distribiiliuii ( klc \':^3S, Spriilcs I'M/). Linear dis- 

 tribution of tisli in streams may be, in part, a result 

 of ditTerences in temperature tolerance. Brook trout, 

 for instance, do best in waters cooler than IQ^C in 

 \'irginia, wiiiie some varieties of introduced rainbow- 

 trout prefer waters above 19°C (Burton and Odum 

 1''45). The altitudinal zonation of various species of 

 invertebrates and fish in mountain streams is well 

 detined, and is in large part contingent on differences 

 in temperature ( I )odds and Hisaw 1025). 



Sl,a,,r uml 



(if indit iiliials 



In tl'.e Tennessee River, riffles snails of the 

 genus lo show a progressive change in shape from 

 the iieadwaters on downstream. There is a decrease 

 in shell diameter, a decrease in globosity, and an in- 

 crease in number and length of spines (Adams 191 .S). 

 flowever, the riffles snail Plcitroccra was found to in- 

 crease in globosity downstream in Michigan (Good- 

 rich 1937). Some pond snails, such as Lymnaea 

 staanailis and Galba paliistris. develop a larger foot 

 and shell aperture when exposed to wave action 

 (Baker 1919). Primitive types of clams, on the other 

 hand, such as Fnscoyiaia. Amblcma, Qiiadnila. Plciir- 

 obcitia. and others, change progressively downstream 

 from a large, compressed, smooth shell to one that 

 is shorter, more obese, and sculptured with tubercles 

 ( Ortman 1 920 ) . Some species of clams show no such 

 changes in shape. In some fish of central Asia (Ni- 

 kolski 1933), the body changes downstream from a 

 torpedo-shape to a flatter, longer form. These 

 changes are probably a result of downstream reduction 

 of water current, increase in amount of calcium in the 

 water, and higher temperatures. The formation of 

 spines and tubercles, for instance, would require an 

 abundance of calcium and quiet water. 



EVOLUTION 



In all probability species inhabiting quiet 

 waters are ancestral to those occurring in running 

 waters (Dodds and Hisaw 1925, Hora 1930). In- 

 vasion of stream habitats requires mechanisms for 

 contending with the force of current, and orientation 

 behavior for maintaining position. Convergent evolu- 

 tion has occurred in many kinds of animals under the 

 influence of current, as shown by similarities in struc- 

 ture and habits (Shelford 1914a). Inducements to 

 the invasion of swift waters have doubtless been new 

 sources of food, escape from enemies, and avoidance 

 of competition with the abundant life of lakes and 

 ponds. As adaptations to stream habitats evolved, ani- 

 mals have largely lost their ability to occupy quiet 

 waters. They no longer can tolerate the lower oxy- 



j;en tension, silt buttums, and the absence of current 

 which brings them food and o.xygen, and, in some 

 forms, such as the Hydropsychidae, helps build their 

 shelters and nests. 



I.ll K lil.vrOKIK.S 



Tiie life-cycle of stream insects is reniark- 

 .■d)le for the long duration of the immature stage in 

 many species and the brief life of the adult. The 

 naiads of mayflies pass through a number of molts 

 (20-40), and this immature stage may last from six 

 weeks to two years. When ready to emerge, the naiad 

 comes to the water surface or crawls out onto a stone, 

 molts into a subimago, and flies away. Within a few 

 minutes, or a period of one to two days at the longest, 

 the subimago undergoes another molt, miique in in- 

 sects, into the fully mature adult. The adult insect 

 does not eat and lives only a few hours or days ; dur- 

 ing this time reproduction takes place. Mating oc- 

 curs in flight, hundreds or thousands of individuals 

 swarming in flight together. The females lay their 

 eggs almost immediately after mating. In some spe- 

 cies, deposition is made upon the water surface. 

 the eggs sinking to the bottom : in other species the 

 female crawls down into the water and attaches the 

 eggs, as they are laid, to a rock surface. The eggs 

 have a viscid surface or filaments and (juickly be- 

 come attached to submerged objects. Embryonic life 

 may last 11 to 23 days, at the end of which time the 

 naiad is fully formed (Needham et al. 1935, Burks 

 1953. Hunt 1953). 



The life-cycle of stoneflies is also 1 . 2, or possibly 

 3 years long in different species, of which time all 

 but a brief interval is spent in the water (Prison 

 1935). Molting into the adult occurs after the naiad 

 crawls out of the water onto a rock or other project- 

 ing object, and there is no subsequent molt in the 

 adult stage. Adult diurnal stoneflies may feed, al- 

 though the adults of nocturnal species apparently do 

 not. It is of great interest that many species emerge, 

 mate. feed, and carry on all essential activities during 

 the coldest months of the year (Prison 1935). At al! 

 seasons, the eggs may be dropped into the water while 

 the female is in flight over the water, or as she alights 

 on its surface. The eggs are mucilaginous and may 

 contain surface filaments or hooks. 



Caddisfly larvae pupate submerged in cases. As 

 the pupa approaches the adult form, it leaves the 

 case ; and. after crawling and swimming, emerges 

 either upon the water surface or on some protruding 

 object. Larval life in different species may be as short 

 as 25 to 80 days, but since overwintering occurs in 

 this stage it may be greatly prolonged. The pupation 

 period is ordinarily shorter than the larval period, 

 and the adults, which probably feed, may live from 



Streams 53 



