ECOLOGICAL FACTORS 403 



indeed, some animals confine their diet entirely to them and are espe- 

 cially equipped with powerful claws and long sticky tongues to open 

 and plunder their nests. Whereas the woodpeckers are the only birds 

 known as ant eaters in the temperate zones, there is a host of ant 

 predators in the tropics. The mammals include the species of Myrme- 

 cophaga and the armadillos in South America, aardvarks {Oryctero- 

 pus) , pangolins, and a correspondingly adapted carnivore, the earth 

 wolf {Protcles cristatus) , in Africa, and Myrmecobius and other 

 marsupials in Australia. Insect-eating birds support themselves mostly 

 upon the swarming sexual forms and destroy them so thoroughly that 

 only a small fraction of the swarming individuals remains. There are 

 also birds that pick up the insects flushed by the ant armies on the 

 march. Many frogs and toads also live on the ants and termites; some 

 have become exclusive termite-eaters, such as the toad Rhinophrynus 

 dorsalis in Mexico. 87 Dragonflies prey upon the flying sexual forms, 

 and a number of ant species, especially among the large ponerines, 

 have become exclusive eaters of termites. 88 



A distinct superiority of the tropical fauna in number of species is 

 shown by the air-breathing vertebrates. The uniformly humid tropical 

 forests afford a very suitable habitat for amphibians. The burrowing 

 caecilians are confined to the tropics. The frogs are much more abun- 

 dant than in the temperate belts. All Europe has only 21 species of 

 tailless batrachians (exclusive of subspecies), while British Guiana 

 with only one-thirty-seventh as great an area has more than 50. The 

 fact that salamanders are almost entirely confined to the north tem- 

 perate regions is a conspicuous exception to the usual rule. Only a 

 single genus, Oedipus, reaches the tropics in Central America. 



Reptiles have their true home in the tropics (cf. p. 49) . Thirty-one 

 species of snakes 89 (again exclusive of subspecies) are found in 

 Europe; there are 38 in Trinidad which is less than one-two thousandth 

 as large. Indo-China and Siam together harbor 221 species of reptiles, 

 Borneo 207, Hindustan and Ceylon and Burma 536 species. The in- 

 crease of snakes from south to north in Australia is as follows: Tas- 

 mania has 3 species of snakes; Victoria, 22; New South Wales, 42; 

 and Queensland, 70. Birds are also much more abundant in the tropics 

 than elsewhere. Europe has only 257 species of land birds that are 

 permanent inhabitants and regular migrants; Borneo has more than 

 twice as many (580 species). The tropical areas are especially rich in 

 birds such as the flycatchers that capture exclusively flying prey. 

 Among the mammals, the apes and the lemurs and the edentates occur 

 principally in the tropics; of the bats, the fruit-eaters are entirely 

 confined to it. The insect-eating bats occur in much greater number 



