226 THE ESSEX NATURALIST. commonly to be found on our ponds, upon the surface of which they can walk and jump with the greatest facility and impunity. As might be expected their bodies are extremely water-repellent, it being quite impossible to make them wet except by first putting them in almost absolute alcohol. In Isotomura palustris the body and legs are covered with a dense coating of hairs, but in Podura aquatica and Sminthurides aquaticus the hairs can only be described as scattered. The surface of the skin in the last-named two species, however, is covered with very small conical or rounded papillae which may possibly have some relation to the wonderfully water- repellent nature of these animals' bodies. The saltatory appen- dage does not seem to have been specially modified for life on the surface of water, except in the case of Sminthurides aquaticus where the leaf-like terminal expansions are certainly larger than in other species of the genus. Another peculiar feature possessed by the Spring-tails is a kind of sucker or adhesive organ which projects from about the middle of the underside of the body and by means of which the animals can adhere to solid bodies, such as the glass sides of an aquarium. It is not known whether this organ has any special function when the creatures are walking and leaping on the surface, but it most certainly enables them to adhere to the stems of water-plants, &c., if they hit them when taking their flying leaps in the air. The value of being able to spring about on the surface is no doubt very great, for these spring-tails are known to be the prey of the Pond- skaters and probably also of other aquatic animals. The animals which swim on the surface are the Whirligig beetles (Gyrinidae). Although when submerged they are lighter than water, owing to the air which they take down with them, when at the surface they undoubtedly make use of the surface-film for support. As can be seen through field-glasses and also demons- trated by the shadow method already referred to, there is normally one large and approximately semicircular depression at each end of the body and an irregular number of much smaller depressions along the sides. The outlines of the larger depressions, especially the posterior one, often show a tendency to be notched in the middle, as though they were the result of two coalescing depressions. The Whirligig beetles are very specially modified for life both on and under the water. Their fusiform bodies are encased in highly