When the piercing organs are at rest they are partly retracted. The external portion is encased in the tubular rostrum. The tube is formed by the two labial palpi which are situated at the apex of the short non-divided labium. The number of segments composing each labial palpus in fleas varies, so far as we know, from two to seventeen. In most fleas, however, the labial palpus consists of five segments. This appears to have been the original state of things in the ancestral flea; the palpus with more and the palpus with less segments being derived from the normal five-jointed one. The rostrum of a flea is not a piercing organ like that of a fly and a bug. The two labial palpi separate and lie flat, right and left, on the skin when the true piercing organ is driven into the host. The labial palpi therefore require to be flexible, and this is attained by increasing the number of segments or by reducing the amount of chitinisation or horniness. We shall find in the chigoes and their allies a rostrum which is pale, weak, soft and scarcely horny. Among other fleas where the rostrum is prolonged and strongly chitinised we shall find greater segmentation. The small bristles at the extreme tip of the rostrum seem to be sensory organs. They are like those at the apex of the maxillary palpus. When a hungry flea is put on one’s arm, it appears to test the skin with these bristles before it ventures to make a puncture. _The mandibles._ These are a pair of sharp lancets with serrated edges. They make the puncture and are interlocked with the labrum to form a sucking tube. _The labrum._ This is the central portion of the mouth-parts and is in fact a prolongation of the upper lip of the flea. It is a hard, sharp, awl-like instrument: in shape like a horny trough. Its edges are more or less toothed. Its apex is pointed and it is as long as the mandibles. The general appearance and the relative positions of the mouth-parts are shown in Fig. 4. [Illustration: Fig. 4. Diagram of the mouth-parts of a flea. The slender awl-like structure at the top is the _labrum_. Beneath are the paired _mandibles_ with serrated edges. The four-jointed hairy _maxillary palpus_ is below, only one being shown. Protruding from the base of the face is the _labium_ which supports the jointed _labial palpi_. The flat obtuse triangular structure from which the palpus springs is the right-hand _maxilla_. The left maxilla is concealed behind.]