Insect mouthparts

Insectshavemouthpartsthat may vary greatly across insect species, as they are adapted to particular modes of feeding. The earliest insects had chewing mouthparts. Most specialisation of mouthparts are for piercing and sucking, and this mode of feeding has evolved a number of times independently. For example,mosquitoes(which are true flies) andaphids(which aretrue bugs) both pierce and suck, though female mosquitoes feed on animal blood whereas aphids feed on plant fluids.

Evolution

Like most external features of arthropods, the mouthparts ofHexapodaare highly derived. Insect mouthparts show a multitude of different functional mechanisms across the wide diversity of insect species. It is common for significanthomologyto be conserved, with matching structures forming from matchingprimordia,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 ofconvergent evolution.

Chewing insects

The trophi, or mouthparts of a locust, a typical chewing insect:
1 Labrum
2 Mandibles;
3 Maxillae
4 Labium
5 Hypopharynx

Examples of chewing insects includedragonflies,grasshoppersandbeetles.Some insects do not have chewing mouthparts as adults but chew solid food in theirlarvalphase. Themoths and butterfliesare major examples of such adaptations.

Mandible

The mandibles of abull 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 arecaudalto the labrum andanteriorto themaxillae.Typically the mandibles are the largest and most robust mouthparts of a chewing insect, and it uses them tomasticate(cut, tear, crush, chew) food items. Two sets of muscles move the mandibles in thecoronal planeof the mouth:abductor musclesmove insects' mandibles apart (laterally);adductor musclesbring them together (medially). They do this mainly in opening and closing their jaws in feeding, but also in using the mandibles as tools, or possibly in fighting.

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 incaterpillars.

In males of some species, such as ofLucanidaeand someCerambycidae,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 someantsandtermites,the mandibles also serve a defensive function (particularly in soldier castes). Inbull 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 manypaper waspshave mandibles adapted to scraping and ingesting wood fibres.

Maxilla

Situated beneath (caudal to) the mandibles, pairedmaxillaemanipulate and, in chewing insects, partly masticate, food. Each maxilla consists of two parts, theproximalcardo(pluralcardines), anddistalstipes(pluralstipites). At the apex of each stipes are two lobes, the innerlaciniaand outergalea(pluralslaciniaeandgaleae). 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 thesagittal 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 heavilysclerotisedthan 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 thesubesophageal ganglia.

Labium

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 duringmastication.

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 thenymphsof theOdonata,thedragonfliesanddamselflies.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]^{[unreliable source?]}

The labium is attached at the rear end of the structure calledcibarium,and its broad basal portion is divided into regions called thesubmentum,which is the proximal part, thementumin the middle, and theprementum,which is the distal section, and furthest anterior.

The prementum bears a structure called theligula;this consists of an inner pair of lobes calledglossaeand a lateral pair calledparaglossae.These structures arehomologousto 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 themusculatureof 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 thehoney 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: thecibariumor dorsal food pouch and ventralsalivariuminto which the salivary duct opens.

Siphoning insects

Butterflies coil the proboscis when not feeding.

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 adultmoths and butterflies.As is usually the case with insects, there are variations: some moths, such as species ofSerrodesandAchaeado pierce fruit to the extent that they are regarded as serious orchard pests.^[8]Some moths do not feed after emerging from the pupa, and have greatly reduced,vestigialmouthparts or none at all. All but a few adult Lepidoptera lack mandibles (thesuperfamilyknown as themandibulate mothshave fully developed mandibles as adults), but also have the remaining mouthparts in the form of an elongated sucking tube, the proboscis.

Proboscis

The proboscis, as seen in adult Lepidoptera, is one of the defining characteristics of themorphologyof 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 preciselyfamilieswithin them) have mouthparts that pierce food items to enable sucking of internal fluids. Some are herbivorous, likeaphidsandleafhoppers,while others are carnivorous, likeassassin bugsand femalemosquitoes.

Stylets

Mouthparts of a female mosquito feeding on blood. The flexiblelabiumsupports the bundle of stylets which penetrates the host's skin.

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]Salivacontaininganticoagulants,is injected into the food item and blood sucked out, each through different tubes.

Proboscis

The defining feature of the orderHemipterais 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 asshield bugs,feed on the fluids of plants. Predatory bugs such asassassin bugshave the same mouthparts, but they are used to pierce the cuticles of captured prey.

Sponging insects

Proboscisof the fly (Gonia capitata): note also the protrudinglabial palps.

Labellum

Thehouseflyis a typical sponging insect. Thelabellum'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 bycapillary 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

^Richards, O. W.; Davies, R.G. (1977).Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development.Berlin: Springer.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 fromthe original(PDF)on 2010-06-10.Retrieved2016-01-08.
^"Hymenoptera: ants, bees and wasps",CSIRO,retrieved8 April2012
^Heppner, John B; Richman, David B; Naranjo, Steven E; Habeck, Dale; Asaro, Christopher; Boevé, Jean-Luc; Baumgärtner, Johann; Schneider, David C; Lambdin, Paris; Cave, Ronald D; Ratcliffe, Brett C; Heppner, John B; Baldwin, Rebecca W; Scherer, Clay W; Frank, J. Howard; Dunford, James C; Somma, Louis A; Richman, David. B; Krafsur, E. S; Crooker, Allen; Heppner, John B; Capinera, John L; Menalled, Fabián D; Liebman, Matt; Capinera, John L; Teal, Peter E. A; Hoy, Marjorie A; Lloyd, James E; Sivinski, John; et al. (2008). "Silkworm Moths (Lepidoptera: Bombycidae)".Encyclopedia of Entomology.pp. 3375–6.doi:10.1007/978-1-4020-6359-6_4198.ISBN 978-1-4020-6242-1.
^Walter Reuther (1989).The Citrus Industry: Crop protection, postharvest technology, and early history of citrus research in California.UCANR Publications. pp. 64–.ISBN 978-0-931876-87-5.
^Zahran, Nagwan; Sawires, Sameh; Hamza, Ali (2022-10-25)."Piercing and sucking mouth parts sensilla of irradiated mosquito, Culex pipiens (Diptera: Culicidae) with gamma radiation".Scientific Reports.12(1): 17833.doi:10.1038/s41598-022-22348-0.PMC9596698.PMID 36284127.
^Mehlhorn, Heinz (2001).Encyclopedic Reference of Parasitology: Biology, Structure, Function.Springer Science & Business Media. p. 310.ISBN 978-3-540-66819-0.

External links

[isbn0-412-61390-5-1] Richards, O. W.; Davies, R.G. (1977).Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development.Berlin: Springer.ISBN 0-412-61390-5.^{[page needed]}

[2] Head, Mandibles, and unusual Labium of Dragonfly Nymph (viewed from below)

[3] Insect Mouthparts

[4] Insect mouthparts - Amateur Entomologists' Society (AES)

[5] "Structure and function of insect mouthparts"(PDF).Archived fromthe original(PDF)on 2010-06-10.Retrieved2016-01-08.

[6] "Hymenoptera: ants, bees and wasps",CSIRO,retrieved8 April2012

[7] Heppner, John B; Richman, David B; Naranjo, Steven E; Habeck, Dale; Asaro, Christopher; Boevé, Jean-Luc; Baumgärtner, Johann; Schneider, David C; Lambdin, Paris; Cave, Ronald D; Ratcliffe, Brett C; Heppner, John B; Baldwin, Rebecca W; Scherer, Clay W; Frank, J. Howard; Dunford, James C; Somma, Louis A; Richman, David. B; Krafsur, E. S; Crooker, Allen; Heppner, John B; Capinera, John L; Menalled, Fabián D; Liebman, Matt; Capinera, John L; Teal, Peter E. A; Hoy, Marjorie A; Lloyd, James E; Sivinski, John; et al. (2008). "Silkworm Moths (Lepidoptera: Bombycidae)".Encyclopedia of Entomology.pp. 3375–6.doi:10.1007/978-1-4020-6359-6_4198.ISBN 978-1-4020-6242-1.

[Reuther1989-8] Walter Reuther (1989).The Citrus Industry: Crop protection, postharvest technology, and early history of citrus research in California.UCANR Publications. pp. 64–.ISBN 978-0-931876-87-5.

[Zahran_Sawires_Hamza_2022-9] Zahran, Nagwan; Sawires, Sameh; Hamza, Ali (2022-10-25)."Piercing and sucking mouth parts sensilla of irradiated mosquito, Culex pipiens (Diptera: Culicidae) with gamma radiation".Scientific Reports.12(1): 17833.doi:10.1038/s41598-022-22348-0.PMC9596698.PMID 36284127.

[10] Mehlhorn, Heinz (2001).Encyclopedic Reference of Parasitology: Biology, Structure, Function.Springer Science & Business Media. p. 310.ISBN 978-3-540-66819-0.

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