SWAMPS AND SHORES 471 



completely aquatic larval stages, with air-breathing adults, as in some 

 Neuroptera, and all may flies, stone flies, caddis flies and dragonflies. 

 A great number of Diptera have aquatic larval stages and even an 

 active aquatic pupal stage as in mosquitoes. These aquatic Diptera are 

 present in such great numbers, in both tropical and temperate zones, 

 that they afford an important food supply to many other creatures. 

 Lake Myvatn in Iceland derives its name from them. Its waters harbor 

 a host of mosquito larvae, whose adults hover over the water in dense 

 clouds in summer. The circumpolar tundra zone is infested with these 

 blood-sucking hordes, and the humid tropical regions are equally re- 

 nowned for their insect plagues. Numerous Hemiptera and Coleoptera 

 have carried these amphibious habits further, and have returned to the 

 water in the adult stage as secondarily aquatic animals. 



All the classes of vertebrates include animals with amphibious 

 habits. The water frequently serves only as a place of refuge from 

 drought, heat, or enemies, as for the common frog or the hippopotamus. 

 Conversely, the water may be the hunting ground, from which the 

 animals retire to land for the rest of their activities, as do the penguin 

 and otter. The amphibious members of the several classes of vertebrates 

 have acquired certain characters in common, such as the relation of 

 eyes, ears, and nostrils in the hippopotamus, crocodile, frog, and 

 Periophthalmus already mentioned (p. 299, Fig. 81) or the develop- 

 ment of webbing between the toes. 



Only a few fishes are in any sense amphibious. The gobies have 

 given rise to a number of forms which leave the water in active search 

 for food in tropical tide flats, notably Periophthalmus and Boleoph- 

 thalmus. Certain blennies (Alticus s aliens, of the tropical Pacific, for 

 example) have similar habits on rocky shores. Fishes like the lung- 

 fishes and mud minnows, which are able to aestivate in the mud of 

 dry swamps and ponds until the return of water necessary for their 

 activity, are more numerous. 



Amphibia are by implication amphibious. Some of them have be- 

 come secondarily completely aquatic. Most frogs and salamanders begin 

 life in the water and spend their adult life either on land or as air- 

 breathers in or near water. Some frogs, like the European Rana 

 esculenta, or the American bullfrog, Rana catesbiana, never wander 

 far from water. Others, like our common North American leopard frog, 

 inhabit fields and meadows during the summer. Similar variation in 

 the degree of attachment to water is found among the salamanders. 

 Hibernation in temperate climates and aestivation in the tropics are 

 the usual means of surviving unfavorable seasonal conditions. 



