There is a constant battle between plants and the caterpillars which eat them, each evolving ways to try and stay ahead in the struggle for survival.

Plants have evolved a variety of methods to protect themselves from being eaten. Many extract minerals from the soil, and chemically convert them into toxic compounds - alleochemics, which in theory will kill caterpillars, or at least discourage them from feeding.

In practice however many caterpillars have evolved ways to avoid being poisoned. One method they use is to bite through leaf veins, stems or petioles to allow the toxic juices to bleed out before eating a leaf. Melinaea glasswing caterpillars for example cut circular trenches in leaves to cut off the flow of toxins, and then devour the enclosed tissue. Brahmaea caterpillars bite through stems to cut off the toxins and then feed on the drooping leaves. Other species simply restrict themselves to nibbling at the edges of leaves where toxicity is minimal.

the larva of the Hawkmoth Isognathus leachi from Peru is poisonous to birds © Adrian Hoskins

In many cases the toxins sequestered from larval foodplants are passed forward to other stages of the lifecycle, e.g. in Ithomiines the toxic qualities are passed to the adult butterflies.

An interesting case is the neotropical Arctiid moth Utetheisa ornatrix, whose caterpillars feed on Crotalaria. From these plants they sequester pyrrolizidine alkaloids, a group of toxins that render the caterpillars unpalatable to birds. These PAs are stored within the bodies of the caterpillar, pupa and resulting adult moth, all of which inherit the toxicity and are thus protected from predators. They are also passed to the eggs, providing them with protection against a variety of predators such as ants, and from egg parasitoids.

Plants fight back !

When caterpillars develop an immunity to the toxins, the plants become threatened, and have to evolve other ways to protect themselves. Some grow thorns to make it difficult for caterpillars to walk on their leaves and stems, or develop tough leathery leaves that are difficult to digest.

Caterpillars of Parides Cattle Heart butterflies feed on Aristolochia vines, but some vines have found ways to defend themselves. They do this by only "permitting" the butterflies to lay a limited number of eggs. If "too many" eggs are laid, the leaf around each extra egg dies, and the dead tissue drops to the ground, carrying the egg with it !

In South America, Passiflora vines have evolved a seemingly "intelligent" means of protecting themselves from being eaten by the offspring of Heliconius butterflies. The butterflies normally only lay a single egg on each Passiflora, so as to minimise competition between siblings for food. Some Passiflora species have "learned" to make use of this fact by randomly producing tiny structures on their leaves or stems which mimic Heliconius eggs. Whenever a Heliconius detects an egg - or a false egg - it is inhibited from laying "further" eggs on the plant, so the vine effectively prevents butterflies from ovipositing on it.

Another vine Passiflora adenopoda has evolved a different trick - its leaves and stems are covered with a coating of sharp microscopic hairs which puncture the skin of browsing caterpillars, rendering them immobile, and killing them by starvation.