The HEXAPODA, or true insects, are characterized by the fact that the adult possesses three pairs of legs. The body is distinctly segmented and is divided into head, thorax, and abdomen. The mouth-parts in a generalized form, consist of an upper lip, or _labrum_, which is a part of the head capsule, and a central unpaired _hypopharynx_, two _mandibles_, two _maxillæ_ and a lower lip, or _labium_, made up of the fused pair of second maxillæ. These parts may be greatly modified, dependent upon whether they are used for biting, sucking, piercing and sucking, or a combination of biting and sucking. Roughly speaking, insects may be grouped into those which undergo _complete metamorphosis_ and those which have _incomplete metamorphosis_. They are said to undergo complete metamorphosis when the young form, as it leaves the egg, bears no resemblance to the adult. For example, the maggot changes to a quiescent pupa and from this emerges the winged active fly. They undergo incomplete metamorphosis, when the young insect, as it leaves the egg, resembles the adult to a greater or less extent, and after undergoing a certain number of molts becomes sexually mature. Representatives of several orders have been reported as accidental or faculative parasites of man, but the true parasites are restricted to four orders. These are the Siphunculata; the Hemiptera, the Diptera and the Siphonaptera. SIPHUNCULATA The order SIPHUNCULATA was established by Meinert to include the true sucking lice. These are small wingless insects, with reduced mouth-parts, adapted for sucking; thorax apparently a single piece due to indistinct separation of its three segments: the compound eyes reduced to a single ommatidium on each side. The short, powerful legs are terminated by a single long claw. Metamorphosis incomplete. There has been a great deal of discussion regarding the structure of the mouth-parts, and the relationships of the sucking lice, and the questions cannot yet be regarded as settled. The conflicting views are well represented by Cholodkovsky (1904 and 1905) and by Enderlein (1904). [Illustration: 64. Pediculus showing the blind sac (_b_) containing the mouth parts (_a_) beneath the alimentary canal (_p_). After Pawlowsky.] Following Graber, it is generally stated that the mouth-parts consist of a short tube furnished with hooks in front, which constitutes the lower lip, and that within this is a delicate sucking tube derived from the fusion of the labrum and the mandibles. Opposed to this, Cholodkovsky and, more recently, Pawlowsky, (1906), have shown that the piercing apparatus lies in a blind sac under the pharynx and opening into the mouth cavity (fig. 64). It does not form a true tube but a furrow with its open surface uppermost. Eysell has shown that, in addition, there is a pair of chitinous rods which he regards as the homologues of the maxillæ.