Why is the water-flea so called?

Because his abode is in quiet, still water; though, in mild weather, we see him gamboling upon the surface of the sheltered pool, in his shining black jacket. Retiring in the autumn, and reposing all the winter in the mud at the bottom of the pond, he awakens in the spring, rises to the surface, and commences his summer sports. They associate in small parties of ten or a dozen, near the bank, where some little projection forms a bay, or renders the water particularly tranquil; and here they will encircle round each other without contention, each in his sphere, and with no apparent object, from morning until night, with great spright-liness and animation; and so lightly do they move on the fluid, as to form only some faint and transient circles on its surface. - Knapp.