Written by Joe Ballenger
For the final entry of the year, I decided to tackle a question that is something that I have a lot of experience with, but one is also really challenging to answer at the same time.
Does this caterpillar change its skin colour when its about to go to the coocoon stage? How would we be able to tell how old the caterpillar is? Is there way to tell the age?
The caterpillar Jessica sent us was one of a Tussock moth, a species I don’t have any experience raising. So we couldn’t answer the question, but I did want to write a post about how to tell the age of caterpillars. It’s a really important topic.
When you’re doing research on insects, you need to be able to tell whether they’re growing properly for a number of reasons. If you’re testing insecticidal proteins or new diets (which is what I do), you need to be able to tell if your protein is stunting the caterpillar’s growth. If you’re looking at host-parasite interactions (which is what I did to earn my degree), the caterpillars need to be at the proper stage because parasites usually only attack caterpillars at very specific parts of the lifecycle. This is a skillset which is integral to the scientist’s toolbox.
When I was in graduate school, I handled hundreds of caterpillars every day. Nowadays, I handle thousands. Weighing each caterpillar individually would be impossible for my current job. Consequently, I need to be able to do this by sight without the use of any tools.
So how can you tell how old a caterpillar is, and more importantly, why is it hard for scientists to do it over the internet?
Growth and development
All insects grow in a very specific way, by shedding their skins. They shed their skins a specific number of times, usually five or six, and grow an enormous amount throughout that time. The video below shows how the process works in the Monarch caterpillar:
The caterpillar can grow hundreds or even thousands of times it’s size in just a few weeks, which makes it easy to tell if the caterpillar is young or old. If it’s very young, it will be very small. If it’s very old, it will be bigger. Monarch caterpillars have antenna-like projections which grow as they age. Other caterpillars, like the Tiger Swallowtail below, change colors as they age. Interestingly, young swallowtails also smell different when they get older.
There’s another way to tell when the caterpillar begins to pupate. Just before entering the pupal stage, the caterpillar expels whatever is in it’s gut. The gut gets remodeled during the pupal stage, and the adult won’t be able to digest the food that’s in there. So the caterpillar just clears it’s gut.
This results in a very obvious color change. You can see the gut’s contents through the caterpillar’s skin, and this is actually a type of camoflauge which helps it blend into it’s environment. Expelling the contents results in a color change, so instead of seeing the diet you’re seeing the caterpillar’s blood through it’s skin.
This stage is called the wandering stage, or sometimes the prepupa. Unfortunately, different lighting can make caterpillars look quite different. You need a side-by-side shot to see this, and this isn’t commonly photographed.
So if you’re familiar with the biology of caterpillars, you can sometimes tell the age by sight or smell if you’re dealing with the right species.
Sometimes, however, you need to be pretty specific. You need to be able to tell whether the caterpillar has shed it’s skin once, twice, three, or four times…depending on what you’re doing.
So how is that done?
Head Capsule Size
The one part of the insect which grows in a stepwise manner is the head, oddly enough. For this exercise, we’re going to be talking about honeybee development. They’re a different group of insects, but close enough to butterflies to show how staging works. There’s also really good video of their lifecycle online.
If you were to measure each of those bee larvae after they’ve shed their skins, you’d get a series of numbers which look like this:
There’s a lot of numbers there…but they don’t matter as much as it might seem at first. The important part of this graph isn’t so much the exact measurements, but how close the different larval stages are in terms of size to one another.
For example, if a larva has shed it’s skin 5 times (L5 above), it’s head will be about one-third larger than a larva that’s only shed it’s skin 4 times (L4) and twice the size of a larva that’s only shed it’s skin 3 times (L3). When you’re routinely working with these insects, the difference is very easy to spot.
However, you need to work with one species a lot in order to be able to recognize this. You need to do this on a daily basis, and practice to keep from forgetting how to do it. It’s difficult at first, but quickly becomes secondhand.
Then, it seems to go away as quickly as you learned it, once you move onto another species. It’s something you have to re-learn every time you switch to another species, or switch back.
The Bottom Line
There’s over a million species of insects, each with their own unique lifecycles and markings. Insects routinely used in research, so-called model organisms, have lifecycles that are very well characterized. Some non model insects have some really general things that can be used to tell age.
However, not all insects are that well characterized. Even though entomologists have been studying insects for hundreds of years, we’re still scratching the surface of their biology. It’s a side-effect of only having so much time to devote to a discipline; you can only be so familiar with so many insects. Unfortunately, this is a really high level of familiarity that we can’t possess unless we work with them daily.
Unfortunately, photographs can also be really tricky. Lighting can make the caterpillar look different in important ways, and judging size this specifically can be very difficult without a ruler nearby. We can go by markings sometimes, but they have to be something really distinct.
If you’re curious about the exact stage of the caterpillar in our inbox, it appeared to be an older larva. However, we weren’t sure if it was about to pupate or had one more molt to go before pupation.
Correction: The fact that insect head capsules grow in this manner is called Dyar’s Law. A rather fascinating man by the name of Harrison Dyar is the one who discovered this, a fact which I was unaware of at the time of publication.
1/8/2016, 7:32 PM
Since I identify diverse butterfly caterpillars worldwide from photos alone all the time, my trained eyes look for:
* Overall size relative to the leaf, venation, stem, etc. (knowing the length is helpful)
* Comparative head-to-body size (generally greater in younger instars, less in older ones)
* Relative size of body tubercles and branching scoli, including horns and tails (nymphalids and some pierids)
And as Joe says, the decision is much easier with the bug itself in front of you. FYI, not all cats undergo a prepupal change in color or bout of wandering, with many butterflies “fading” little, if at all, and pupating more or less where they last fed. For those wanting to learn more about the developmental progression of head capsules, please Google Dyar’s Rule/Law.
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