SWIFT STREAMS 95 
to stones in the current, (ii) avoiding the current by creeping under 
stones, (ili) 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 
sonale) (Fig. 44), the fan-tailed darter (EF. 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 fantail). With them 
are sometimes found the Johnny darter (Boleosoma nigrum), the black- 
sided darter (Hadropterus aspro) (Fig. 46), and the small bullhead or 
stonecat (Schilbeodes exilis). These fish are all positively rheotactic. 
_ 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 
