Insect Mandibulate Mouthparts: Comparative Morphology & Larval Types

 

Comparative Morphology of Insect Mandibulate Mouthparts and Larval Adaptations 

Mouthparts are the head appendages of segments 3-6, surrounding the mouth.

In omnivorous insects like, cockroaches, crickets and earwigs have a biting and chewing type or Mandibulate type of Mouthparts which are most closely resembling the basic design of ancestral pterygote insects.

During insect evolution, an array of different mouthpart types has derived from the basic type.

( Knowledge of mouthparts is useful for taxonomic classification and identification, and for ecological generalisation.)

Generally, mouthpart structure is categorised according to feeding method, but mandibles and other components may function in defensive combat or even make to male sexual contests.

(Example: enlarged mandibles on certain male beetles, Lucanidae)

The mandibulate mouthparts enable the insect to bite and chew hard substances, consume soft stuffs and lap up liquids.

Mandibulate mouthparts are composed of highly sclerotised mandibles that move transversely and can bite off and chew particles of food.

Detailed structural morphology of the primitive chewing (mandibulate) mouthparts of the American cockroach, Periplaneta americana (ventral view). This setup serves as the classic foundational blueprint for insect head appendages.

Components of Mouthparts of Periplaneta americana (Cockroach)

There are five basic components of the mouthparts.

1. LABRUM or upper lip ( with ventral surface called epipharynx)

2. HYPOPHARYNX, a tongue-like structure.

3. MANDIBLES or jaws.

4. MAXILLAE ( singular - maxilla), a pair.

5. LABIUM or lower lip.

Labrum

Labrum or upper lip is the appendage of the 3rd head segment. It forms the roof of the pre-oral cavity and mouth. Also, it covers the base of the mandibles.

It is a broad flap-like lobe located below the clypeus on the anterior side of the head, in front of other mouthparts.

At its junction with the clypeus, it bears a small sclerite on each inner side called a torma.

The inner surface of the labrum also bears two rows of sensory (gustatory) setae, one on each side.

On the posterior or ventral side of the labrum may be a swollen area, the epipharynx.

[Until recently the labrum was considered to be associated with head segment-1, before the studies of embryology. General expression and nerve supply of the labrum showed that it is innervated by the tritocerebrum of the brain ( the fused ganglia of the third head segment) and is formed from the fusion of parts of a pair of ancestral appendages on head segment-3.]

Mandibles

These are appendages of the 4th segment. These are small triangular and heavily sclerotised unsegmented jaws, lying immediately behind the labrum. They are attached to the sides of the head capsule and articulate by means of a condyle with the gena of the same side.

Function: The mandibles cut and crush food and may be used for defence. Generally, they have a typical cutting edge and a more basal molar area that grinds the food.

The inner margin of each mandible is made of two small cutting or serrated lobes, the distal incisor and the proximal molar, each bearing 3 tooth-like denticles of thickened cuticle.

Proximal to the molar lobe is a membranous lobe, the prostheca, that bears sensory hairs. They work like jaws.

Denticles act as the interlocking structures whereas smooth molar areas act as masticatory surfaces. 

Both mandibles show horizontal sideways movements brought about by the action of the adductor and abductor muscles.

In the winged insects and the order Thysanura, they articulate with the head capsule at two points, one on each anterior and posterior and move transversely ( and therefore these two taxa are classified together as Dicondylia).

The mandibles of chewing insects may vary somewhat in structure. In some insects (ex. Cricket), they bear both cutting and grinding ridges, while in others ( ex. Predaceous beetles) they are long and sickle-like.

They can be extremely hard ( approx. 3 on Moh's scale of mineral hardness) and thus many termites and beetles can bore easily through foils of metals such as copper, lead, tin and zinc.

Maxillae

These are appendages of the 5th head segment and are known as the first pair of maxillae. They are paired and lie behind the mandibles and articulate with the posterior surface of the head capsule, one on either side. 

They are biramous ( divided or branched into two parts) in structure. 

It consists of a basal part called Cardo and a distal cylindrical stipe.

Attached to the stipes are three processes,

1. Inner lacinia (sclerotised with a pair of sharp denticles) and blunt lacinula,
2. Middle galea (lobe like structure that acts as a hood of the lacinia)
3. Lateral segmented maxillary palp.

It is 5-segmented and sensory in nature. The palp is borne on a lobe of the stipes called the palpifer.

Function:

It assists the mandibles in processing food.

The pointed and sclerotised lacinae hold and macerate the food, whereas galea and palps bear sensory setae (mechanoreceptors and chemoreceptors) that sample items before ingestion.

[ Maxillary palp represents exopodite while lacinia and galea represent endopodite.]

Both maxillae act as accessory jaws. They work sideways to masticate food and pass it on to the oral cavity.

They also serve to clean the antenna and forelegs.

Labium

It is the appendage of the 6th head segment. Also called the lower lip.

[ The appendages of the 6th head segment fuse with the sternum to form the labium].

It is formed by the fusion of two embryonic appendages or the second pair of maxillae.

The protopodite is three-segmented with a large proximal submentum, a small middle mentum and a distal prementum.

The partially fused endopodites form a tongue-like lingula. Each half of the lingula consists of the inner glossa and the outer paraglossa corresponding to the lacinia and galea respectively.

Prementum bears a 3-jointed sensory labial palp borne on a basal projection, the palpiger. The terminal segment of the palp is covered thickly with sensory setae.

Hypopharynx 

It lies between the maxillae and above the labium, inside the mouth cavity. It is a median somewhat cylindrical structure also known as lingua. Its distal end bears two comb-like plates the superlingua. The common salivary duct opens at its front end.

Comparative Morphology of the Labium

Comparative evolution of the insect labium (lower lip) across five major mandibulate orders. Note the transition from the balanced lobes of the cockroach to the expanded paraglossae of the grasshopper, the specialised sensory palps of the cricket, the fused grinding plate of the wood-boring termite, and the distinct anterior cleft and primitive superlinguae hidden in the earwig.

While the mandibles cut and the maxillae manipulate, the labium (lower lip) is the true taxonomic differentiator of the mandibulate mouthpart suite. As seen in the comparative diagram, the structural variations across orders are distinct:

Cockroach (Generalized): 

Features a primitive, highly balanced ratio between the glossa and paraglossa.

 Grasshopper & Cricket (Orthoptera): 

Characterised by massively expanded, leaf-like paraglossa to manage plant tissue.

 Termite (Isoptera):

 Modified into a highly sclerotised plate with fused lobes designed to withstand the friction of wood boring.

Earwig (Dermaptera): 

Easily identified by a prominent deep cleft splitting the prementum, an adaptation for its forward-facing (prognathous) feeding habits.

In prognathous insects, such as earwig, the labium is attached to the ventral surface of the head via a ventromedial sclerotised plate called the gula. In earwig, the labium consists of a fused protopodite ( 3-segmented), a large proximal submentum, a small middle mentum, and a distal prementum.

The mandibulate mouthparts are the most primitive and occur in adult Thysanura, Diplura, Collembola, Orthopteran, Dermaptera, Psocoptera, Mallophaga, Odonata, Placoptera, Isoptera, Neoroptera, Mecoptera, Tricoptera, Coleoptera and Hymenopteran, as well as in the larval stages of many insects.

A few mandibulate insects obtain their food in liquid form by sucking it through a channel of some sort of mouthparts; Such mouthparts are sometimes described as chewing-sucking and are found in bees and some larvae.

Adaptive Variations in Mandibulate Larvae

Functional morphology of specialised mandibulate larvae's head capsules. (A) Owlfly larva (Ascaloptynx appendiculatus) showing fused mandibulo-maxillary sucking canals. (B) Predaceous diving beetle larva (Dytiscus verticalis) in ventral and dorsal orientations illustrating adaptations for extra-oral digestion.

(A) The larvae of some Neoptera, for example, Owlfly Larva (Ascaloptynx appendiculatus): showcase an extreme modification where the mandibles and maxillae fuse to form elongated, sickle-shaped suction tubes to suck up body fluids through a channel between the mandibles and maxillae. This allows the ambush predator to impale prey and inject paralytic toxins instantly.

(B) Predaceous Diving Beetle Larva (Dytiscus verticalis): 

Features an advanced prognathous head capsule with hollow, perforated mandibles. They suck the body fluids of their prey through channels in the mandibles.

 This configuration facilitates extra-oral digestion, allowing the larva to liquefy and ingest the internal tissues of aquatic prey efficiently.

 In the ventral view diagram, notice how tightly packed the labium and maxillae are to seal the bottom of the pre-oral cavity while swimming.

Notice the Dorsal view, which emphasises the massive overlapping jaws and the simplified upper head capsule.

Both the adult and larvae are highly predatory and feed on a variety of small aquatic animals including small fish.

The larvae are often called water tigers; they have long sickle-like jaws that are hollow and when they attack a prey they suck outs body fluids through the channels in the jaws. These larvae are very active and can attack an animal much larger than themselves.

Mandibular sucking channels occur in larvae of some groups of beetles (for example Dytiscidae, Lampyridae).

The mandibles form a sucking apparatus together with the laciniae in neuropteran larvae.

[Know the how the mandibulate mouthparts enable feeding in cockroach.]

 

About the Author: This educational content on Zoology is written by Rekha Debnath, M.Sc. & M.Phil. in Zoology, with a foc us on university-level academic topics. Read the full Author Credentials and Background here.