Feeding I: Structure And Function Of Mouthparts

Feeding I: Structure and Functionof MouthpartsOrientation of the mouthparts: Hypognathous. Mouthparts are directedventrally downward. Primitive condition inInsecta. Common in phytophagous groupsinhabiting open habitats (Thysanura,cockroaches, mantids, phytophagous beetles). Prognathous. Mouthparts directed forward.Common in carnivorous groups that activelypursue prey (earwigs, larval neuropterans,coleopterans, termite soldiers and ants). Opisthognathous. Mouthparts directedbackward. Common in hemipteroid orders withsucking mouthparts.

Generalized Structure & Function Generalized chewing mouthparts arefound in the Thysanura, Odonata, allPolyneoptera orders, Neuroptera,Coleoptera, Mecoptera, manyHymenoptera, and immatures in theEmphemenoptera, Siphonaptera,Trichoptera and Lepidoptera. Modified sucking mouthparts are foundin all the Paraneoptera orders, adultSiphonaptera, Diptera, bees andLepidoptera. Basal or generalized insect mouthpartsconsist of 5 basic structures. Movingfrom anterior to posterior thesestructures are: the labrum, the pairedmandibles, the paired maxillae, andthe labium. The hypopharynx islocated medially to the mandibles andthe maxillae.

The Labrum The labrum is the anterior-mostmouthpart. It forms a broad lobesuspended from the clypeus andprotects the more delicatemouthparts behind it. The innersurface is frequently membranousand may be produced into amedian lobe called theepipharynx. The epipharynx iscovered with chemoreceptorsand mechanoreceptors used infood selection. Muscles attached to the labrummove it outward, away from theother mouthparts and inwardtoward the other mouthpartsduring feeding. Some limitedlateral movement is also possible.

The Mandibles The mandibles are used to capture food andbreak it up into smaller pieces. Size and shape ofthe incisors and molar cusps varies with diet.Predators have strong shearing, pointed cusps.Grasshoppers feeding on vegetation other thangrasses have a series of sharp pointed cusps,while grass-feeding species have chisel-edgedincisor cusps and molar cusps with flattenedridges for grinding. Mandibles of primitively wingless insects(Archaeognatha) have only one articulation withthe head capsule (monocondylous). Movementabout the point of articulation is accomplished withanterior and posterior rotator muscles, remotormuscles and transverse adductor muscles thatdirectly connect the two mandibles. Mandibles of the Pterygota have two points ofarticulation with the head capsule (dicondylous).Movement about the points of articulation isaccomplished by abductor muscles (opening)and adductor muscles (closing). Adductormuscles in many pterygotes are extremelypowerful. These muscles are homologous with therotator muscles in the archaeognathans.

The Maxillae The maxillae (single maxilla) are positionedlaterally behind the mandibles. They are heldin place by a single point of articulation withthe head capsule and membraneousconnections that allow freedom ofmovement. The main body of the maxilla is composed ofthe proximally located cardo and the distallylocated stipes. Distally on the stipes are twolobes, an inner lacinia and an outer galea.More laterally on the stipes is a jointed leglike palpus. The whole maxilla, including the lacinia andgalea, move food particles backwards intothe preoral cavity. The maxillary palps aresensory organs used to test the quality of thefood. In addition to its role in directing foodinto the mouth, the galea is used to cleanthe palps, antennae and forelegs. Muscle attachments are roughly similar tothose of the mandibles. Lateral adductormuscles are attached to the tentorium.

The Labium The labium is the posterior-mostmouthpart and is derived from the fusionof two maxillae-like appendages. It formsthe protective “lower lip” to the mouth. The main body of the labium consists ofthree plate-like segments (submentum,mentum and prementum). The mostdistally located segment bears four lobes,two inner glossae and two outerparaglossae. A pair of palps also ariselaterally from this most distally locatedsegment. These labial palps typically actas sensory organs similar to the maxillarypalps. In some larval odonates thesepalps are modified into raptorialappendages to capture prey. Muscle attachments are roughly similar tothose of the maxillae, with some attachedto the tentorium.

The Hypopharynx The hypopharynx is locatedmedially to the mandibles and themaxillae. It is behind the preoralcavity and in front of the labium. The hypopharynx is mostlymembraneous and closelyassociated with the salivary glandsand/or salivary ducts. The hypopharynx functions as atongue, moving food around in thepreoral cavity. This isaccomplished by muscleattachments that move it back andforth.

Common Features of SuckingMouthparts Presence of a sucking tube. Thesucking tube is the means by whichliquid food is drawn into the mouth.Mouthparts used to form thisstructure varies among groups. Presence of a sucking pump. Thesucking pump creates the negativepressure necessary to draw thefood into the mouth. How thesucking pump is formed variesamong groups. Many insects withsucking mouthparts also have asalivary pump for injecting salivainto the preoral cavity.

Paraneoptera Mouthparts The sucking tube (beak) in theHemiptera is formed from the mandibles,maxillae and the labium. The labiumforms the protective sheath that foldsback during feeding. The mandibles formthe outer stylets that surround theinner stylets. The laciniae of themaxillae form the inner stylets with thefood and salivary channels. The sucking pump is formed in thecibarium. Negative pressure isgenerated by the enlargement of thecibarial dilator muscle attached to theclypeus. The sucking tube in the Anoplura isformed by the labium.

Honeybee Mouthparts Maxillae and labium are fused into asingle structure. The sucking tube(tongue) is formed by the fusion of theglossae of the labium, sometimestogether with the paraglossae. Maxillaegaleae are enlarged and modified toform a cutting appendage used by shorttongue bees to cut holes in flowers.Laciniae are lost and the maxillarypalps reduced. Mandibles are flattened and used forgrasping and manipulating objects,rather than for biting and cutting. The sucking pump is formed by thecibarium, the pharynx and the buccalcavity. Muscles associated with thesucking pump are attached to the fronsand the clypeus.

Lepidoptera Mouthparts The sucking tube (proboscis) is formedfrom the maxillae galeae with the foodchannel running down the center. Outerwalls of the galeae alternate scleritizedand membranous rows. Thisarrangement facilitates coiling (think of avacuum hose). Extension of theproboscis is accomplished by bloodpressure. Coiling is automatic. Mandibles are completely lost in mostderived groups. Labrum is reduced to a small transversesclerite and the labium is reduced to asmall flap. Labial palps however arelarge. Sucking pump is formed by the cibariumand the pharynx. Muscle attachmentsare similar to those in the Hymenoptera.The sucking pump is absent inLepidoptera that do not feed as adults.

Diptera Adult MouthpartsBiting flies (mosquitos, black flies, deer flies, horseflies) Mandibles are present and used for piercing the host’sskin. They are long and sharply pointed. The sucking tube is formed by the labrum anteriorly andthe mandibles posteriorly. Food is drawn up the foodchannel which is a groove on the posterior side of thelabrum. The maxillae retain most of the components of the typicalbiting form, but the galeae are long and bladelike and thepalps are enlarged. The hypopharynx is styletlike and contains the salivaryduct. The labium is a large, thick appendage with a deepanterior groove into which the other mouthparts normallyfit. Distally the labium bears two large labellar lobes withpseudotracheae, which directs the blood towards thefood canal. In mosquitos the food canal is formed between thehypopharynx and the labium. The sucking pump is formed in the cibarium.

Diptera Adult MouthpartsLapping Muscids (houseflies, blowflies) Sucking tube (proboscis) is a composite structurethat includes the labrum, hypopharynx and labium.The tube is divisible into a basal rostrum bearing themaxillary palps, a median flexible haustellum and twoapical labellae. The labellae are broad sponging pads,equipped with pseudotracheae along which foodpasses to the oral aperture Mandibles are completely lost. The sucking pump is formed from the cibarium and itsdilator muscles.Biting Muscids (tsetse flies, stable flies) Piercing, sucking tube is a composite structure as inthe lapping Muscids. However, the haustellum iselongate and rigid, and the distal labellar lobes aresmall and bear rows of prestomal teeth on their innerwalls. The labrum and labium interlock to form the food canalwithin which lies the hypopharynx enclosing thesalivary duct.

Diptera Larval Mouthparts Nematoceran larvae have typicalchewing mouthparts as do the larvaeof most holometabolous insects. Basal Brachycera larvae showmodifications away from the typicalchewing mouthparts. Derived Brachycera larvae in theCyclorrhapha have highly modifiedmouthparts in which the typicalmouthparts are lost or fused.Mandibles are modified into verticallydirected mouth hooks.

Polyneoptera orders, Neuroptera, Coleoptera, Mecoptera, many Hymenoptera, and immatures in the Emphemenoptera, Siphonaptera, Trichoptera and Lepidoptera. Modified sucking mouthparts are found in all the Paraneoptera orders, adult Siphonaptera, Diptera, bees and Lepidoptera. Basal or generalized insect mouthparts