Arthropod mouthparts

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The face of a caterpillar with the mouthparts showing. Caterpillar face close up.jpg
The face of a caterpillar with the mouthparts showing.

The mouthparts of arthropods have evolved into a number of forms, each adapted to a different style or mode of feeding. Most mouthparts represent modified, paired appendages, which in ancestral forms would have appeared more like legs than mouthparts. In general, arthropods have mouthparts for cutting, chewing, piercing, sucking, shredding, siphoning, and filtering. This article outlines the basic elements of four arthropod groups: insects, myriapods, crustaceans and chelicerates. Insects are used as the model, with the novel mouthparts of the other groups introduced in turn. Insects are not, however, the ancestral form of the other arthropods discussed here.

Contents

Insects

Insect mouthparts exhibit a range of forms. The earliest insects had chewing mouthparts. Specialisation includes mouthparts modified for siphoning, piercing, sucking and sponging. These modifications have evolved a number of times. For example, mosquitoes (which are flies) and aphids (which are bugs) both pierce and suck; however, female mosquitoes feed on animal blood whereas aphids feed on plant fluids. This section provides an overview of the individual mouthparts of chewing insects.

Figure 1: Chewing mouthparts of a grasshopper. Legend: lr, labrum; md, mandibles; mx, maxillae; hp hypopharynx; lb, labium. Chewing mouthparts.JPG
Figure 1: Chewing mouthparts of a grasshopper. Legend: lr, labrum; md, mandibles; mx, maxillae; hp hypopharynx; lb, labium.

Labrum

The labrum is a flat extension of the head (below the clypeus), covering the mandibles. Unlike other mouthparts, the labrum is a single, fused plate (though it originally was—and embryonically is—two structures). It is the upper-most of the mouthparts and located on the midline. It serves to hold food in place during chewing by the mandibles and thus can simply be described as an upper lip.

Mandible

Chewing insects have two mandibles, one on each side of the head. They are typically the largest mouthpart of chewing insects, being used to masticate (cut, shred, tear, crush, chew) food items. They open outwards (to the sides of the head) and come together medially.

Maxilla

Paired maxillae cut food and manipulate it during mastication. Maxillae can have hairs and "teeth" along their inner margins. At the outer margin, the galea is a cupped or scoop-like structure, which sits over the outer edge of the labium. They also have palps, which are used to sense the characteristics of potential foods.

Labium

The labium is a single structure, although it is formed from two fused secondary maxillae. It can be described as the floor of the mouth and functioning in close the mouth of the insect. With the maxillae, it assists manipulation of food during mastication.

Hypopharynx

The hypopharynx is a somewhat globular structure, arising from the base of the labium. It assists swallowing. It performs the role of the tongue found in large vertebrates.

Myriapods

Figure 2: Ventral view of forcipules of a centipede, arising from the first body segment Scolopendra fg02.JPG
Figure 2: Ventral view of forcipules of a centipede, arising from the first body segment

Myriapods comprise four classes of arthropod, each with a similar morphology: Class Chilopoda (centipedes); class Diplopoda (millipedes); class Pauropoda; and class Symphyla. Myriapod mouthparts are similar to those of chewing insects, although there is some variation between the myriapod classes. A labrum is present but sometimes is not obvious and forms an upper lip, often in association with an epistome. The labium is formed by first maxillae in diplopoda forming the gnathochilarium. The preoral cavity so-formed contains paired mandibles and any maxillae which are present.

Forcipules

Centipedes, in addition to their mouthparts, possess a pair of "poison claws", or forcipules. These, like the maxillipeds of crustaceans, are modified legs and not true mouthparts. [1] The forcipules arise from the first body segment, curving forward and to the midline. The tip is a pointed fang, which has an opening from a venom gland. The forcipules are used to capture and envenomate prey.

Crustaceans

Crustaceans comprise a number of classes, with various feeding modes supported by a range of adaptations to the mouthparts. In general, however, crustaceans possess paired mandibles with opposing biting and grinding surfaces. The mandibles are followed by paired first and second maxillae. Both the mandibles and the maxillae have been variously modified in different crustacean groups for filter feeding with the use of setae.

Figure 3: The mouthparts of an edible crab: the third maxillipeds conceal the remaining mouthparts Yaquina Bay Cancer productus Macro.JPG
Figure 3: The mouthparts of an edible crab: the third maxillipeds conceal the remaining mouthparts

Maxillipeds

Up to the first three pairs of legs are modified to maxillipeds, which assist manipulation of food items by passing food forward to the mandibles for chewing or to the maxillae for cutting into smaller pieces.

Setae

Filter feeding crustaceans have setae on modified appendages that act as filters. Filter feeding may have developed in association with swimming, with early morphological adaptations occurring on the appendages of the body trunk. Subsequent adaptations appear to have favored forward filtering appendages. Filtering appendages generate water currents that bring food items into reach for collection by setae. Other setae may be used to brush the filtering setae clean, and yet other setae may transport food items to the mouth.

Cirri

Barnacles have thoracic appendages modified for feeding, the cirri, which filter suspended food particles from water currents and pass the food to the mouth.

Chelicerates

Chelicerates comprise four classes of arthropod, with similar gross morphology but defining differences: Class Xiphosura (horseshoe crabs); class Eurypterida (the extinct eurypterids); class Arachnida (spiders, scorpions, ticks and mites); and class Pycnogonida (sea spiders). Chelicerates are in part defined by possessing chelicerate appendages, although crustaceans also possess chelate appendages. Chelicerates are more easily distinguished from other arthropods in lacking antennae and mandibles.

Figure 4: Types of chelicerae: (A) jackknife, (B) scissor, and (C) 3-segmented chelate Chelicerae.svg
Figure 4: Types of chelicerae: (A) jackknife, (B) scissor, and (C) 3-segmented chelate

Chelicerae

Chelicerae are chelate appendages that are used to grasp food. For example, in horseshoe crabs, they are like pincers, whereas in spiders, they are hollow and contain (or are connected to) venom glands and are used to inject venom to disable prey prior to feeding. In some spiders, the chelicerae have teeth, which are used to macerate prey items to assist digestion by secreted enzymes. Those spiders without toothed chelicerae inject digestive enzymes directly into their prey. Mites and ticks have a range of chelicerae. Carnivores have chelicerae that tear and crush prey, whereas herbivores can have chelicerae that are modified for piercing and sucking (as do parasitic species). In sea spiders, the chelicerae (also known as chelifores) are short and chelate and are positioned on either side of the base of the proboscis or sometimes vestigial or absent.

Proboscis

Sea spiders possess a tubular proboscis forward from the body trunk, at the end of which is the opening to the mouth. In those species that lack chelifores and palps, the proboscis is well developed and more mobile and flexible. In such cases, it can be equipped with sensory bristles and strong rasping ridges around the mouth.

See also

Related Research Articles

Chelicerata

The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, arachnids, and several extinct lineages, such as the eurypterids and chasmataspidids.

Arachnid Class of arthropods

Arachnida is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes orders containing spiders, scorpions, ticks, mites, harvestmen, and solifuges. In 2019, a molecular phylogenetic study also placed horseshoe crabs in Arachnida.

Centipede

Centipedes are predatory arthropods belonging to the class Chilopoda of the subphylum Myriapoda, an arthropod group which also includes millipedes and other multi-legged creatures. Centipedes are elongated metameric creatures with one pair of legs per body segment. Most centipedes are generally venomous and can inflict painful bites, injecting their venom through pincer-like appendages known as forcipules. Despite the name, centipedes can have a varying number of legs, ranging from 30 to 354. Centipedes always have an odd number of pairs of legs. Therefore, no centipede has exactly 100 legs. Like spiders and scorpions, centipedes are predominantly carnivorous.

Jaw Opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food; structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of most animals

The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of humans and most animals.

Pedipalp

Pedipalps are the second pair of appendages of chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to the chelicerae ("jaws") and anterior to the first pair of walking legs.

<i>Yohoia</i>

Yohoia is a genus of tiny, extinct animals from the Cambrian period that has been found as fossils in the Burgess Shale formation of British Columbia, Canada. It has been placed among the arachnomorpha, a group of arthropods that includes the chelicerates and possibly the trilobites. Their sizes range from 7 to 23 mm. 711 specimens of Yohoia are known from the Greater Phyllopod bed, where they comprise 1.35% of the community.

Appendage External body part or natural prolongation, that protrudes from an organisms body

In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body. An appendage is any of the homologous body parts that may extend from a body segment. These include antennae, mouthparts, gills, walking legs (pereiopods), swimming legs (pleopods), sexual organs (gonopods), and parts of the tail (uropods). Typically, each body segment carries one pair of appendages.

The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa, trochanter, femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus, patella.

Mandible (insect mouthpart)

Insect mandibles are a pair of appendages near the insect’s mouth, and the most anterior of the three pairs of oral appendages. Their function is typically to grasp, crush, or cut the insect’s food, or to defend against predators or rivals. Insect mandibles, which appear to be evolutionarily derived from legs, move in the horizontal plane unlike those of vertebrates, which appear to be derived from gill arches and move vertically.

Spider anatomy

The anatomy of spiders includes many characteristics shared with other arachnids. These characteristics include bodies divided into two tagmata, eight jointed legs, no wings or antennae, the presence of chelicerae and pedipalps, simple eyes, and an exoskeleton, which is periodically shed.

Mandible (arthropod mouthpart) Pair of mouthparts used either for biting or cutting and holding food

The mandible of an arthropod is a pair of mouthparts used either for biting or cutting and holding food. Mandibles are often simply referred to as jaws. Mandibles are present in the extant subphyla Myriapoda, Crustacea and Hexapoda. These groups make up the clade Mandibulata, which is currently believed to be the sister group to the rest of arthropods, the clade Arachnomorpha.

Arthropod head problem Uncertainty regarding the evolutionary relationship of the segmental composition of the head in various arthropod groups

The (pan)arthropod head problem is a long-standing zoological dispute concerning the segmental composition of the heads of the various arthropod groups, and how they are evolutionarily related to each other. While the dispute has historically centered on the exact make-up of the insect head, it has been widened to include other living arthropods such as the crustaceans and chelicerates; and fossil forms, such as the many arthropods known from exceptionally preserved Cambrian faunas. While the topic has classically been based on insect embryology, in recent years a great deal of developmental molecular data has become available. Dozens of more or less distinct solutions to the problem, dating back to at least 1897, have been published, including several in the 2000s.

Maxilla (arthropod mouthpart)

In arthropods, the maxillae are paired structures present on the head as mouthparts in members of the clade Mandibulata, used for tasting and manipulating food. Embryologically, the maxillae are derived from the 4th and 5th segment of the head and the maxillary palps; segmented appendages extending from the base of the maxilla represent the former leg of those respective segments. In most cases, two pairs of maxillae are present and in different arthropod groups the two pairs of maxillae have been variously modified. In crustaceans, the first pair are called maxillulae.

Insect mouthparts

Insects have a range of mouthparts, adapted to particular modes of feeding. The earliest insects had chewing mouthparts. Specialization has mostly been for piercing and sucking, although a range of specializations exist, as these modes of feeding have evolved a number of times (for example, mosquitoes and aphids both pierce and suck, however female mosquitoes feed on animal blood whereas aphids feed on plant fluids. In this page, the individual mouthparts are introduced for chewing insects. Specializations are generally described thereafter.

Arthropod Phylum of invertebrates with jointed exoskeletons

An arthropod is an invertebrate animal having an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids, myriapods, and crustaceans. The term Arthropoda as originally proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora.

Insect morphology

Insect morphology is the study and description of the physical form of insects. The terminology used to describe insects is similar to that used for other arthropods due to their shared evolutionary history. Three physical features separate insects from other arthropods: they have a body divided into three regions, have three pairs of legs, and mouthparts located outside of the head capsule. It is this position of the mouthparts which divides them from their closest relatives, the non-insect hexapods, which includes Protura, Diplura, and Collembola.

This glossary describes the terms used in formal descriptions of spiders; where applicable these terms are used in describing other arachnids.

Labrum (arthropod mouthpart)

The labrum is a flap-like structure that lies immediately in front of the mouth in almost all extant Euarthropoda. The most conspicuous exceptions are the Pycnogonida, which probably are chelicerate-relatives. In entomology, the labrum amounts to the "upper lip" of an insect mouth, the corresponding "lower lip" being the labium.

Hexapoda Subphylum of arthropods

The subphylum Hexapoda constitutes the largest number of species of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura. The Collembola are very abundant in terrestrial environments. Hexapods are named for their most distinctive feature: a consolidated thorax with three pairs of legs. Most other arthropods have more than three pairs of legs.

Subcapitulum

The subcapitulum, also known as infracapitulum, hypognathum or hipognatum, refers to the ventral part of the gnathosoma or the fusion of the palpal coxae and the labrum complex present in some arthropods on which the mouth, pedipalps, mouthparts and pharynx are generally located. It is delimited by the subcapitular apodeme, which separates it from the cheliceral frame.

References

  1. Rowland Shelley & Paul Marek (2005-03-01). "Centipedes: general information". East Carolina University. Archived from the original on 2004-09-11.