When is a caterpillar not a caterpillar?

Today I want to concentrate on the humble caterpillar.

We have all seen at least the more commonplace ones from time to time, but do we actually know what we're looking at?

For instance, is this a caterpillar?

What about this example then?

Or how about this weird looking creature?

Here's a face-on view of this one if it helps?

One more chance to get it right...do you think this is a caterpillar?

Unsure? Do you perhaps think that these are all caterpillars? Maybe, none are?

The truth is that you would need to have chosen the third & fourth pictures (The weird creature!) as being the only true caterpillar amongst those examples.

If you did, then congratulations, you probably know your caterpillars and won't need to bother reading the next section of this blog.

If that has surprised you, or confused you, then I'll try and explain.

Let's start by taking a look at a diagram of the parts of a caterpillar...

Caterpillars come in many colours, shapes and sizes. Some are hairy, some not. Regardless of these differences they all share certain morphological features.


The diagram above shows these features.
1. Head
2. Thorax
3. Abdomen
4. Segment 
5. Horn
6. Prolegs
7. Spiracle
8. True legs
9. Mandible
10. Anal prolegs


The important part of this information, as far as a quick identity check goes is the amount of legs.
If you are unsure as to whether you are looking at a caterpillar or not, count the prolegs. Caterpillars may have up to five pairs of abdominal prolegs, but never have more than five.


Caterpillars are the larvae of butterflies or moths and belong to the order 'Lepidoptera'.


If the caterpillar/larva you are looking at has more than five pairs of prolegs, then it's likely to be a sawfly larva.
Sawfly larvae look very similar to caterpillars but are an entirely different kind of insect.
Sawflies are related to bees and wasps and belong to the order 'Hymenoptera' Like caterpillars the sawfly larvae also feed on plant foliage.

Close-up of caterpillar prolegs

And so, it's a fair bet that if the insect has 6 pairs of legs or more, it's probably going to be a sawfly but definitely isn't going to be a caterpillar.

A sawfly larva (count the legs)

Caterpillars don't breathe as we do of course. They use the spiracle to 'exchange gases'. They do require oxygen to live, just as we do and produce carbon dioxide as a waste, just as we do.They don't however have lungs to transport oxygen around the body. Instead they use a series of tubes called the tracheal system to perform the gas exchanges.


This system can't be controlled by the insect and only proves efficient for small  organisms. If this were not so, we might find ourselves living with giant insects (now there's a thought).


So there you have it. The rough guide to caterpillars.


Until the next time then...