We have already remarked that this is one of the most neglected of the families of Coleoptera, and its classification is not satisfactory. It is usually divided into Hydrophilides and Sphaeridiides. The Sphaeridiides are in large part terrestrial, but their separation from the purely aquatic Hydrophilides cannot be maintained on any grounds yet pointed out. Altogether about 1000 species of Hydrophilidae are known, but this probably is not a tenth part of those existing. In Britain we have nearly ninety species. Some taxonomists treat the family as a series with the name Palpicornia. The series Philhydrida of older authors included these Insects and the Parnidae and Heteroceridae. [Illustration: Fig. 101—Larva of _Spercheus emarginatus_. (After Schiödte).] FAM. 13. PLATYPSYLLIDAE.—This consists of a single species. It will be readily recognised from Fig. 102, attention being given to the peculiar antennae, and to the fact that the mentum is trilobed behind. This curious species has been found only on the beaver. It was first found by Ritsema on American beavers (_Castor canadensis_) in the Zoological Gardens at Amsterdam, but it has since been found on wild beavers in the Rhone in France; in America it appears to be commonly distributed on these animals from Alaska to Texas. It is very remarkable that a wingless parasite of this kind should be found in both hemispheres. The Insect was considered by Westwood to be a separate Order called Achreioptera, but there can be no doubt that it is a beetle. It is also admitted that it shows some points of resemblance with Mallophaga, the habits of which are similar. Its Coleopterous nature is confirmed by the larva, which has been described by both Horn and Riley.[99] Little is known as to the food and life-history. Horn states that the eggs are placed on {220}the skin of the beaver amongst the densest hair; the larvae move with a sinuous motion, like those of Staphylinidae. It has been suggested that the Insect feeds on an Acarid, _Schizocarpus mingaudi_; others have supposed that it eats scales of epithelium or hairs of the beaver. [Illustration: Fig. 102—_Platypsyllus castoris._ A, Upper side; B, lower side, with legs of one side removed; C, antenna. (After Westwood.)] [Illustration: Fig. 103—_Leptinus testaceus._ Britain.]