Saddleback or Packsaddle Caterpillar
A Stinging Slug Caterpillar with Warning Colouration
The Saddleback Caterpillar Acharia (formerly Sibine) stimulea, is easier to recognise than the adult moth. This is unusual, and probably explained best by the fact that it is diurnal (active by day) and therefore easily seen by predators. The normal solution here is to be camouflaged, but in this case the caterpillar can sting and evolution has equipped it with a conspicuous warning! (In contrast the adult is nocturnal and well camouflaged.)
Eyespots
Many adult butterflies carry ‘eyespots’ on their wings. They fly by day and it is thought that the sudden ‘flash’ of a pair of ‘eyes’ unsettles predators. See the ‘Fish and Insects’ blog - eyespots. Adult moths (which normally fly by night) have no need for this sort of deception, and their caterpillars – if they are poisonous – usually carry the universal yellow/black warning markings. The Saddleback Caterpillar has evolved unique markings which work by giving potential predators a glimpse of a single large ‘eye’ and the further confusion of two ‘heads’. Any predator that goes on to eat a saddleback will be badly stung and probably ‘remember’ the distinctive pattern. (This will not help the individual that was eaten of course, but it will benefit the species.)
Nasty Sting
The caterpillars have stinging hairs all over their body, especially on the ‘horns’ that poke out at either end. These hairs can inject a venom from poison sacs carried at their base, and the very painful sting can cause humans problems for days.
Two Heads
The ‘double-ended’ look of the saddleback caterpillar has two uses. First there is probably an element of ‘confusion’ – the predator is left wondering which end to attack – and second there is the fact that any attack is likely to encounter the stinging hairs. The fleshy ‘horn’ might well be damaged, but it is not essential for life. The nasty sting is likely to discourage the would-be predator, and the experience might affect future behaviour.
Food Plants and Disrtibution
The Saddleback Caterpillar is found across much of Eastern North America. It is a solitary feeder and will feed on many trees and shrubs. It often lurks in the shade under leaves, and its striking pattern can make it irresistible for children. They should be warned not to touch it because the sting is as bad as that of a bee and the swelling, sick feeling and rash can last a long time.
There is at least one record of this caterpillar crossing the Atlantic – probably on imported plants – and stinging a French toddler . This is just one example of the way humans can affect the distribution of animals – other ways can be found here.
Saddleback Caterpillar
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Packsaddle Caterpillar
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