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:

Grasshoppers

Grasshoppers have mandibulate mouthparts that are directed downward toward the leaves of their host plant.  The labrum is a broad flap that serves as a front lip. Mandibles operate from side to side with scissor-like edges for cutting and molar surfaces for grinding or crushing. Paired maxillae manipulate the food with fork-shaped laciniae and spoon-shaped galeae. The hypopharynx is a fleshy, tongue-like process between the maxillae. The labium functions as a back lip. Its large outer lobes are paraglossae and the 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.

Cockroaches

Cockroaches are omnivores.   They feed opportunistically on a variety of foods including decaying matter, plant material, organic wastes, and even dead insects.   The mouthparts are well-adapted for a scavenging lifestyle.   They include the labrum, a flattened sclerite that serves as an upper lip, a pair of mandibles with toothed inner surfaces for grinding or crushing food, a pair of maxillae with sensory palps for tasting, and a labium with a second pair of sensory palps that serves as a lower (or posterior) lip.   A tongue-like structure, the hypopharynx, is centrally located near the mouth where it helps mix food with saliva.

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.  Dragonfly naiads, honeybees, and weevils all illustrate this modification of some structural components while still retaining the overall ability to bite, chew, or grasp a potential food resource.

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.

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.

Weevils

Weevils have a narrow, elongated snout (rostrum) that protrudes from the front of the head and may be as long or longer than the rest of the body. The mouthparts (mandibles, maxillae, labium, etc,) are located at the very tip of this rostrum and by necessity are much reduced in size. This structural arrangement allows weevils to use their mouthparts as a tool for boring or tunneling into a substrate for the purpose of feeding or, in some cases, for the preparation of an oviposition site.

Haustellate Mouthparts

In some of today’s more “advanced” insects the mouthparts have lost the ability to chew or bite and have become entirely 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

Sponging-lapping:  The mouthparts of house flies and blow flies are specialized for soaking 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

Piercing-sucking:  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

Probing-sipping:  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.