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.
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.
larvae, e.g. Danaines and Ithomiines, have developed an immunity to
the poisons, but store them in their bodies, or convert them into
even more toxic substances which deter predators. The hawkmoth
Isognathus leachi is toxic, and uses
bold stripes to "advertise" its noxious properties to potential
the larva of the Hawkmoth Isognathus leachi
from Peru is poisonous to birds ©
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
An interesting case is the neotropical
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.
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.
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
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.
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.