354 ANIMALS IN INLAND WATERS 



ing; Ampullaria, Melania, Melanopsis, Bythinia thus survive the dry 

 season in African, south Asiatic, Australian, and South American 

 waters. It is perhaps for this reason also that in the inland waters of 

 East Africa the Pectinibranchiata, with 112 species, far outnumber 

 the inoperculate Limnaeidae, with 46 species. 86 In the temperate zones 

 the Limnaeidae take refuge in the mud when the water basins dry up, 

 and thus survive. Limnaea and Planorbis survive in temporary pools; 

 and small Limnaea came out of the dried-up ground of a flood region 

 of the lower Danube on the first day after water had been added. 

 Caddis-fly larvae up to a length of 8 mm. also were found in this mud, 

 and experiments with the larva of the dragonfly Libellula depressa 87 

 have shown that it can remain alive for 50 days or more in dry air, 

 and therefore can live in temporary water basins. 



Numerous kinds of fishes bury themselves in mud in the tropics. 

 In Ceylon, in a very few days after the beginning of the rainy season, 

 the natives fish industriously in the ponds and pools which were re- 

 cently dried up; 88 in Celebes, eels {Anguilla mauritiana) are dug out 

 of the dried-up clay of the rice fields during the dry season.* 89 Fishes 

 that possess special adaptations for air-breathing, the lungfishes Pro- 

 topterus, and Lepidosiren, have this same habit and live in similar 

 niches in Africa, South America, and Australia. In Europe, Misgurnus 

 fossilis buries itself in the mud when the water it inhabits dries up. 90 

 Carp and tench (Cyprinidae) have been found in the dry bottoms of 

 flood regions of the Danube; barbels can be dug out of the dried-up 

 brooks in Macedonia. 91 The common mud minnows of North America 

 (Umbra) survive in the mud of dried-up ponds in the same way. It 

 has recently been observed that the eggs of small, short-lived fish 

 (Cynolebias) can survive for five months in the muddy bottoms of 

 dried-up water basins, and can develop 92 with a renewed water supply. 



Finally, in such temporary water basins there are many inhabi- 

 tants which are only immigrants, such as water beetles and other 

 water insects, frogs, and salamanders, and these migrate to other 

 ponds when the water dries up. In tropical South America fishes simi- 

 larly migrate out of waters which are drying and travel across dry 

 land to new water basins, e.g., the catfishes Callichthys^ and Doras. 94 

 The frequently temporary nature of water basins in the tropics also 

 explains the fact that so many tropical frogs do not place their eggs 



* The ability to survive the drought by burying themselves in mud is found 

 among fishes of most diverse relationships; of the Symbranchia, Symbranchus 

 and Monopterus, in the rice fields of Java; in the Siluridae, Callichthys in South 

 America; and among Cyprinidae, Discognathus (near Aden). 



