The _Plecoptera_ contain only one family.
_Family 4, Perlidæ._--The _Perlidæ_, or Stone Flies, are chiefly
inhabitants of the temperate regions; they are of small size, and not
very numerous in species, about thirty being indigenous to the British
Isles.
The _larvæ_, which are aquatic in their habits, closely resemble the
perfect insect in form. Their food consists of other insects and they
prey especially upon the _larvæ_ of the Day Flies.
The perfect insects are usually found resting quietly on plants and
other objects on the banks of streams. The large brown species,
_Perla bicaudata_, appears in the spring, and is very common in the
neighbourhood of pools, streams, and canals.