Caterpillars and how to identify them by Martin Robinson

  • One of the miracles in the lives of butterflies and moths are the dramatic transformations they make during the various stages of their life cycle. They begin life as eggs the size of pinheads which hatch into caterpillars, then in turn transform into pupae and then finally emerge as the adult flying insect.

    Identifying caterpillars can be tricky as their appearance gives no indication about what the adult insect will look like. Caterpillars are basically eating machines. They will increase their weight thousands of times, building the body mass needed for the final stage of their life cycle. The attrition rate among caterpillars is huge and most of them will not make it to the pupal stage. They are very valuable food sources for their various predators such as birds, small mammals and other insects. Caterpillars are slow moving and so have two main ways of defending themselves - camouflage or attack, some of them even using chemical warfare! Because many caterpillars are masters of blending in with their surroundings, they are very difficult to spot. Some caterpillars also change their appearance as they develop. The caterpillars of the grassland butterflies such as the Meadow Brown and Ringlet are so well camouflaged that it is very easy to walk through a grassy meadow and not see a single one, despite these butterflies being very common in the Marlow area.

    Many caterpillars will feed exclusively on their particular food plant, specific to their species. This food plant can be used as one way of helping to identify the caterpillar.  For instance, the caterpillar of the White Admiral, a woodland butterfly in the Marlow area, feeds only on Honeysuckle. A common butterfly in our area and the whole of the British Isles is the spectacular Peacock butterfly with distinctive eyespots on its wings. Its black spikey caterpillars are a common site on nettles, particularly growing up against hedgerows. When young, the caterpillars lead a communal lifestyle and spin a web which they use for collective protection. If the young caterpillars feel threatened by a would-be predator, they respond in a coordinated way by twitching their bodies simultaneously. This gives the impression that their commune is a much larger and potentially menacing animal. As they get older and bigger, they disperse but maintain their threatening appearance by growing formidable looking spines.

    A very distinctive caterpillar is that of the Cinnabar Moth. This beautiful black and red adult insect is day flying. The caterpillar is very easy to find as it flaunts its black and yellow hooped livery unashamedly as it feeds on ragwort. This common yellow flowering plant is well known for being very toxic, particularly to horses, due to the alkaloids it contains. As it feeds on ragwort leaves, the Cinnabar Moth caterpillar absorbs these toxins and its yellow and black colouring serves as a warning to any hungry passing creature.

    The best way to identify a caterpillar is to take a picture in the environment in which you find it and then look in a reference book or use an online resource such as the Butterfly Conservation website. It is always best to leave the caterpillar in its native environment rather than take it home. By chance you may have found the caterpillar on open ground or not on its food plant, in which case you may not know what to feed it on. Maintaining a food supply for the caterpillar may be difficult as some food plants may be challenging to grow out of their native environment 

    One of the other ways of not only identifying but also observing butterfly caterpillars is to attract them to your garden, if you have one, by growing their food plants and letting nature take its course. There are some helpful tips on the Butterfly Conservation website https://butterfly-conservation.org/how-you-can-help/get-involved/gardening/gardening-for-butterflies about which plants to grow and which species they will attract. Good luck and happy hunting!

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