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. [Illustration: Fig. 8. Pregnant female of _Dermatophilus cæcata_, a South American chigoe which burrows into the flesh of the host. The abdomen swells until it surrounds the head and thorax, which are shown in the centre. The natural size is about equal to a small pea.] The swollen abdomen of the female chigoe is a strange and a conspicuous object, which is not, however, found occurring to the same extent in all the members of the family. Having burrowed into the flesh of a man, or other mammal, the pregnant female swells and enlarges until she reaches the gigantic proportions of a small pea. Itching and inflammation ensue unless the whole insect be skilfully removed with a needle. To such an extent does the abdomen swell that the segments and the horny plates are driven asunder and the connecting membrane between is exposed to view. In the extreme case of _Dermatophilus cæcata_ from South America the abdomen swells until it completely envelopes the head and thorax after the manner shown in Fig. 8. Dr Enderlein found seventeen specimens of this species in the skin behind the ears of a rat from Brazil.