surface. The Imlilile is really a physical gill niecha- 

 iiism, hut functions only as long as tlie nitrogen 

 present provides an adeciuate surface for oxygen dif- 

 fusion. The insect's trip to the surface is as much to 

 get a fresh supply of nitrogen as it is to get a fresh 

 supply of oxygen (Wolvekamp 1955). 



The air-breathing respiratory mechanisms of 

 other aquatic insects are equally remarkable. In 

 many larvae, Pytisciis. Culicidae. and other Diptera, 

 and in tlie a(|uatic Hemiptera, only the terminal ab- 

 dominal spiracles are functional. The tracheal trunks 

 of mosquito larvae and Dytisciis larvae among others, 

 store considerable air so that the animal may remain 

 submerged for long ])eriods. Ranatra and other 

 N'epidae have long respiratory tubes extending from 

 the tip of the abdomen so that they can cling to vege- 

 tation well below the water surface yet respire di- 

 rectly into the air. 



Dragonfly naiads ]Hini]i water through the anus, 

 in and out of an enlarged rectum. The walls of the 

 rectum are abundantly supplied with a network of 

 tracheae for interchange of gases directly with the 

 water. In the larvae of midge flies, black flies, and 

 corixid beetles, the general body surface is richly 

 supplied with fine tracheae for exchange of gases di- 

 rectly with the water. The anal papillae of midge 

 flies and mosquito larvae are not respiratory in func- 

 tion, as formerly supposed, rather they serve for 

 osmoregulation. Tracheal gills, plates, or filaments 

 are found on many immature insects, Ephemeridae. 

 Plecoptera, Zygoptera. Trichoptera, Neuroptera, and 

 some Diptera, and effectively increase the area of 

 surface available for oxygen absorption. The larvae 

 of the beetle Donacia and certain Diptera including 

 mosquitoes have a unique ability to puncture the 

 walls of submerged plants and collect air from the 

 intercellular spaces (Miall 1934). 



tom-feeders — suckers, bullheads, bufifalo, and carp; 

 the last, a species introduced from Europe. By feed- 

 ing on the submerged vegetation and stirring up the 

 bottom they may control the habitat and the compo- 

 sition of species present in the community. This con- 

 dition, however, does not last indefinitely. Vegetation 

 encroaches on the margins of ponds, and the fish are 

 gradually eliminated because of the disappearance of 

 suitable breeding sites. The mudminnow, bowfin, and 

 bullhead are usually the last to disappear before the 

 pond becomes a dry marsh (Shelford 1911). 



Amphibians and reptiles 



Terrestrial invertebrates 



The terrestrial insects found in marsh vegeta- 

 tion are in the main adult mosquitoes, midges, 

 dragonflies, damselflies. mayflies, and alderflies, whose 

 immature stages live submerged. On bare ground 

 around ponds may be found toad bugs, shore bugs, 

 springtails, tiger beetles, and sometimes ground 

 beetles and pigmy locusts. Spiders become numerous 

 throughout the vegetation, and the snail Succinea ap- 

 pears. In addition to these true marsh and pond spe- 

 cies, invertebrates belonging to the forest-edge biocies 

 may occasionally be found. 



Fish 



Fish are often very abundant (Table 7-^). In- 

 cluded among the species that occur are several bot- 



Salamanders and frogs are basically aquatic 

 animals, although they show varying degrees of adap- 

 tation to terrestrial life. Siren and A'cctiirus have 

 permanent external gills and spend all their lives in 

 the water. Most other forms lay their eggs and pass 

 through their earlier development in water, but the 

 adults are air-breathing and wander over the land. 

 Since their skins must be kept moist, they are con- 

 fined to the vicinity of water, to humid climates, or 

 to damp humus. A few species, such as Plcthodon 

 cincreiis and P. glutinosns. lay their eggs in the cavi- 

 ties of well-rotted logs and seem largely non-depend- 

 ent on standing water. The ability of salamanders 

 and frogs to live temporarily away from water ap- 

 pears positively correlated with thickness, cornifica- 

 tion, and relative impermeability of the skin. The 

 aquatic tadpoles and larvae are scavengers or herbiv- 

 orous in their food habits, the adults feed on in- 



Ponds, marshes, swamps, and bogs 85 



