FIG. 7-2 The beetle Dryops freshly submerged, crawling along 

 a stem, encased by a bubble of air (after Thorpe 1950). 



forms, especially the beetles and some of the hemi- 

 pterans, have evolved oar-like legs for rapid pro- 

 pulsion. 



Respiratory adaptations 



Air-breathing aquatic insects, as well as the 

 pulmonate aquatic snails Lymnea, Helisoma, Gyra- 

 nlus, Physa, Laevape.v, have evolved special mech- 

 anisms and behavior for respiration. Most species 

 rise to the surface of the water at intervals to re- 

 plenish their supply of air. Insects are so buoyant 

 that they must cling to the vegetation or some other 



object to maintain a submerged position. As soon as 

 they let go of the substratum, they float to the surface 

 and must return by swimming. Snails commonly 

 creep to the surface along plant stems or other suli- 

 merged objects, or suddenly emit mucous threads 

 that float them to the surface (Dr. Max Matteson, 

 personal communication). They find their way to the 

 surface, at times of oxygen need, by negatively geo- 

 tactic behavior. After they have obtained a fresh 

 supply of oxygen, they become positively geotactic 

 (Walter 1906). Pulmonate snails probably also ab- 

 sorb some oxygen from the water : indeed, some spe- 

 cies appear never to come to the surface. The gill- 

 bearing or branchiferous species of snails are seen to 

 be segregated into rather distinct niches when their 

 iiabitat relations are analyzed in detail (Baker 1919). 

 Diving beetles carry a bubble of air beneath the 

 elytra, and the entire body of Dryopa is enclosed in 

 air. The hemipteran notonectids and corixids carry 

 a bubble over the ventral surface of the body, trapped 

 there by hair-like setae. The spiracles of the tracheal 

 system open into these bubbles. The body surface 

 encompassed by and setae holding the bubble are 

 water-repellant, or hydrofugoits. The fresh air- 

 bubble contains 21 per cent oxygen and 78 per cent 

 nitrogen, the same proportion as the atmosphere. 

 The nitrogen dissolves into the water very slowly. 

 The carbon dioxide given off by the insect passes 

 quickly into the water. As the insect uses up the 

 oxygen in the bubble, it shrinks. The oxygen content 

 of the bubble may be reduced to one per cent, or less, 

 before the insect rises to the surface for a fresh 

 supply; in water containing little or no oxygen, ris- 

 ing may occur every three or four minutes. If the 

 water contains ample, however, oxygen will diffuse 

 into the bubble as rapidly as it is used, and perhaps 

 three times as fast as the nitrogen diffuses out. Un- 

 der these conditions backswimmers, Notonecta, have 

 survived for nearly 7 hours without coming to the 



""^ 



■^^ 



FIG. 7-3 Dragonfly niches (after Needham 1949): ( I ) on sand, 

 Macromia; (2) in sand, Gomphm; (3) in muck, Libellula, Neuro- 

 cordulia; (4) on massed W,7e//o, damselflies; (5) on tips of 

 Websferio, Enallagmo laurenfi; [b] in open tangles of blad- 

 derwort, Erythemis and damselflies; (7) in fallen brown leafage, 



Pachydiplax longipennis: (8) at sides of ditch, Tefragoneuria, 

 Cetifbemis, Eryfbrodlplax: (9) on invading roots of woody plants, 

 Argia fumipenn'n; (10) at water-line rooted green plants, dam- 

 selflies; (II) in rafts of fallen pine needles, aquatic Hemipfera 

 that are enemies of dragonfly naiads. 



Habitats, communities, succession 



