Why Do Caterpillars Have Hair?

While walking in your garden, you’ve probably noticed all sorts of caterpillars in varying shapes, sizes, and colours. Some look smooth with squishy exteriors, while other caterpillars appear to have a fluffy or fuzzy appearance, covered with hair. So, why do caterpillars have hair?

The tiny hairs, quills, or spines on a caterpillar are a defensive mechanism that gives them a fighting chance against predators. 

These bristles or setae are what protect larvae from predators Most of these squishy insect larvae are completely harmless, while some of them are covered with numerous tiny bristles that can inject dangerous toxins into you upon coming in contact with it.

The poisonous bristles on a caterpillar can cause several symptoms depending on the caterpillar species. Symptoms may include nausea, pain in the stomach area, rashes, and irritating allergic reactions.

Around 50 varying species of these hairy and spiky poisonous species exist in the US. The puss caterpillar and the saddleback caterpillar are among the most common of these species. 

Saddleback Caterpillar

Why Do Caterpillars Have Bristles?

The bristles called setae are what keep a caterpillar protected against predators. Some species have bristles that won’t irritate and are rather harmless, but other species have urticating hairs that are hollow and may cause irritation.

Envenomating hairs are the type of bristles that have venom and are hollow. The venom comes from a gland at its follicle base. When you or any predator comes in contact with these types of bristles, the tips come off, and the venom is released into your body.

Depending on the species of the caterpillar, the venom can have varying effects on you. You can get irritating rashes, itching, or something serious as nausea, pain in the surrounding area, or a burning sensation. 

Other types of bristles may not have venom in them, but they’re very sharp and get stuck in your skin, penetrating your skin. This can cause rashes and itching and may cause an allergic reaction also.

When predators come too close, they feel very uncomfortable upon contact with the caterpillar’s bristles, and the spines often hurt the smaller predators such as other insects or reptiles, amphibians more so they leave the caterpillars alone. 

In the wild, several types of caterpillars have very bright colours accompanied by bristles. This bright colouring indicates that the caterpillar is poisonous and predators should stay away from it. This way, most predators don’t even try to come close to them. 

Are Caterpillars Spiky or Soft?

Some caterpillars are soft, while some are spiky, covered with bristles and hair-like quills called setae. The soft ones are safe to handle and pose no threat.

Swallowtail caterpillars and painted lady caterpillars are some of the examples of smooth-bodied caterpillars that aren’t dangerous and won’t have spiky bristles to hurt you.

The monarch caterpillar is another smooth-bodied caterpillar that doesn’t sting you with venom. It’s harmless. However, monarch caterpillars are toxic to eat. Otherwise, they don’t possess any external defensive mechanisms that can hurt you. 

Soft and smooth-bodied caterpillars generally pupate into butterflies, while fluffy, fuzzy, and spiky caterpillars mostly grow into becoming moths.

The fuzzy and spiky caterpillars are often poisonous and can sting you like a bee or a wasp if you touch them, so a safe distance should always be kept whenever you see a fluffy caterpillar. Some hairy caterpillars can be harmless, but often are dangerous. 

How to Identify a Poisonous Spiky Caterpillar

It’s important to identify poisonous caterpillars for obvious reasons. To know if a caterpillar is dangerous or not, you must note if their colouring is bright or dull, what type of hairy covering they have and if they have any specific markings on their body.

The caterpillars that are dangerous are brightly coloured, and their bright colouring indicates that their bodies have toxins to warn predators not to bother them.

The caterpillars with bristles and spikes are usually the ones that often have very bright colours. Their spikes are hollow and have venom in them that, upon contact, gets injected, as mentioned earlier. 

The saddleback caterpillar is a common type of spikey caterpillar that has several thorns and spikes on its body. It has several identifications through which you can figure it’s a poisonous caterpillar, and handling it or going close to it may be risky.

Its identifications include a very obvious green patch on the back of the caterpillar. 

The patch is bright green with white borders, and the caterpillar has brown colours at its rear and frontal ends. There’s a large brown spot in the middle of the green patch. The saddleback caterpillar also has two intimidating fake eyes on one side of its body.

The caterpillar has big thorns on both of its ends. The thorns have several bristles or spikes on them that the caterpillar stings with. These bristles are very poisonous. The saddleback caterpillar’s fake eyes, spikes, and bright colouring are all defensive manoeuvres. 


Caterpillars come in all peculiar shapes, sizes, and colours. Caterpillars can be smooth and soft with squishy bodies, and they can also be spiky and fluffy with lots of hair or quills.

A caterpillar’s bristles or hair can protect it from predators trying to eat them as bristles often have venom in them, and upon contact, they get dislodged and release toxins. Depending on the caterpillar species, the toxins will have varying effects on the animal or person that came in contact with it. 

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