Insects are cold blooded or, more accurately, are poikilothermic, a term which means having a body temperature that is in line with the temperature of their surroundings.  If the air temperature is 20 degrees, then their body temperature is 20 degrees and they are active, at 5 degrees they are barely able to move and at zero in winter they would be expected to freeze, but they do not. 

Like many insects these centipedes over-wintered under a log. Photo by Charles Sinclair. poikilothermic
Like many insects these centipedes over-wintered under a log.
Photo by Charles Sinclair

The shorter days and cooler temperatures in the autumn trigger a change in metabolism so that they store extra food reserves and, more importantly, produce chemical antifreezes in their tissues, so that in extremely cold conditions their organs do not freeze.  Therefore, as winter approaches insects will find somewhere out of sight to hibernate without freezing to death.  The name for this type of hibernation is diapause and each species of insect will always diapause at the same stage in its life cycle.  Examples are the Brimstone, which is the first butterfly to emerge in the spring, then soon after that the Comma and Small Tortoiseshell.  These all diapause as adults.  Next are the Small and Large Whites which diapause as pupae.  The Browns and Fritillaries, are seen in high summer as they diapause as larvae, and finally the Hairstreaks most of which diapause in the egg stage and these are on the wing in late summer or autumn.  Hibernating adults may choose crevices in tree bark or dense cover such as ivy, hibernating larvae will be beneath tufts of grass, and eggs out in the open or underground on their food plant.

There are exceptions to the usual diapause.  Queen bumblebees will fly in early spring because they can warm up their bodies by contracting their thoracic muscles to produce heat. Ants and the Honeybee are the only insects to pass the winter as a complete colony.  Each colony behaves like one superorganism and can survive without hibernating.  A colony of honeybees forms a winter cluster and eats stored honey to keep themselves warm.  The temperature of the cluster will be maintained at 5 degrees and bees on the colder outside of the cluster will continually change places with those on the warmer inside so they do not fall off and die. 

When working to manage the countryside for the benefit of ourselves and wildlife, a risk assessment is carried out for our own work and an environmental impact assessment for reducing damage to wildlife.  We can all do this to some degree by considering where we walk.  Tarmac paths or other hard surfaces along defined routes are best.  If these are not available then although compacted soil becomes damaged, it is still better to limit damage and walk on a well-used path rather than across open ground.  As we ramble, mow, slash, prune, coppice, lay hedges and fell trees, we should be mindful that there is a world beneath our feet and above our heads that is full of dormant life.  

Tag: Poikilothermic.

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