FAM. 16. SCYDMAENIDAE.—_Minute Insects allied to Silphidae, but with the hind coxae separated, and the facets of the eyes coarser; the tarsi are five-jointed; the number of visible abdominal segments is six._ These small beetles are widely spread over the earth's surface, and about 700 species are now known, of which we have about a score in Britain; many live in ants' nests, but probably usually rather as intruders than as guests that have friendly relations with their hosts. Nothing is known as to their life-histories, but the food of the imago, so far as is known, consists of Acari. _Mastigus_ is a very aberrant form, found in moss and dead leaves in Southern Europe. By means of _Brathinus_ the family is brought very near to Silphidae; Casey, however, considers _Brathinus_ to belong to Staphylinidae rather than to Scydmaenidae. The South European _Leptomastax_ is remarkable on account of the slender, long, sickle-shaped mandibles. The Oriental genus _Clidicus_ is the largest and most remarkable form of the family; it has a very slender neck to its broad head, and is more than a quarter of an inch long. FAM. 17. GNOSTIDAE.—_Minute Insects with three-jointed antennae, five-jointed tarsi, and three apparent ventral segments, the first of which, however, is elongate, and consists of three united plates. Elytra entirely covering the after-body._ The family consists of two species which have been found in the nests of ants, of the genus _Cremastogaster_, in Brazil.[102] FAM. 18. PSELAPHIDAE.—_Very small Insects; the elytra much abbreviated, usually leaving as much as half the abdomen uncovered; the maxillary palpi usually greatly developed, and of a variety of remarkable forms; the segments of the abdomen not more than seven in number, with little or no power of movement. Tarsi with not more than three joints._ These small Coleoptera mostly live in the nests of ants, and present a great diversity of extraordinary {224}shapes, and very peculiar structures of the antennae and maxillary palpi. Owing to the consolidation of some of its segments, the abdomen frequently appears to have less than the usual number. In the curious sub-family Clavigerides, the antennae may have the joints reduced to two or even, to all appearance, to one; the tarsi suffer a similar reduction. There are about 2500 species of Pselaphidae known; many of them have never been found outside the ants' nests; very little, however, is known as to their natural history. It is certain that some of them excrete, from little tufts of peculiar pubescence, a substance that the ants are fond of. The secretory patches are found on very different parts of the body and appendages. _Claviger testaceus_ is fed by the ants in the same way as these social Insects feed one another; the _Claviger_ has also been seen to eat the larvae of the ants. They ride about on the backs of the ants when so inclined. The family is allied to Staphylinidae, but is easily distinguished by the rigid abdomen.