An immature butterfly, known as a caterpillar or larva, has chewing mouthparts built for biting and grinding plant material. This is the opposite of the coiled, straw-like proboscis adult butterflies use to sip nectar. A caterpillar’s mouth is made up of six main structures: a labrum (upper lip), a pair of mandibles, a pair of maxillae, a labium (lower lip), a hypopharynx, and a spinneret for producing silk.
Mandibles: The Primary Chewing Tools
The mandibles are the most prominent mouthparts on a caterpillar. They sit just below the labrum and work as paired, hardened, tooth-like jaws that move side to side rather than up and down. Each mandible hinges at two points on the head, allowing it to swing inward and outward along a lateral axis to bite, cut, and crush leaf tissue. If you’ve ever seen the neat half-moon bites along the edge of a leaf, that’s mandible work.
Depending on what a caterpillar eats, the mandibles may have distinct regions for cutting and grinding. Most butterfly caterpillars are herbivores, so their mandibles are shaped to slice through tough plant fibers efficiently.
Labrum: The Upper Lip
The labrum sits above the mandibles and acts as an upper lip. It helps hold food in place while the mandibles do the heavy work of chewing. In many caterpillars, the labrum has a notch in the center that functions as a leaf guide, orienting the edge of a leaf so the mandibles can grip and cut it cleanly. It also closes off the top of the mouth cavity, keeping food from falling out during chewing.
Maxillae and Their Sensory Palps
Behind the mandibles sits a pair of maxillae. These smaller appendages help push food toward the mandibles, but their most important job is sensory. Each maxilla has a small, finger-like extension called a maxillary palp. Caterpillars “drum” these palps against the surface of leaves before and during feeding, essentially tasting the plant to decide whether it’s worth eating.
This tasting ability is remarkably concentrated. A caterpillar’s maxillary palps contain over 65% of its total taste receptor cells. Research on caterpillars exposed to bitter plant defense chemicals found that the maxillary palps were the structures responsible for detecting those compounds and triggering food rejection. The other taste sensors on the caterpillar’s body didn’t respond to the same chemicals. So the maxillary palps act as a caterpillar’s primary quality-control system, preventing it from eating toxic or unsuitable leaves.
Labium: The Lower Lip and Silk Spinneret
The labium forms the floor of the mouth, functioning as a lower lip. It’s actually two fused appendages that meet along the midline of the head. The labium carries its own pair of small sensory extensions called labial palps, which provide additional touch and taste information during feeding.
Tucked between the two labial palps is a structure unique to caterpillars: the spinneret. This small, nozzle-like opening is where silk is extruded. Caterpillars use silk for a variety of purposes, from anchoring themselves to leaves and branches to building shelters and, eventually, forming the chrysalis. The spinneret’s placement right at the mouth means the caterpillar can lay silk precisely as it moves its head.
The Hypopharynx
Inside the mouth cavity, a tongue-like structure called the hypopharynx sits on the floor between the other mouthparts. It helps direct food into the throat and is associated with saliva delivery, which begins breaking down food as the caterpillar chews.
How These Mouthparts Transform in Adulthood
During metamorphosis inside the chrysalis, the caterpillar’s chewing mouthparts undergo a dramatic reorganization. The mandibles shrink to nonfunctional remnants. Most of the maxillary structures disappear as well, with one critical exception: the galeae, which are small lobes on the outer edge of each maxilla. These two galeae elongate enormously, become concave along their inner surfaces, and lock together to form the adult butterfly’s proboscis, a hollow tube used to suck up nectar and other liquids.
The labial palps survive metamorphosis too, becoming the fuzzy, forward-facing sensory structures that flank the proboscis on an adult butterfly’s face. Everything else, the labrum, the powerful mandibles, the hypopharynx, is either lost or reduced to near nothing. It’s one of the most complete mouthpart transformations in the insect world: a set of jaws built for demolishing leaves is replaced by a delicate drinking straw built for sipping from flowers.