Flow to obtain it. 211 
hide, and this they do under stones, or water plants, or anything 
that comes handy, and they feed chiefly at night. 
The most favourable places for them to thrive and increase 
seem to be shady, overgrown, shallow streams, where they are 
seldom disturbed, and in such places they multiply very rapidly 
indeed. I have met with them, too, in reservoirs and lakes, 
where they also seem to thrive ; and in open brooks they usually 
find plenty of shelter amongst submerged marginal plants or their 
roots, or under stones, etc. Aquatic birds generally know where 
to find them, and should not be allowed to enter any places where 
gammari are cultivated. In open streams and lakes it does not 
matter so much, as they can there take care of themselves. It is 
most suggestive of shrimps to see the way in which a water hen, 
for instance, goes poking about in their hiding places among the 
marginal grasses, or the stones at the head of a shallow. Decaying 
animal and vegetable matter constitutes the food of this crustacean, 
and it seems to thrive equally well in cold spring water or in that 
of a higher temperature, though it usually attains a rather larger 
size in water that warms up well in the summer. 
Fig. 28. Dytiscus marginalis and larva. 
There are a great many aquatic beetles, but they do not 
possess a very high food value. They are hard cased and exceed- 
ingly voracious, and probably destroy far more food than they 
yield, so there is no economy in encouraging their presence. The 
two largest are Dytiscus marginalis and Hydrophilus piceus. The 
former is carnivorous and very fierce, and will attack and kill fish 
up to the size of minnows, both in its larval and perfect form. As 
a beetle it is well known, being a conspicuous object, but in its 
