COCOON comes from ‘coque’, French for ‘egg shell’. ‘Un oeuf à la coque’ is a boiled egg and ‘se renfermer dans sa coque’ is ‘to retreat into one’s shell’. For zoologists, a cocoon is the container protecting an insect during the adolescent stage of development.
We humans go through Shakespeare’s seven ages, but insects experience only three or four. They begin life as eggs deposited on stems and branches of plants; the hatchlings will feed on the leaves in due course. A larva eats voraciously, shedding several skins.
Those of some species moult up to 50 times, although most caterpillars wear out fewer than five outfits. Then it’s time for either ‘partial’, or ‘complete’, metamorphosis.
Dragonflies are of the ‘partial’ persuasion. After months under water as a larva, the nymph climbs a stem, extends newly-grown wings, and takes to the air.
The Bombyx mori moth, of silkworm fame, was domesticated in China 5,000 years ago. Its white caterpillar undergoes ‘complete’ transformation.
Feeding on mulberry leaves, it sheds the skin four times. Then it weaves a cocoon from a single silk thread, 800 to 1,200 metres long, secreted from spinnerets located below the mouth. After ten days transforming itself inside the cocoon, the pale brown moth emerges, mates, lays eggs, and dies.
Mystics retreat from the world. John the Baptist preached in the wilderness; ‘clothed in camel’s hair and with a girdle of skin about his loins, and he did eat locusts and wild honey’.
The prophet Muhammad experienced his revelations in a cave on Mount Jabal al-Nour near Mecca. However, we ordinary mortals, being highly social creatures, are not natural cocooners. The rise in alcohol sales, and calls to domestic-abuse charities, during the current lockdown, attest to that.
Hibernation is another way small creatures minimise risks during difficult transitions, but it’s not an option for us; we are too big. Ireland’s largest ‘true’ hibernator is the hedgehog. Its body-temperature drops to a few degrees above freezing and the pulse rate sinks to 3% of normal.
Big bodies retain heat longer, so creatures larger than hedgehogs would be too slow cooling down in the autumn, and warming again in the spring, to make complete shutdown viable.
Some mammals opt for partial hibernation, which biologists call ‘dormancy’. A female polar bear’s temperature drops by 10ºC and her heart beats at half its ‘normal’ rate, but she still gives birth and suckles her cubs in her winter den.
Our current ‘cocooning’ is more akin to dormancy; pregnancy rates are expected to rise.
Bats and birds face a dilemma at the onset of winter.
Many birds avoid its hardships by migrating but, despite being able to fly, Irish bats hibernate instead. It was thought that cocooning was totally alien to birds, but native Americans knew better.
In 1947, ornithologists discovered that the poorwill of North America hibernates. The announcement didn’t impress Arizona native people. Their name for the bird translates as ‘the sleeper’.
One Irish bird cocoons. In their nests, high in buildings, swift chicks risk starvation when inclement weather prevents the parents gathering sufficient insect food for them.
By becoming torpid, they save enough energy to get them out of a tight corner.