Evolution needs three factors – a means of reproduction, a
method for change, and a struggle for life.
Remove any one of these factors, and evolution stops. Remove the ‘struggle for life’ and you end up
with species like the horseshoe crab that have no effective predators or
parasites and have stayed the same for millions of years. Quite often though, the struggle for life
ends up as a sort of arms race between predator and prey, as one evolves an
effective defence and the other gets round it.
Nothing exemplifies this better than the humble caterpillar.
The role of caterpillars is to eat. Their purpose is to put on as much weight as
possible because adult moths and butterflies don’t need or have the ability to
eat any protein, they can only top up their energy reserves with nectar while
looking for the chance to put into practice the ‘means of reproduction’. Of course the fatter and juicier they get,
the more attractive caterpillars are to predators like birds that need to feed
their growing brood. The first defence
is to hide, which many species do quite effectively by burrowing into stems,
under bark, or underground. But this
limits their food supply and stops them getting at most of the nutrients
concentrated in the leaves of plants. So
an effective way of eating leaves without being eaten is to look like a leaf,
or failing that to look like a twig. I
have long marvelled at the camouflage displayed by adult moths that appear to
look like lichens or tree bark, or even broken birch twigs (buff-tip moth), but
caterpillars take an equally bizarre range of colours and forms.
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| Buff-tip Moth - or Birch twig |
Another path that evolution has taken is to make some caterpillars
unpalatable. One of the ways this
manifests itself is in the hairiness of some caterpillars. A fine example of this is the woolly bear,
the caterpillar of the garden tiger moth.
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| Garden Tiger moth caterpillar - Woolly Bear |
Whilst eating something hairy may be unpalatable, if the hairs
themselves contain irritating chemicals, then so much the better. One caterpillar that has such hairs is that
of the brown-tail moth. So irritating
are those hairs that they can even penetrate human skin and cause rashes. A young bird may not have come across a
brown-tail caterpillar before and so the caterpillar gets eaten, or partially
eaten, but part of the species success is due to the two bright orange dots on
its back. The bird may forget the hairs
but the warning dots make a powerful and obvious reminder.
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| Brown-tail moth caterpillar |
If you have fuchsia in your garden, you may well find a
bright green caterpillar with what looks like large eyes near its head. It can retract its head, which puffs out its
body and displays the large eyes to warn off any potential predator. As the caterpillar gets bigger and the leaves
get eaten, the body fades to a dark grey colour, which with the mobile and
retractable head gives the moth its name – the elephant hawkmoth. If you don’t have fuchsia, then another
favourite plant is rose-bay willow-herb.
In this discussion about prey and predators, I am forgetting
the prey of the caterpillar – the plants that they prey upon. Evolution has equipped these plants too with
some remarkable defences. They are not
able to hide, but they do thorny and unpalatable quite well. One plant, ragwort, is so poisonous that it
reputedly kills horses and cattle, though not if their owners look after them properly
(whether or not they have ragwort growing in their pastures). If you have ragwort, you may see a very
conspicuous caterpillar on it with striking black and orange stripes. This is the cinnabar moth caterpillar which
is immune to the poison, but will happily pass it on to any predator that
ignores the bright warning stripes.
