SWIFT STREAMS 95 



to stones in the current, (ii) avoiding the current by creeping under 

 stones, (iii) self-maintenance by strong swimming powers. 



Upper surface of stones (stratum 1) : Here again we find the black-fly 

 larvae, particularly in the smaller streams. They are provided at the 

 posterior end of the body with a sucker surrounded with hooks (Figs. 

 27-32). The salivary glands are, as is common in insects, modified into 

 silk glands and the silk is of such a nature that when it is brought into 

 contact with a stone it adheres. The animals are usually found attached 

 to the rock by the sucker, with the head downstream. The fans are 

 extended and serve to catch diatoms and other floating algae. If for 

 any reason the sucker gives way, the animal starts to float downstream. 

 If the mouth can be brought into contact with a stone, the silk is exuded 

 and the animal is held until it can make the sucker fast again. The 

 pupae of this fly are also attached to the stones. They are surrounded 

 with a cocoon. We have removed them from the stream and have 

 found that they cannot make this cocoon in the absence of the current, 

 but make a shapeless tangle instead. The adults deposit their eggs at the 

 sides of the streams (96). 



On the tops of stones caddis- worms (Hydropsyche sp.) usually have 

 cases made of pebbles stuck together with silk (Figs. 39, 40). They also 

 have a net for catching floating food. The net faces the current (usually 

 upstream) (Fig. 40). The river snail (Goniobasis livescens) (Fig. 54) is 

 common on the upper surfaces of the larger rocks and is distinguished 

 by a strong adhesive foot. These snails are usually headed upstream. 

 When placed in a long piece of eave-trough into which the tap water was 

 running at one end, they nearly all made their way to the upper end 

 within a short time. They are ecologically equivalent to the caddis-worms 

 and the black-fly larvae. 



Among the stones (stratum 2) : Of the animals living among stones, 

 the darters are most important. Of these the banded darter (Etheostoma 

 zonale) (Fig. 44), the fan-tailed darter (E. flabellare), and the rainbow 

 darter (E. coeruleum) (97) (Fig. 45) live among and under the stones or 

 in the algae which cover the rocks (especially the fan tail). With them 

 are sometimes found the Johnny darter (Boleosoma nigrum), the black- 

 sided darter (Hadropterus as pro) (Fig. 46), and the small bullhead or 

 stonecat (Schilbeodes exilis). These fish are all positively rheo tactic. 

 They apparently orient because of unequal pressure on the two sides of 

 the body when it is not parallel with the direction of the current. 



Under the stones (stratum 3): There are many more forms living 

 under and among the stones than on the tops of them. Here are the 



