Caterpillar Anatomy: Senses and Defenses

Vision:
The caterpillar’s head contains the mouth parts along with eyes and antennae. Caterpillars have poor vision. Though many appear to have large eyes, these are actually false eyes designed to fool predators. (Click here to see great examples.) A caterpillar actually has six pairs of very simple eyes called ocelli, which provide a general picture of the world around it. Instead of using their eyes to find food, the caterpillar relies on its antennae, which once again can fool the casual observer.

Antennae,Tentacles, and Tubercles:
Most people assume the spiky projections they see sticking out from a caterpillar’s head are antennae. In fact, a caterpillar’s antennae are very short projections found close to the caterpillar’s mouth, as shown in the drawing above. Along with the maxillary palp, these small projections are used to find and identify food.

Many caterpillars also have one or more pairs of tentacles, which are also used as sensory organs. These fleshy tentacles are often found at the front and back ends of a caterpillar, but several more pairs may be found along a caterpillar’s body.

Some caterpillars also have tubercles along their bodies. These fleshy knobs are sometimes topped with spikes, providing a caterpillar with protection from predators. The Pipevine Swallowtail caterpillar (Battus philenor) to the right has both long tentacles and short, light-colored tubercles.

Setae:
In addition to tentacles and tubercles, caterpillars also have setae, short hairs covering all or a part of their bodies. These hairs are very sensitive to touch. They can also serve as a form of protection, as the setae can come off and irritate membranes around the eyes and mouth of an attacker. On the Gulf Fritillary (Agraulis vanillae) caterpillar to the left, notice the tubercles topped with black spikes. Each spike is further covered in tiny hairy setae.

Caterpillar Defenses:
Caterpillars are food for almost anything larger than themselves: birds, lizards, even wasps will prey on these creatures. Caterpillars have a variety of defenses, including camouflage, toxicity, and mimicry.

Camouflage allows caterpillars to blend into their surroundings. Sulphur caterpillars are often the exact color of the leaf they are eating, and they lie along the center of the leaf, blending into the mid-vein. Other caterpillars, such as the Long-Tailed Skipper (Urbanus proteus) shown to the right, roll leaves around themselves to hide as they eat.

Host plants can also help caterpillars by making them toxic to predators. Some caterpillars isolate toxins from the plants they eat, causing the predators who eat them to become sick or even die. Caterpillars in the swallowtail family use these toxins to coat their osmeterium, a fleshy organ that is extended from the head and emits a foul smelling liquid. (Click here to learn more.) Toxic caterpillars, including monarchs (Danaus plexippus), are often brightly colored to warn predators to stay away. This behavior is known as using aposematic signals.

Caterpillars use several types of mimicry for defense as well. If a caterpillar is not toxic itself, it may mimic aposematic signals to make predators think that it is. Caterpillars may also mimic less-appetizing objects to trick predators. Giant Swallowtail (Papilio cresphontes) caterpillars, shown to the left, are camouflaged not as leaves, but instead to resemble bird droppings.

Illustration Credit: Caterpillar Head Drawing via MonarchLab.org