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thern soon finds out, detect the presence of a moving object at a distance of at least twenty feet. When it be remembered that the surface of water in the sunshine produces a strony dazzle the acuteness of their vision seems the morę remarkable.

On the surface of the water, exposed to the fuli blaze of the sun, organisms are subjected to the strong influence of actinic light. This is not only derived directly from the sun's rays but indirectiy by reflection from the water. With the exception of snowfields and, possibly, of some deserts it is the most fiercely illuminated of all environments. The shade loving forms are not immune to the destructive action of light; Re-GIMBART indeed records that Orectochilus uillosus is killed in a few minutes by bright sunlight. It is, therefore, unlikely that any species has tissues which are immune to the destructive action of light. Species which swim in the open have always, I believe, a thick and deeply pigmented exoskeleton, often highly polished. Gyrinus, the most numerous and wide spread of open water genera, is densely pigmented and the majority of the species are glossy; South European species being morę aenescent than those of morę northerly distribution. This is noti-ceable even amongst individuals of the same species; the Southern races of G. substriatus and G. thomsoni are morę aenescent than the northern races.

The Gyrinidae are gregarious:    the instinct being best de-

veloped in diurnal forms. In the Genus Gyrinus the habit is strongly developed in all species with the possible exception of G. suffriani. Of that species I have captured only four spe-cimens and, although these were all solitary, the number is too smali to have any significance. In the other species which I have observed the degree of gregariousness appears to vary. G. urinator, which is usual found in running water and is to be taken in faster flowing streams than the other species of the genus, is often found solitary but morę usually occurs swimming in smali schools in which the individuals do not aggregate closely. G. paykulli appears to have the habit better developed but, even wEen they are numerous, schools of morę than a dozen rarely occur and they are poorly maintained, the individuals freąuently cruising off alone after a short interval. Their habit of swimming amongst reeds, no doubt. accounts for