Swallowtail caterpillars of the genus Papilio are found virtually everywhere in the United States. This genus is very successful in terms of survival and fitness because the caterpillars possess unique structures called osmeterial glands. When a swallowtail caterpillar feels threatened, two antenna-like protrusions emerge out of a small pocket just above the caterpillar’s eyes. They look pink, shiny, and wet, and produce a strong fruity smell. The osmeterial glands confer a unique biological advantage.
These special glands are key in two distinct mechanisms that the swallowtail caterpillar uses to ward off predators. Two eye-like spots make the front of the caterpillar’s body look just like a snake’s head, and the color, shape, and wet appearance of the osmeterial protrusions closely resemble a snake’s tongue. Together, these features constitute a convincing disguise that scares away birds, which are perhaps the most common predator of caterpillars.
The other function of the osmeterial glands is to secrete a pungent liquid containing high concentrations of aromatic compounds. Alpha-Pinene, myrcene, limonene, beta-phellandrene, ocimene, beta-caryophyllene, farnesene, and germacrene are just a few of the terpene molecules secreted by the osmeterial glands.1 Do any of these compounds sound familiar? They are primary chemical constituents of many essential oils.
It turns out that caterpillars secrete aromatic compounds to repel other insects that may be caterpillar predators. One study found that the osmeterial secretion of caterpillars was the single factor that prevented them from being attacked by red ants.2 One particular study demonstrated that the osmeterial secretions were actually toxic to ants.5 Other studies have shown similar repellent effects on small spiders and also on mantids.3,4
Plant leaves and shoots in the caterpillar’s diet are the source of the aromatic compounds. The caterpillar’s digestive organs somehow sequester plant-derived terpenes and store them in the osmeterial glands, where they remain until they are ultimately released in order to ward off a predacious insect. For over 50 million years, Papilio caterpillars have used these natural compounds as their own bug repellent.6
This lesson in caterpillar biology suggests that some essential oils can provide reliable outdoor protection. Next time you are preparing for a camping trip or a hike, take advice from the swallowtail caterpillar and pack yourself some essential oils.
https://www.doterra.com/US/en/blog/science-research-news-caterpillars-use-essential-oil-constituents?