Insect mouthparts
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Evolution
Like most external features of arthropods, the mouthparts of Hexapoda are highly derived. Insect mouthparts show a multitude of different functional mechanisms across the wide diversity of insect species. It is common for significant homology to be conserved, with matching structures forming from matching primordia, and having the same evolutionary origin. However, even if structures are almost physically and functionally identical, they may not be homologous; their analogous functions and appearance might be the product of convergent evolution.
Chewing insects
Examples of chewing insects include dragonflies, grasshoppers and beetles. Some insects do not have chewing mouthparts as adults but chew solid food in their larval phase. The moths and butterflies are major examples of such adaptations.
Mandible
A chewing insect has a pair of mandibles, one on each side of the head. The mandibles are
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.
Maxilla
Situated beneath (caudal to) the mandibles, paired
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
Labium
The labium typically is a roughly quadrilateral structure, formed by paired, fused secondary maxillae.
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
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
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]
Hypopharynx
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.
Siphoning insects
This section deals only with insects that feed by sucking fluids, as a rule without piercing their food first, and without sponging or licking. Typical examples are adult
Proboscis
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.
Piercing and sucking insects
A number of insect orders (or more precisely
Stylets
In female mosquitoes, all mouthparts are elongated. The labium encloses all other mouthparts, the stylets, like a sheath. The labrum forms the main feeding tube, through which blood is sucked. The sharp tips of the labrum and maxillae pierce the host's skin. During piercing, the labium remains outside the food item's skin, folding away from the stylets.[9] Saliva containing anticoagulants, is injected into the food item and blood sucked out, each through different tubes.
Proboscis
The defining feature of the order
Sponging insects
Labellum
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.[10]
References
- ISBN 0-412-61390-5.[page needed]
- ^ Head, Mandibles, and unusual Labium of Dragonfly Nymph (viewed from below)
- ^ Insect Mouthparts
- ^ Insect mouthparts - Amateur Entomologists' Society (AES)
- ^ "Structure and function of insect mouthparts" (PDF). Archived from the original (PDF) on 2010-06-10. Retrieved 2016-01-08.
- ^ "Hymenoptera: ants, bees and wasps", CSIRO, retrieved 8 April 2012
- ISBN 978-1-4020-6242-1.
- ISBN 978-0-931876-87-5.
- PMID 36284127.
- ISBN 978-3-540-66819-0.