Caterpillars – the shapes and sizes that they come in and for many the urge to touch, pick up and hold is almost irresistible. Yet although most butterfly and moth larvae are quite harmless, preferring to curl up in a ball when threatened, some will make it quite plain that they do not like to be touched. They will sting: here is a selection of the stinging caterpillars of the United States.The saddleback moth caterpillar,, pictured above has a 'face' that scares off many a potential predator. Yet it will also send you a definite message that it is unhappy with your sticky fingers on it. In a purely defensive tactic it will give you a sting that will dissuade you from picking up another. You can see the ‘horns’ that the caterpillar has on each end of its body – these are barbed spines which are also known as urticating setae.The setae funnel poison from glandular cells. When you pick up a saddleback some of the spines will stick in to your skin and break off from the caterpillar’s body. The poison will then spill on to your skin. Although it is a small almost invisible amount the results can be very painful. You will get a rash and swelling – and if you have sensitive skin you may even need hospital treatment.Put yourself in the position of the caterpillar. If something many thousand times your size picked you up, you would want some sort of defensive capability, surely? After all you do not want to enjoy your lunch, not be something else’s. Yet you might think that some of these caterpillars would never look like a tasty morsel in a million years. Take the one below, for example.While the saddleback could possibly pass as a roll of sushi , the Hag Moth caterpillar () hardly passes for what you might expect a caterillar to look like – more like a leaf bound starfish than a caterpillar. Yet pick it up and you will get a nasty surprise from this creature which is also known as the. Perhaps if a monkey and a slug could possibly reproduce with each other this is what we might get.This little guy is covered in setae and you will receive a nasty sting if you pick one up. If you live near the fields and forests of Florida to Arkansas, and north to Quebec and Maine then you might come across one of these on an apple or a walnut tree.is the best advice that can be given.This caterpillar looks something like a tribble from Star Trek but do not let appearances be deceptive. While it looks cute and fluffy this is the most dangerous stinging caterpillar in the US. The larvae of the southern flannel moth () it is also known as the puss caterpillar.However, if you come across something like this, which grows to about an inch long, dotouch it however much the temptation. Even when the spikes molt they remain dangerous and a sting from this will leave you with a lifelong memory of agonizing pain.The stinging rose caterpillar () is very appropriately named. It comes in a variety of colors but all of them have the pinstripe effect – four dark stripes down the back with a cream color in between. It may look harmless, but touch at your peril!Red spells danger and the Spiny Elm Caterpillar () has a series of red dots on its back to tell predators to back off. This, unlike most others in this collection, will turn in to a butterfly rather than a moth.For added safety this caterpillar remains and feeds in groups, which makes it potentially more dangerous than many of the others featured here. Can you imagine the consequences for Homer if the Simpson family came across a group of these on a camping trip? Cue an appearance by Doctor Hibbert within minutes.Although the adult is white, the caterpillar of the White Flannel Moth () is black, yellow and orange and it feels nothing like flannel. This caterpillar has a particularly nasty sting despite its harmless appearance. Look out for it in Virginia to Missouri, and south to Florida and Texas.Another caterpillar which does not look quite like our common idea of how one should appear is the Crowned Slug (). Its spines are arranged around its perimeter and act as a painful decoration around its flat body. Found from Florida to Mississippi and up to Massachusetts it can also develop red or yellow spots along its back. Best to avoid this squat caterpillar if you come across one.Everything about the Io Moth caterpillar () says! Found from Canada all the way down to Texas this caterpillar should be approached with extreme caution. It has a multitude of branches coming off its body, all ready to sting in defense.The eggs of this moth are laid in clusters, so the caterpillars are often seen together, looking more like the fronds of some peculiar alien plant than creatures. Yet if you touch one you will soon realise that it is for real.The White Marked Tussock Moth Caterpillar () is quite easy to spot because of its red head, with a black back and the yellow stripes down its side. As well as having a nasty sting this caterpillar is considered a tree pest. They will eat anything woody and decimate the area in which they breed and grow.Found from Canada all the way down to Florida this ravenous little beasty can be found on pretty much any tree, both deciduous and evergreen. It simply doesn’t care – it is one hungry and indiscriminate little caterpillar. Remove it from its food source, however, and it will sting you.The Buck Moth caterpillar () is found in Maine all the way down to Florida. A startling black and white color this caterpillar has a sting which is still visible up to two weeks later.Let’s return finally to the White Flannel Moth (). Although it looks harmless enough this caterpillar will hurt – badly – as the photographer involved discovered when he picked one up. A good rule of thumb is if a caterpillar looks spiny, spiky or furry, dopick it up as it most likely able to sting you.Yet the aim of this post has not been to frighten the life out of you about caterpillars in general. Most species of moth and butterfly caterpillars do not sting. However, some have adapted to fight back when touched. The best rule of thumb withcaterpillar is not to pick one up, just as much because they don’t like it very much than for our own sakes.