ECOLOGICAL FACTORS 413 



burrowing insects, such as the mole cricket (Gryllotalpa) , the bumble- 

 bees, spiders, amphibians, snakes, and mammals. Associated with these 

 active burrowers are many other species which are unable to burrow 

 extensively for themselves but occupy the excavations made by others. 



It is evident that solid rock offers no habitat for such animals; 

 no air-breathing animals can burrow into rocks as do the boring 

 mussels and sea urchins. Other soils, composed of more or less compact 

 particles, are favored by various animals according to their compact- 

 ness. Clay soils are too hard for many burrowers; the rabbit, for 

 example, does not dig readily in clay; butterfly larvae and beetle 

 larvae are seldom found there. However, many animals build their 

 nests in clay because a certain durability is supplied by the firmness 

 of the earth. Perpendicular clay walls are often perforated by the 

 brood burrows of bees and wasps. Birds likewise establish their nests 

 in such clay walls, particularly if near water, as for example, bank 

 swallows and kingfishers {Alcedo) . Since such places are limited, these 

 birds nest together and often in such great numbers that the cliffs are 

 riddled. The nests of the bank swallows are so close together in the 

 Transcaspian oases Pinde and Mero that there is not more than 3 cm. 

 between them, 127 and on the banks of the Essequibo (British Guiana) 

 the four species of kingfishers (C hloroceryle amazona, aenea, inda, 

 americana) breed close together in entire harmony. 



Humus of more or less spongy nature is the especial habitat of 

 the earthworms, enchytraeids, and nematode worms; nevertheless, it 

 must not contain free humic acid. If it is especially spongy, mole 

 crickets and insect larvae may also occur in it; however, the last are 

 also found in sand, for example, in dunes. Earthworms and insect 

 larvae compose the greater part of the food of the moles, which share 

 their subterranean habitat. In humus and sandy humus soils, the 

 pocket gopher and ground squirrels build their tunnels ; mice and voles 

 have their runways here, as do the larvae of many of the tiger beetles 

 (Cicindela) which build round burrows in which to lie in wait for 

 their prey. Many predatory and solitary wasps (Cerceris, Bembex, 

 Ammo'phila) dig their brood holes in sand. In South America and in 

 hot parts of Africa the sand flea {Sarcopsylla) lives in sandy areas. 

 The ant lion (larva of Myrmeleon) forms its pit traps in the sand; 

 rhagionid fly larvae of the genus Vermileo build similar sand pits. 

 Tenebrionid beetles burrow into the sandy soil of the African dune 

 regions and in the American desert. In certain places, these sand in- 

 sects determine the character of the fauna. 128 Many reptiles conceal 

 themselves in sand, like the skink (Scincus) and the horned viper 

 (Cerastes) . Sea birds, including many petrels, dig their brood holes in 



