What is the best way to identify the sex of caterpillars?


I am a fairly new user here (but I love it), and I have been recording a good amount of moths and caterpillars. I just want to know if there’s a good way to identify the differences between the sexes. I apologize if this seems like an unnecessary question.

If you know of any good material to read up on, please feel free to link it below. I did a quick search of the forums but I didn’t find anything related but feel free to move this if necessary.

Thank you for your time!



Caterpillars do not have any sexual organs. Only during the pupal stage do the reproductive organs form. So really the only way to tell is to wait until the adult has eclosed! Although for many species you can tell the gender by looking at the pupa too.


The nature talk section here is meant to be a place to discuss or ask questions about the natural world, not a section about how to use iNat, so no need to apologize at all.

So fire away with the questions or comments, hopefully there will be someone with an answer among the forum readers.


Most caterpillars are not sexually dimorphic, and the adults aren’t either (apart from antenna form), so sexing them is probably not possible.

But, good news!, lepis that have strongly sexually dimorphic adults, often have sexually dimorphic caterpillars. In my experience (in Africa) these are mostly lasiocampids, lymantriines and arctiines. Usually it’s just a size difference (females larger than males).

We have one species with obvious sexual dimorphism - Mountain Whitespot - the females in later instars are orange while the males stay black. Only the adult male is winged, the female is a wingless grub-like beastie who never even leaves the cocoon. Males find the females by following their pheromones, chew their way into the cocoon (with what, one wonders), and mate with her inside the cocoon. She then lays eggs in there, and when the pillars hatch, their first meal is their mum. This is a fascinating species all round, and there are some populations that are completely parthenogenetic without any males at all! But I digress…:-)


I realise I didn’t answer your question very well, bookworm86, because you asked for references.

I don’t know of any references for sexing caterpillars specifically, but there are some good references for caterpillars in general.

You don’t say where you’re based, so I’m going to assume North America. There is an excellent book by David L. Wagner called Caterpillars of Eastern North America which is good for pillar IDs and also family characteristics.

Good online resources include:
UK Leps - caterpillar morphology
Lepidoptera - keys & descriptions (focused on British Columbia, Canada)
iNat Pillar Parade project (southern Africa)

The last link has many other links under the About section to the published records of the African Caterpillar Rearing Group (CRG).


As I read your comment about pupae it conjured up memories of sexing moth pupae. I don’t remember much more than that!
@bookworm86 I know of no way to sex larval lepidoptera. Even with adults the most reliable way is dissection - males have claspers &etc (which are used in identification - some moth species can only be separated by dissection). This requires a dissecting microscope amongst other things. Since most of the observations on iNat are photographs this is not possible. And unless there is a good reason to kill and dissect a lepidoperan, I would discourage that process. Generally with moths and butterflies, the sex is not really important. Assume a 50/50 ratio, unless otherwise informed!


An interesting adjunct to the question is that when moths mate, the male builds a spermatheca inside the female. It contains the sperm of the male, and I assume she doles it out as necessary. I have dissected impregnated female moths and have seen the spermatheca (it was at a reserch facility, and I don’t remember why it was necessary).


Definitely not an unnecessary question! I actually didn’t even know it was possible to sex any caterpillars from a photograph, so I learned something today.


Thank you for replying! I’ll take a look at the references when I have time (probably not till Monday cause I’m busy attending a wedding).

Glad to help out @tiwane :smile_cat:. Also, to @karoopixie, yes I’m in North America (Virginia to be exact).

I’ll check again when I have time.

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6 posts were split to a new topic: Sexing Lepidoptera: Conversation and references

@mamestraconfigurata, @karoopixie, @kiwifergus… just a heads up: I made a new topic so that’s where your posts disappeared too. No worries, I’ll make great use of the new lepi-topic, personally!


I also just purchased the kindle version of the David L. Wagner book, have not been able to look at it very closely just yet though.



I assume the sex of a caterpillar can be identified by a genetic analysis. However, this may need a proper molecular biology lab to do! And I am not sure how to take a DNA sample and leave the caterpillar alive.

Much easier it would be to (1) document each caterpillar separately during its lifetime, in separate cages, by photography, and then (2) check out the sex of the emerging butterfly / moth. I am pretty sure this hasn’t been done a lot… at least I currently cannot find any scientific litrature on it.

@amzamz yes, there is a lot of info lacking. But, there are caterpillar rearing groups, and ours in Africa has been very active and has published - check out the Pillar Parade link in my comment further up.

When the CRG was started in 2011/12 we knew something about the life-histories and host-plants for about 7% of southern African lepis (info gathered from all published material), and much less than that for parasitoid associations.

By the end of our rearing season this year (June) we had records for 13.6% of our lepis (10,636 records), effectively doubling, in seven years, the available information which was accumulated in over a cenury before. And we have more than 600 lepi-parasitoid associations now too.

Some of the data gathered includes sexual dimorphism in caterpillars and cocoons, as well as pupae and adults. All from rearing experiments as you have suggested :-)

Why not start a rearing group in your area? If you need any help with this, just ask :-)


This isn’t quite true. Last-instar male caterpillars have testes that are often externally visible. A Google image search for “caterpillar testes” brought up several examples; here is one: http://apaturairis.blogspot.com/2014/02/larval-sexing.html


I follow that group on FB, although I am nowhere near the African continent. Lots of great stuff coming out of that group. It’s really important to get a life cycle worked out.

I am interested, but I have no clue on how to start one, what to record, and also I do not have a lot of money at the moment, so I can’t buy really expensive equipment. But, if there’s something affordable and easy, then I’m willing to do it. Also, do you let the butterflies go free afterwards?

Ok, I was mainly wondering for the annotations on the site. I like to be specific as possible, I obviously will not be dissecting any of the specimens in my photos mainly due to the fact that I do not have the tools and of course I don’t want to harm them. Thank you for the answer.

As far as I know, the SA Caterpillar Rearing group lets the catch go. Best way to find out is to ask them - https://www.facebook.com/groups/caterpillarrg/ The two people I know there are Allison Sharpe, and Suncana Bradley, both highly experienced.
I agree with your reluctance to harm moths just to get the sex. There are a few moths that are sexually dimorphic and a few moths that can only be identified with a dissection, but in the greater scheme of things it’s better that they do their thing than wind up in a collection.

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@bookworm86 some answers to your questions:

  • You can start the group on your own, but I would suggest getting together a team of people to help you. You are going to need taxon specialists to help with the IDs. Or join an existing group which already has the systems etc. in place.
  • Read our paper about how we went about it and our results up to 2015 - free download http://www.metamorphosis.org.za/?p=articles&s=Details&pt=527&i=1354
  • You will need to set up some kind of recording system. Excel would be great but unfortunately it doesn’t handle images well. We use Word to set up the Master Lists (a summary of the submissions). Some kind of database would be good if you have skills and time to maintain it. A DB is also better for analysing the data.
  • There is no equipment necessary for maintaining the group’s submissions apart from a computer and appropriate software. Unless you are going to curate a collection of voucher specimens (euphemism for drawers of dead insects) - then you need all sorts of stuff unless you can deposit vouchers at an existing collection/museum. I do not kill my moths or parasitoids, but many members do. If the beasties have emerged and died during the night then I will send them to the collection (moths usually last more than 8 hours, but many parasitoid wasps don’t). Put moths in a baggy in the freezer (with all their details written on a piece of paper), and preserve parasitoids in a small vial of 70-100% alcohol. Do some more internet searches for advice.
  • For rearing, the ‘equipment’ consists of containers, caterpillars, host-plant (or lichens or whatever else they may be eating (carnivorous caterpillars are rare, but they do exist)), soil, notebook & pen, & camera :-) Some people use tissue paper instead of soil (I prefer to try and keep things as natural as possible under the circs). Do an internet search and you will find loads of advice about rearing. If you are rearing species not reared before, then you have to wing it.

I’ve probably left out something important above because it’s the end of a very long day. Anyway, read the journal article linked above and then decide what direction you’d like to take.