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INSECT MOUTHPARTS

A group of appendages associated with the mouth which form collectively a feeding

apparatus of an insect commonly called the mouthparts. Mouthparts are some of the most

distinctive features of insects, and their structure tells a great deal about the feeding habits of

a species. Insect mouthparts are variously adapted for the ingestion of many types of foods.

Mouthparts vary not only among different insect groups, but also among different stages of

the same species (for example, note the mouthparts of a butterfly and a caterpillar). In some

insects (e.g., mayflies) mouthparts are not developed; this condition is termed as vestigial.

Importance of studying mouthparts

Different insects can be identified by observing their mouthparts.

The feeding mechanism of insect pests can be known by studying their mouthparts.

Nature of damage of various insect pests can be recognized by learning their

mouthparts.

Appropriate control measures can be applied for specific insect pests by

understanding their mouthparts.

Types of mouthparts

Eight major types of mouthparts based on the feeding habit of insects.

A. Chewing type (eg. Grasshopper, cockroach, beetles etc.)

B. Rasping sucking type (eg. Thrips)

C. Piercing sucking type (eg. Rice bug, bed bug, stink bug, leaf hopper, female mosquito etc.)

D. Sponging type (eg. House fly

E. Siphoning type (eg. Butterfly and moths)

F. Cutting sponging type (eg. Horse fly)

G. Chewing lapping type (eg. Honey bee)

H. Degenerate type (eg. Larvae of mosquito, fruit fly etc.)

A. CHEWING TYPE MOUTHPARTS

Insects like ground beetles and grasshoppers with chewing mouthparts have heavy crania,

adapted for muscles involved in capturing prey and biting off leaf tissue. Chewing type

mouthparts consist of a labrum (upper lip), a pair of chewing mandibles (upper jaws), a pair

of maxillae (lower/second jaws), and a labium (lower lip) [Figure 1]. These structures

surround the mouth and form the pre-oral cavity. In addition, a central tongue-like

hypopharynx drops from the membranous floor of the cranium, behind the mouth, and bears

the opening of the salivary ducts. The interior, fleshy-surface of the labrum, endowed with

numerous sensory structures, is referred to as the labrum-epipharynx (roof of the mouth).

Labrum: The labrum is a sclerite or plate that acts as the 'upper lip' in insects. It is a flap-

like structure that lies immediately in front of the mouth. There is a notch at the middle of

the labrum which hold food material during cutting or grinding. Labrum pushes food into the

mouth cavity.

Mandibles: Mandibles are strongly chitinized structures and are commonly known as the

jaws. They always articulate in a transverse plate. Mandibles are provided with strong

adductor and abductor muscles arisen from the dorsal and ventral walls of the cranium.

Teeth are blunt like molar in phytophagous insects, slightly pointed like incisors in

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carnivorous insects. Teeth are molar near the base and incisor as blade at the tip in

omnivorous insects for cutting and grinding the food material.

Maxillae: They are also known as second pair of jaws of the insect. Each maxilla is

composed of proximal cardo, middle stipes, lateral palpifer, inner subgalea or parastipes and

the distal sclerite consisting two lobes, outer galea & inner lacinia. On the outside, the

palpifer bears the maxillary palp, which is sensory appendages and composed of 1-7

segments. The maxillae possess various types of cuticular processes, hairs and bristles etc.

Mandibles and maxillae are adapted for cutting and grinding the solid food material.

Labium: It is formed from the fusion of paired 2nd

maxillae pair. It closes the preoral cavity

and known as lower lip of insect. It is composed of distal prementum, proximal

postmentum, separated by labial suture. Postmentum is divided into proximal submentum

and distal mentum. Palpiger: lateral lobe arises from prementum bears 1-4 segmented labial

palp, which is sensory in function. Paraglosae: arise from distal margin of prementum

outward as lobe. Glossae: arise from distal margin of prementum innerward as lobe.

Hypopharynx: In some Apterygota, it is considered as composite gnathal appendages. At

embryonic development, hypopharynx is formed by a pair of lateral lobes (Superlinguae)

fused with a median lobe (Lingua) in Apterygota and some primitive Pterygota. In higher

Pterygota, it develops as a ventral median lobe from the ventral wall of the labium. At the

base it bears an orifice of salivary duct. Various muscles serve to swing the hypopharynx in

forward and backward directions.

Epipharynx and hypopharynx are membranous lobulated structures which push the food

material into the pharynx.

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B. RASPING SUCKING TYPE MOUTHPARTS

Rasping sucking mouthparts are commonly found in the Order Thysanoptera (Figure 2).

Head bears a short, thick conical beak projected downward.

Externally, the beak is formed by labrum in front, maxillae on sides and labium behind.

Labrum is a triangular structure. The maxillae are long and triangular structure bearing

palpi distally. Labium is wide with a pair of short palpi.

Within beak are enclosed (a) single elongated slender mandibular stylet, (b) two

maxillary stylets, (c) a short median hypopharynx. The stylets are adapted to move in and

out of beak. All these parts form piercing apparatus. Maxillary stylets are longer than the

mandibular stylets. The lacinia fit together to form the food channel.

These mouthparts are intermediate between chewing-biting and piercing-sucking type. They

are asymmetrical due to absence of right mandible. They do not enter the wound deeply. The

sap exuded on the surface is sucked up by the cone-shaped mouth rather than stylets. Food

channel lies between labrum and hypopharynx within the head and saliva passes to the tip

of stylets between the hypopharynx and labium.

C. PIERCING SUCKING TYPE MOUTHPARTS

In these mouthparts, a tube or jointed beak encloses needle like stylets. Labrum is short and

covers the basal part of the beak like a mall flap. Labium is elongated and formed a beak

which is dorsally grooved and 3-4 segmented, hold styles. It lacks palpi and terminal lobes.

Mandibles and Maxillae are elongated, needle like and form long slender stylets. Maxillary

palpi are absent. Food Channel and salivary duct are formed by closely apposed inner

surface of Maxillae. Stylets are longer than labium and coiled within head. Stylets may be

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variously equipped. The hypopharynx is a median conical lobe projected into the proximal

part of the beak between the bases of the stylets (Figure 3).

In female mosquito, there are 6 stylets – two mandibles, two maxillae, one each hypopharynx

and labrum epiphyrynx. Food channel formed between labrum epiphyrynx and

hypopharynx. Salivery duct extends length of hypopharynx. Maxillary palpi present but

labial palpi absent. Labium unjointed except for distal pair of lobes called labellum (Figure

4). Stylets are not retractile and protractile, entire beak inserted during feeding.

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D. SPONGING TYPE MOUTHPARTS

They are incapable of piercing skin and found in adult house fly. The mandibles are wanting.

Feeding apparatus is proboscis which is a composite structure formed by labrum,

hypopharynx and labium. The proboscis is anteriorly associated with maxillary palpi and

distally bears a pair of labellar lobes called labellum. Labella are broad, soft pads with teeth

for rasping food.

The proboscis has 3 parts: (1) basiproboscis or rostrum which bears maxillary palpi, (2)

mediproboscis or haustellum which is cylindrical and its anterior surface is covered by

labrum, (3) distiproboscis fromed labellae or oral suckers which occur as non-piercing

spongy pads. It bears pseudotrachae. Food channel lies between labrum and hypopharynx.

The maxillae are represented by maxillary palpi (Figure 5). The hypopharynx conveys saliva

into the wound which contains anticoagulant.

E. CUTTING SPONGING TYPE MOUTHPARTS

They are found in female horse flies (Tabanidae) only. Total number of stylets is six similar

to that of mosquitoes. Stylets are flattened blade like. Labium is similar to the sponging type

terminating into a pair of large lobes, the labellae. Mandibles are well-developed, forming

sharp blades. Maxillae are long slender tapering forming probing stylets. The mandibles cut

the tissues like scissor and the maxillae thrust and retract repeatedly. The blood vessels are

ruptured by the teeth armed at the tip of mandibles. The hypopharynx is long, narrow,

tapering stylet arising from the ventral wall of the head (Figure 6). The saliva contains

powerful anticoagulant.

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F. SIPHONING TYPE MOUTHPARTS

It is the common mouthparts in nectar feeding insects. Labrum is a small, present as a

narrow transverse band at lower edge of clypeas. Mandibles are usually absent or ill-defined

in small Lepidoptera. Maxillary palpi remain rudimentary. The feeding organ is proboscis

formed by the extension of the gelea. Lacinia is reduced or absent. Labium is represented by

3 segmented large scaly labial palpi. Food channel is formed from concave walls of the

apposed galeae (Figure 7).

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G. CHEWING LAPPING TYPE MOUTHPARTS

Labrum and mandibles are similar to general chewing type. Mandible flattened, lose bitting

capability, spoon shaped. Maxilla-labial complex lengthened and free part modified for

sucking. In maxillae, lacinia is lost and galeae are enlarged. Stipes well developed bears a

small pair of rudimentary palpi. Cardo long, rod-like, V-shaped, small postmentum, articulate

with well develop prementum. The glossae of the labium greatly elongated to form a hairy,

flexible tongue that can be rapidly protracted and retracted. Paraglossae are small and

present at the base of the glossae. Labial palp 4 segmented with 1st and 2

nd segment blade-

like. Flabllum is the spoon-like lobe at the end of the glossae (Figure 8). Bee thrusts out

tongue and licks the nectar with its tip. Glossae smear up nectar rapidly and retract between

the labial palpi and galeae. Nectar is squeezed off the tongue by glossae and deposited so as

to accumulate in small cavity formed by paraglossae at the base of glossae.

H. DEGENERATE TYPE MOUTHPARTS

a) Larvae of Aquatic Nematocera

True mouthparts are absent. External mouthparts form brush like structures that bring water

current with suspended particles into pharynx (Figure 9A). Pharynx develops into a pump

with internal screen. Comb like screens are closely arranged with spacing that permit removal

of very fine suspended materials.

b). Muscoid or Higher Diptera

Usual mouthparts are totally suppressed. All of the head is invaginated into thorax. Circular

fold of neck projects forward to from a conical snout or functional head. Only external

feeding organs are a pair of strong movable hooks derived from cervical region. Musca

domestica has only single mouth hook (Figure 9B). Hypopharynx, clypeas and associated

parts and sclerites of head collectively formed the cephalopharyngeal skeleton.

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