“Of necessity, he who pursues a very specialised task will do it best.” Xenophon
On a still, warm summer day if you lie in the grass or stick your nose into the herbaceous border you can almost hear the sound of munching. A whole army of insect larvae are chewing their way through the foliage. In amongst them are the woolly bears, large and hairy and, to the unwary predator, deadly. As the summer progresses they will disappear and re-emerge as garden tigers. This is not the work of Greek gods or shamans, this is evolutionary magic. The woolly bears are the caterpillars of moths of the Arctiidae (from the Latin arcticus representing the classical Greek ἀρκτικός ‘of the Bear’) family which includes the tiger moths. The beautiful Garden Tiger was once common in gardens throughout the UK, but sadly this beautiful creature, like many other moths, is becoming increasingly scarce.
The transformation of one creature into an apparently totally different life form, as in a caterpillar to a butterfly or a tadpole into a frog, is an amazing process. In some animals the juvenile form is a smaller version of the adult, in others, they are unrecognisable members of the same species e.g. the aquatic dragonfly nymphs (larvae) which dwell in ponds and are liberated as free flying acrobats on gossamer wings.
In the insect world metamorphosis is a common phenomenon. It is, therefore, not difficult to understand why the mystery of shape changing has intrigued and fascinated mankind, permeating the classical literature from Ovid to Kafka. Scientifically it is a simple case of post-embryonic reprogramming of gene expression i.e. the genes which make larval cells are switched off and those which make adult cells are switched on. So when the caterpillar begins to pupate and forms a chrysalis all the larval cells are destroyed except a small group of stem cells which differentiate to form the body and organs of the adult.
This explains the how, which is probably only of interest to the nerdy brigade (yes I am a fully paid-up member), but not the why which is equally as interesting. Superficially it seems to be a lot of trouble to go through at least two additional stages to get from egg to adult so there must be an a good reason. Nature is not always “red in tooth and claw” but the name of the game is to out-compete your rivals and maximise your chance of getting your genes into the next generation. You don’t really need to know about evolutionary strategies and game theory to work out why metamorphosis is a successful strategy and why it is so common. It is a case of division of labour between the adults and the larvae, hence the evolution of two very different specialised forms within the life cycle. The larva is the eating machine which accumulates energy and raw materials when there is ample food around, and when the time is right the focus moves from food to sex and the adult reproductive form takes over. Some adult insects only live for a very short time, concentrating on procreation and not feeding at all.
So the next time you see a small green caterpillar munching its way through your cabbages, pause for a moment and marvel at nature’s ingenuity. Remember there are no butterflies without caterpillars. At which point I will remind everyone that it will soon be time for the Big Butterfly Count (20 July – 11 August).