, a large snake four or 

 five feet in length, which is not itself poisonous, but preys 

 exclusively upon other snakes, many of which are poisonous, 

 being itself immune to the venoms of lachesis and rattlesnake, 

 but not to colubrine poisons. In the United States the niche 

 is filled by the king-snake which has similar habits, while in 

 India there is a snake called the hamadryad which preys upon 

 other (in this case non-poisonous) snakes.®^* 



19. Another widespread niche among animals is that 

 occupied by species which pick ticks off other animals. For 

 instance, the African tick-bird feeds entirely upon the ticks 

 which live upon the skin of ungulates, and is so closely de- 

 pendent upon its mammalian " host " that it makes its nest 

 of the latter's hair {e.g. of the hartebeest).^^^ In England, 

 starlings can often be seen performing the same office for sheep 

 and deer. A similar niche is occupied on the Galapagos Islands 

 by a species of scarlet land-crab, which has been observed 

 picking ticks off the skin of the great aquatic lizards {Ambly- 

 rhynchus).^^^ Another niche, rather analogous to the last one, 

 is that occupied by various species of birds, which follow 

 herds of large mammals in order to catch the insects which are 

 disturbed by the feet of the animals. Chapman ^^^ saw 

 elephants in the Sudan being followed by kites and grey 

 herons ; Percival i^s says that the buff-backed egret follows 

 elephants and buffalo in Kenya for the same purpose ; in 

 Paraguay ^^^ there are the Aru blackbirds which feed upon 

 insects disturbed by the feet of cattle ; while in England 

 wagtails attend cattle and sheep in the same way. 



20. There is a definite niche which is usually filled by earth- 

 worms in the soil, the species of worm differing in different 

 parts of the world. But on coral islands their place may be 

 largely taken by land- crabs. Wood- Jones ^^^^ states that on 

 Cocos-Keeling Island, coconut husks are one of the most 

 important sources of humus in the soil, and in the rotting 

 husks land- crabs (chiefly of the genus Cardiosoma) make 

 burrows and do the same work that earthworms do in our own 

 country. (There are as a matter of fact earthworms as well 

 on these islands.) On the coral reefs which cover such a large 



