The chigoes and their allies are of special interest for more than one reason. The females are to a greater or less degree stationary; they fix themselves firmly to their hosts and become veritable parasites. Several of the earlier zoologists believed that the animal was a mite; and it is somewhat remarkable that Oviedo himself should have so promptly detected the relationship of the insect he saw with the fleas. By reason of the parasitic habits of the females, more is known about their appearance and life than in the case of the more active males. In some species the males remain, for the present, quite unknown: and not very much is recorded about the early life-history, eggs, larvæ and pupæ of either sex. The parasitic habits of the chigoes and other allied fleas lead one to expect peculiar modifications of form such as are usually to be observed when an animal passes from an active to a stationary life. These modifications are the more easily understood as the various species are not all stationary to the same degree. It is fairly plain that this family of fleas is a development from the less specialised and less parasitic family Pulicidæ. In fact the gradual development of the organs from a generalised to a more specialised stage is strikingly shown in these insects. To follow this in detail would, however, require a very minute and technical knowledge of their form.
The chigoe family is so well characterised that a student of fleas cannot possibly have any doubt whether a flea belongs to this family or not. Yet there is great diversity in general appearance, as well as in details of structure. One very peculiar character, namely the enormous swelling of the abdomen in pregnant females is, moreover, shared with certain other fleas. The most distinguishing character of the family, however, is the rostrum. This organ, which it may be well to remind the reader, consists of the under-lip and the labial palpi, sheaths the piercing and sucking mouth-parts. In the chigoes and their allies the rostrum is reduced, not in length, but in stiffness and in number of segments. In this group there are never more than three segments to the rostrum, whilst in the main group of fleas, with one or two exceptions such as the rabbit-flea (Spilopsyllus cuniculi), there are never less than five. There is no indication of a comb on the head, but all the family, without exception, have a large triangular projection, which is more or less curved backwards, at each side of the head. These two organs doubtless discharge the same functions as the combs of other fleas, and prevent the insect from slipping back as it works its way through the fur or feathers of the host.
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.
The two methods by which fleas keep in touch with their hosts have already been alluded to. The two main fixing and clinging organs are the mouth and the claws. Weak mouth-parts accompany strong legs. We observe, accordingly, two lines of development. The chigoes and their allies present an extreme case: for the legs are practically useless for holding on. The bristles, and the claws as well, are exceedingly thin. In this family the mandibles serve the purpose of claws. The other line of development is best seen in a genus of fleas from South America (Malacopsylla), where the piercing organs are short and weak, whilst the claws and bristles of the legs are very strongly developed.
The modification found in the rostrum of the chigoes has already been referred to, and the explanation of this will now be understood. The rostrum is a sheath, on either side of the piercing organs, consisting of an under lip and two labial palpi. When the flea sucks, the labial palpi are pushed asunder, as the piercing organs are driven in, and lie flat on the skin of the host. In this family the rostrum is almost white in colour and soft instead of being horny or chitinised. Where the rostrum is strongly chitinised or very horny the flea has to use a certain amount of force to counteract the spring-like action of the labial palpi. It is conceivable that rigid labial palpi would prove inconvenient to fleas which remain permanently attached to their host by their mouth organs.