Tracheate Arthropoda with the body divided into three regions', a head, thorax, and abdomen. The head carries a pair of antennae and three pairs of oral appendages, the thorax three pairs of jointed locomotor appendages, and in most instances one or two pairs of wings. Abdominal limbs are rarely present.

The head shows no trace of segmentation and is sharply marked off from the thorax, to which it is moveably connected by a neck in the imago. It is also generally distinct in the larva except in some Diptera. The thorax consists of a pro-, meso-, and nieta-thorax. The pro-thorax may be free, e. g. Coleoptera, many Rhynchota, but as a rule the three somites are firmly united. The pro-thorax is generally small, the meso-and meta-thorax either equal in size or one larger than the other, the variations depending on the peculiarities, etc. of the wings. The thoracic somites, especially the meso- and meta-thorax, are rarely ring-like (Aptery-gogenea), but have well-developed tergal and pleural regions. The sternal region varies much in size. The abdomen consists typically of eleven somites, but the number may be reduced in certain groups, e.g. to ten in Lepidoptera etc, to nine or eight in many Diptera. The successive somites are connected by soft intersegmental membranes, which admit of contraction and expansion of the abdomen in respiration and of its distension by the sexual products. Each somite of the abdomen may have the form of a ring (Apterygogenea; Plecopterd), or may have a firm tergal and sternal plate connected laterally by soft pleural membranes.

The shape of the abdomen varies much. The first somite may remain independent, e. g. Dermaptera, Ephemeridae, or it may lose its sternal plate whilst the tergal plate becomes connected to the meta-thorax, e. g. Coleoptera, Trichoptera, a change that may extend to more or fewer of the following somites. In Hymenoptera with a pedunculate abdomen the peduncle is formed by the contracted second, or second and third somites. The meta-thorax in Macro-Lepidoptera, Diptera, and Rhynchota Homoptera is so closely united to the first abdominal somite that in most instances it appears to be a portion of it, the meso-thorax being very large.

The head bears the antennae, mandibles, maxillae, labium or second maxillae. The antennae, processes as in Myriapoda of the procephalic lobes and not appendages, are borne on the margin of the head in front of the eyes in more primitive forms, or upon the vertex, i. e. summit of the head. They are jointed, and their length, shape, and other secondary characters are very variable. They lodge peculiar nerve-endings and appear to be olfactory in function. The mouth-parts may be similar in all stages of life and then either adapted for biting (Menognathd) or for sucking (Menorhyncha), or else in the larva they are adapted for biting, in the imago for sucking (Metagnathd), the change commencing in the pupal, and rarely affecting the larval, stage. The mandibles are one-jointed and never possess a palp, and are furnished in biting insects with a variously conformed biting or masticatory surface, whilst in sucking insects they may be reduced to a pair of stilets (i. e. are setaceous), e. g. Rhynchota, become rudimentary as in many Diptera, or even completely lost as in Lepidoptera with rare exceptions (see p. 150). The maxillae have a complicated structure when fully developed as in biting insects (see p. 140), and are provided with jointed palpi1. The two galeae are lengthened out, and each one forms one half of the antlia in Lepidoptera. In other sucking insects the maxillae may be reduced to stilets, e. g.

Rhynchotay may be lost as in some Diptera, their palpi being either lost or preserved. The labium is formed from the second pair of maxillae. The basal joints of the limbs are united invariably, but the remaining parts may be more or less completely retained or lost. It possesses jointed labial palpi rarely absent. In the Rhynchota it is modified into a four-jointed tube containing the mandibles and maxillae. An upper lip or labrum, the nature of which is not quite certain, forms a chitinoid plate well-developed in typical biting insects, and lies transversely in front of the oral cavity in continuity with the head. It usually bears on its oral surface an internal process or epipharynx. The oral surface of the base of the labium also bears an internal process or hypopharynx, beneath which the ducts of the labial salivary glands open. The labrum and hypopharynx are produced into stilets in some Diptera, and the fleshy proboscis of certain members of this group (e. g. Muscidae) is formed from the hypopharynx and exterior surface of the labium.

Varieties of detail in the structure of the mouth-parts are very numerous.

1 Cf. Beauregard on Vesicant Insects, Journal de l'Anat. and Physiol, xxii. 1886, p. 95 et seqq.

The mouth-parts are attached to the head by articulation as well as by muscles. In the Collembola (Apterygogened) as in Scolopendrella (see p. 519) the mouth-parts, mandibles as well as maxillae, are retracted within the head. They project from the head in other Insecta either forwards in a direction prolonging the axis through the so-called occipital foramen, or downwards in a direction at right angles to it. The insect in the first case is said to be ortho- or pro-gnath, in the second hypo-gnath, and the distinction applies equally to larva and imago.

The thoracic limbs consist typically of a coxa, trochanter, femur, tibia, and tarsus. The trochanter may be subdivided into two parts or fused with the femur. The tibia is often armed with spines or calcaria, and the tarsus is composed of a series of joints, usually not more than five in number. The terminal joint bears two bent hooks or claws, between which are often lodged one, two, or three membranous pads or pulvilli. Hairs often clothe the under surface of the tarsal joints, and in jumping and climbing Insects certain of them are connected to glands and exude a sticky fluid. The form, size, etc. of the limbs depend upon the mode of locomotion of the insect, e. g. running, swimming, jumping, etc. In addition to limbs the meso- and meta-thorax in the majority of Insecta give origin dorsally to wings. The Thysanura and Collembola appear to represent a primitive group of Insecta in which wings have never been developed; hence Insecta Apterygogenea. Other Insecta may be designated Pterygogenea, and wingless forms such as the Mallophaga, Siphonaptera, etc. must be regarded as having lost their wings.