Tobacco hornworm caterpillars can grow to a large size and make a large meal for a vertebrate predator. The caterpillars may receive some protection from the tobacco inside their gut. The alkaloids in the tobacco they consume can be distasteful or poisonous to predators.
A toxic defense works best if it is accompanied by a warning to a predator. The predator associates the warning with the poisoning and avoids the caterpillar prior to attacking and possibly killing the caterpillar. Individuals derive the greatest benefit if they can deter the attack without suffering damage. Thus, we observe numerous adaptations that warn potential predators before an attack is made.
A common means for insects to deliver a warning is through aposematic or “warning” coloration. Bright colors such as reds or yellows against a black background commonly signal, “I am toxic.” However, some insects such as hornworms have multiple enemies and use cryptic coloration (camouflage) to hide from predators. What are the options?
Tobacco hornworms, Manduca sexta may use sound to warn potential predators. Late instar caterpillars can make sounds by grinding their mandibles (mouthparts) together. This previously unreported phenomenon is described by Yack and colleagues in the Journal of Insect Behavior.* The sound is made when the caterpillars are disturbed by poking them with forceps. Each movement of the mandibles against each other produces a single click. A train of clicks can be produced by rapid grinding of the mandibles against each other. The clicks are typically accompanied by regurgitation of the gut contents. The gut contents contain toxins that could deter predators. The caterpillar can aim its regurgitant in the direction of the disturbance for maximal effect. Trash talk and a spit in the eye can be an effective defense for vulnerable caterpillars.
*J Insect Behav (2012) 25:114–126 DOI 10.1007/s10905-011-9282-8