Insect mouthparts

The mandibles of a bull ant

European honeybee (Apis mellifera) lapping mouthparts, showing labium and maxillae

A chewing insect has a pair of mandibles, one on each side of the head. The mandibles are caudal to the labrum and anterior to the maxillae. Typically the mandibles are the largest and most robust mouthparts of a chewing insect, and it uses them to masticate (cut, tear, crush, chew) food items. Two sets of muscles move the mandibles in the coronal plane: abductor muscles move insects' mandibles apart (laterally); adductor muscles bring them together (medially). This they do mainly in opening and closing their jaws in feeding, but also in using the mandibles as tools, or possibly in fighting; note, however, that this refers to the coronal plane of the mouth, not necessarily of the insect's body, because insects' heads differ greatly in their orientation.

In carnivorous chewing insects, the mandibles commonly are particularly serrated and knife-like, and often with piercing points. In herbivorous chewing insects mandibles tend to be broader and flatter on their opposing faces, as for example in caterpillars.

In males of some species, such as of Lucanidae and some Cerambycidae, the mandibles are modified to such an extent that they do not serve any feeding function, but are instead used to defend mating sites from other males. In some ants and termites, the mandibles also serve a defensive function (particularly in soldier castes). In bull ants, the mandibles are elongate and toothed, used both as hunting and defensive appendages. In bees, that feed primarily by the use of a proboscis, the primary use of the mandibles is to manipulate and shape wax, and many paper wasps have mandibles adapted to scraping and ingesting wood fibres.

Situated beneath (caudal to) the mandibles, paired maxillae manipulate and, in chewing insects, partly masticate, food. Each maxilla consists of two parts, the proximal cardo (plural cardines), and distal stipes (plural stipites). At the apex of each stipes are two lobes, the inner lacinia and outer galea (plurals laciniae and galeae). At the outer margin, the typical galea is a cupped or scoop-like structure, located over the outer edge of the labium. In non-chewing insects, such as adult Lepidoptera, the maxillae may be drastically adapted to other functions.

Unlike the mandibles, but like the labium, the maxillae bear lateral palps on their stipites. These palps serve as organs of touch and taste in feeding and in the inspection of potential foods and/or prey.

In chewing insects, adductor and abductor muscles extend from inside the cranium to within the bases of the stipites and cardines much as happens with the mandibles in feeding, and also in using the maxillae as tools. To some extent the maxillae are more mobile than the mandibles, and the galeae, laciniae, and palps also can move up and down somewhat, in the sagittal plane, both in feeding and in working, for example in nest building by mud-dauber wasps.

Maxillae in most insects function partly like mandibles in feeding, but they are more mobile and less heavily sclerotised than mandibles, so they are more important in manipulating soft, liquid, or particulate food rather than cutting or crushing food such as material that requires the mandibles to cut or crush.

Like the mandibles, maxillae are innervated by the subesophageal ganglia.

The labium typically is a roughly quadrilateral structure, formed by paired, fused secondary maxillae.[1] It is the major component of the floor of the mouth. Typically, together with the maxillae, the labium assists manipulation of food during mastication.

Dragonfly nymph feeding on fish that it has caught with its labium and snatched back to the other mouthparts for eating. The labium is just visible from the side, between the front pairs of legs.

The role of the labium in some insects, however, is adapted to special functions; perhaps the most dramatic example is in the jaws of the nymphs of the Odonata, the dragonflies and damselflies. In these insects, the labium folds neatly beneath the head and thorax, but the insect can flick it out to snatch prey and bear it back to the head, where the chewing mouthparts can demolish it and swallow the particles.[2]

The labium is attached at the rear end of the structure called cibarium, and its broad basal portion is divided into regions called the submentum, which is the proximal part, the mentum in the middle, and the prementum, which is the distal section, and furthest anterior.

The prementum bears a structure called the ligula; this consists of an inner pair of lobes called glossae and a lateral pair called paraglossae. These structures are homologous to the lacinia and galea of maxillae. The labial palps borne on the sides of labium are the counterparts of maxillary palps. Like the maxillary palps, the labial palps aid sensory function in eating. In many species the musculature of the labium is much more complex than that of the other jaws, because in most, the ligula, palps and prementum all can be moved independently.

The labium is innervated by the sub-esophageal ganglia.[3][4][5]

In the honey bee, the labium is elongated to form a tube and tongue, and these insects are classified as having both chewing and lapping mouthparts. [6]

The wild silk moth (Bombyx mandarina) is an example of an insect that has small labial palpi and no maxillary palpi.[7]

The hypopharynx is a somewhat globular structure, located medially to the mandibles and the maxillae. In many species it is membranous and associated with salivary glands. It assists in swallowing the food. The hypopharynx divides the oral cavity into two parts: the cibarium or dorsal food pouch and ventral salivarium into which the salivary duct opens.

The proboscis, as seen in adult Lepidoptera, is one of the defining characteristics of the morphology of the order; it is a long tube formed by the paired galeae of the maxillae. Unlike sucking organs in other orders of insects, the Lepidopteran proboscis can coil up so completely that it can fit under the head when not in use. During feeding, however, it extends to reach the nectar of flowers or other fluids. In certain specialist pollinators, the proboscis may be several times the body length of the moth.

The defining feature of the order Hemiptera is the possession of mouthparts where the mandibles and maxillae are modified into a proboscis, sheathed within a modified labium, which is capable of piercing tissues and sucking out the liquids. For example, true bugs, such as shield bugs, feed on the fluids of plants. Predatory bugs such as assassin bugs have the same mouthparts, but they are used to pierce the cuticles of captured prey.

A mosquito biting a human finger

In female mosquitoes, all mouthparts are elongated. The labium encloses all other mouthparts like a sheath. The labrum forms the main feeding tube, through which blood is sucked. Paired mandibles and maxillae are present, together forming the stylet, which is used to pierce an animal's skin. During piercing, the labium remains outside the food item's skin, folding away from the stylet. Saliva containing anticoagulants, is injected into the food item and blood sucked out, each through different tubes.

The housefly is a typical sponging insect. The labellum's surface is covered by minute food channels, formed by the interlocking elongate hypopharynx and epipharynx, forming a proboscis used to channel liquid food to the oesophagus. The food channel draws liquid and liquified food to the oesophagus by capillary action. The housefly is able to eat solid food by secreting saliva and dabbing it over the food item. As the saliva dissolves the food, the solution is then drawn up into the mouth as a liquid.[9]