Mouthparts

 

The ancestors of present-day insects were probably worm-like arthropods with a simple mouth opening near the front of a bilaterally symmetrical body.  Over many eons of time, tissues and appendages near the mouth opening came to be adapted for gathering and manipulating bits of solid food.  As insects evolved, they became more complex, expanded in range, and adapted to new food resources.  The structure and function of their mouthparts changed right along with their evolving diet and life style.  This is an excellent example of adaptive radiation  (an evolutionary process in which two or more populations, exposed to different selective pressures, diverge from a common ancestor).  Examples of adaptive radiation can be found just about everywhere in the insect world (think about variability in legs, wings, and antennae, for example).  Entomologists pay close attention to mouthparts because their structure allows us to infer what type of food is consumed — plant or animal, solid or liquid, dead or alive.  (Remember, “Form Follows Function” in biology).  Knowing something about an insect’s diet leads us to even more information about it’s ecology and natural history.

Mandibulate Mouthparts

In all “primitive” insects, the mouthparts are adapted for grinding, chewing, pinching, or crushing bits of solid food.  These are known as “mandibulate” mouthparts because they feature prominent chewing mandibles.  There are five basic components that form these mouthparts:

1. Labrum — a simple plate-like sclerite that serves as a front lip to help contain the food.
2. Mandibles — a pair of jaws for crushing or grinding the food.  They operate from side to side, not up and down.
3. Maxillae — paired appendages with the following parts:
   • Cardo — basal sclerite that articulates with the head capsule
   • Stipes — medial sclerite that supports a sensory palp
   • Galea and Lacinia — distal sclerites that act as fork and spoon to manipulate the food.
4. Hypopharynx — a tongue-like process that helps mix food and saliva.
5. Labium — a back lip that is derived from a pair of appendages that have fused together along the midline.  It is subdivided into the following parts:
   • Postmentum — fused basal sclerites that articulate with the head.
   • Prementum — distal sclerites that support another pair of sensory palps and divide apically to form four lobes; the two innermost lobes are called glossae and the two outermost lobes are called paraglossae.

Examples of insects with basic mandibulate mouthparts include grasshoppers, cockroaches, and ground beetles. Immature stages of many holometabolous insects (like beetle larvae and lepidopteran caterpillars also have mandibulate mouthparts.

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  Show Mouthparts Assembled
  
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  Show Mouthparts Disassembled
  

Modified Mandibulate Mouthparts

As insects evolved to feed on a wider variety of food resources, their mouthparts adapted accordingly through natural selection.  In some cases, an individual component of the mouthparts became specialized for a new function.  In weevils, for example, the front of the head is elongated into a long, slender proboscis.  The mandibulate mouthparts are reduced in size allowing the insect to excavate a deep narrow hole that is used for feeding, and perhaps later, as a site for oviposition.   In dragonfly naiads (immatures), the labium has become adapted as a prehensile tool that can be rapidly extended forward to catch prey.

 

Haustellate Mouthparts

Some of today’s more “advanced” insects have mouthparts that have become adapted for ingesting liquid food.  These are collectively known as “haustellate” mouthparts (derived from the Latin verb “haustor” meaning to draw up or suck).  They function in various ways:  probing/sipping, sponging/lapping, piercing/sucking, etc.  But regardless of how they work, they are still constructed from the same five building blocks found in mandibulate mouthparts:  labrum, mandibles, maxillae, hypopharynx, and labium.  Through natural selection and adaptive radiation, these parts have sometimes undergone radical changes in shape and function but they still occupy similar positions relative to each other (i.e. the labrum is always in the front and the labium is always in the back).  Examples of insects with haustellate mouthparts include true bugs, aphids (and their relatives), butterflies and moths, fleas, mosquitoes and many other types of flies.

Blow Flies

The mouthparts of house flies and blow flies are specialized for sponging up liquid food.  The labium has large lobes (labellae) with sclerotized grooves (pseudotracheae) on the under surface.  During feeding, liquid food collects in these grooves and moves upward by capillary action until it can be sucked into the food canal on the backside of the labrum.  The hypopharynx is hollow and surrounds the salivary canal.  A pair of large, hairy maxillary palps are usually present on the upper part of the proboscis.  When a fly lands on solid food, it may regurgitate a droplet containing digestive enzymes and then sponge up the residue moments later.  Yum yum!

Mosquitoes

The mouthparts of a female mosquito are highly modified to form a proboscis that is adapted for piercing skin and sucking blood.  Males have similar mouthparts, but they feed only on nectar.  The proboscis is similar to a sword within a scabbard.  The mosquito’s labium is the scabbard (sheath).  It surrounds six slender, sclerotized stylets that interlock to form a “sword” that probes for blood vessels.  The stylets include two mandibles, two maxillae, the labrum, and the hypopharnyx.  The labium retracts during feeding and all six stylets penetrate the host’s body.  The hypopharnyx is hollow and encloses the salivary canal.  The food canal runs through a groove on the back side of the labrum.  A pair of maxillary palps (long in males and short in females) arise near the base of the proboscis.  They are sensory in function.

Moths or Butterflies

Butterflies and moths have mouthparts specialized for probing into a flower and sucking out nectar.  A long, slender proboscis is formed by the two galea of the maxillae which interlock to enclose a central food canal.  At rest, this tubular structure remains coiled beneath the head; it uncoils by hydrostatic pressure when the insect feeds.  Large, conspicuous labial palps are usually present near the base of the proboscis.  The labrum and mandibles are completely absent (vestigal) in most of the Lepidoptera.

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Honey Bees

Honey bees collect nectar and pollen from flowers. The labrum is a short, wide flap that partially covers the other mouthparts and serves as a front lip. Mandibles are generalized grasping tools used for gathering pollen, handling wax, and grooming. The maxillae and labium interlock to form a hinged proboscis that can be extended from beneath the head to lap up nectar. The central-most part of the proboscis is a tongue-like structure containing the salivary canal. It is derived from the fused glossae of the labium. Long labial palps on the anterior side of the glossae are sensory in function. The galea of the maxillae flank the labium on each side, overlapping behind to form a channel through which nectar passes to the mouth. The lacinia is vestigal and the maxillary palp is very small, but the rod-shaped cardo and oblong stipes are easy to find.

Dragonfly Naiads

Dragonfly naiads (immatures) are underwater predators that feed on a variety of small aquatic prey.  Their mouthparts have stout mandibles and multi-segmented maxillae for chewing solid food — rather typical for a mandibulate insect.  The labium, however, is modified into a hinged scoop that can be projected forward rapidly to catch prey.  When at rest, the labium forms a “mask” that covers the insect’s labium, mandibles, and maxillae.  One hinge at the base of the postmentum and another hinge at the base of the prementum allow the labium to stretch forward so a pair of apical lobes can grab the prey.  These apical lobes are thought to be derived from the insect’s labial palps.

Grasshopperss

The grasshopper has mandibulate mouthparts that are directed downward for biting and chewing the leaves of a host plant. Its labrum is a broad flap that serves as a front lip. Mandibles operate from side to side. They have overlapping edges that cut like scissors and molar surfaces for grinding or crushing. Paired maxillae help manipulate the food with fork-shaped laciniae and spoon-shaped galeae. The hypopharynx is a fleshy, tongue-like process that hangs down between the maxillae. The labium functions as a back lip. Its large outer lobes are paraglossae and the very small inner lobes are glossae. Five-segmented maxillary palps and three-segmented labial palps serve primarily as touch and taste receptors.

Ground Beetles

Ground beetles are predators. Their mouthparts are directed forward to catch prey. The labrum is relatively short and close to the head capsule. Mandibles are long and curved with sharp tips for impaling a struggling victim. Maxillae have finger-like laciniae and spatulate galeae that are covered with a dense brush of sensory hairs. The hypopharynx is a simple, fleshy lobe. The labium’s large paraglossae and smaller paraglossae cover and protect the underside of the mouthparts. Like most mandibulate insects, ground beetles have a pair of maxillary palps as well as a pair of labial palps — both pairs serve primarily as touch and taste receptors.