The thorax of a chigoe is exceedingly short. Two reasons for this may be suggested. In the first place, the jumping power of these fleas is very small and the muscles in the thorax are consequently reduced. In the second place, the value to the insect of a contracted thorax is obvious; for the abdomen of a chigoe fixed on or in the skin of an animal does not project so much as it would were the thorax of normal length. The troublesome parasite is, therefore, less likely to be rubbed off by the host. In most fleas the piercing organs of the mouth (upper lip and mandibles) are directed obliquely downwards. In the chigoes they are directed obliquely forwards. It has been suggested that this forward movement of the mouth is connected with the stationary life which the females assume. Fleas which fasten themselves permanently to the skin of their host, do so in a manner similar to ticks. The mouth-parts are in a line with the longitudinal axis of the body. This attitude, so far as we know, is assumed by the females of all the family. The females of one genus, _Dermatophilus_, actually go head foremost right into the skin of their host. The shape of the head is also beautifully adapted to enable the insect to fix itself firmly in a tick-like posture. The fore-part is remarkably obtuse, and almost has the appearance of being truncate and abruptly cut off. When the piercing organs have been thrown forward horizontally it must be a great support to the insect, which is fixed by them, if it can press its head down firmly against the skin of the host. The wider the extent of forehead which can be pressed against the skin of the host the less the strain on the upper lip and mandibles, which serve as anchors, when the host scratches. All the chigoe family have eyes; but in one recently discovered species the eye is very small and devoid of pigment. Like those of other fleas, the antennæ fit into grooves at the side of the head, and the club, which is the sensitive part of the organ, consists of eight segments. In a good many fleas the antennæ are different in the two sexes, but there is no obvious sexual distinction in this family. The peculiar development of the mouth-parts is one of the most singular features in the structure of the chigoes and their relatives. These important organs are modified in a fashion not to be found in any fleas outside the family. Here, as in other fleas, the mandibles are piercing organs which penetrate the skin of the host, the upper lip serving in conjunction as a sucking tube. In ordinary fleas these organs are retracted when the insect has done its meal; in the present family they remain, in the case of the females, apparently permanently fastened in the skin. The piercing organs are broader and the serrate edges of the mandibles more solid and heavy in this family than in the case of other fleas.