bugbaer's Journal

Introduction

I was inspired to make this by the some of the Forum discussions this summer about getting more people involved in identifying in general and in identifying Lepidoptera in particular. This is not a field guide and I rarely show how to distinguish between two species. Instead, this is intended to help you figure out what group (family/subfamily, occasionally genus) of caterpillars something belongs to. Once you know that, it will be much easier to use a field guide, iNaturalist taxon photos, or another resource to make a specific ID. The genera and species I do show are almost always in the top 100 species of the Caterpillars of Eastern North America project. Because of this, there may be other similar genera/species in your area that are not shown, especially if you are at the edges of the project region. I've used scientific names rather than common names because they do not vary by region and are usually shorter.

I drew this because it was the easiest way to fit lots of caterpillars in a small space and show the characteristics that are common to groups without including unimportant or variable details. If something is colored, that mean that color is consistent within the group. I am not a trained scientific illustrator. If you are a better illustrator than I am and want to redraw this whole thing, please do, just share it freely and acknowledge the inspiration (Non-Commercial CC with Attribution).

If a caterpillar is vertically oriented, the caterpillar's head is at the top and I am showing a view from above (dorsal view). If a caterpillar is horizontally oriented, I am showing a view from the side (lateral view) and the caterpillar's head is almost always at the right. If you are observing caterpillars, these 2 views are usually the most helpful to document. If you are identifying caterpillars, there are cases when you need both of these to make the best possible ID. If an observation is recent, it may be worth asking if the observer has any other photos that show the missing view, even if they are blurry, etc.

In a few places, I have included a "no CV" symbol. This indicates that, as of summer 2024, I do not trust the Computer Vision model to accurately identify species within the group. The CV is often good at recognizing that caterpillars belong to the group, but more specific suggestions should be treated with caution.

If you want to zoom in on the drawings, right-click on the picture and choose "Open in a new tab".

Step 1: What does the caterpillar generally look like?

For ones where you can't really see the caterpillar because it's inside something, they generally need to be identified either by rearing or by identifying distinctive features of the leafmine, tunnel, or construct. Some resources for this are:
Leafminers of North America Project; you can also use the new Leafmine Evidence of Presence Annotation
Global Caterpillar Construct Project; you can also use the new Construction Evidence of Presence Annotation
Reared or bred insect larvae-nymphs Project: Any caterpillar (or other insect immature) that is accompanied by documentation of the adult form can be added to this project.
As far as I know, there aren't any projects specifically for caterpillars that bore into plant stems rather than mining just below the surface.

In all these cases, host plants can really help with caterpillar identification. If the observer is active, it can be good to ask them if they know the host plant or have an observation for it.

A. Hairy/Velvety

Does the caterpillar have hair tufts?

Note that hairy caterpillars can lose hair tufts and even basic body hairs fairly frequently. If you think something looks like it could be ___ except for some missing hairs, you may be right.

B. Bristly

C. Spiny

No CV for Limenitis: In eastern North America, L. archippus and L. arthemis are extremely similar. There may be some scoli characteristics that can distinguish between the species (Flaim 1993), but more data are needed and it's often hard to get a really clear view of the scoli in iNaturalist photos anyway. The best clue is that L. arthemis can eat plants that aren't in Salicaceae (the willow family). Since the CV still suggests L. archippus for Limenitis on plants from other families, I really don't trust it to distinguish the species.

D. Smooth

How many visible pairs of prolegs does the caterpillar have?

Final Thoughts

I hope this is useful, especially if you are just starting to identify caterpillars! If you're trying to decide between two IDs, feel free to tag/mention me with your question. You can also plug a taxon into the search bar here to see the ID Leaderboard for that taxon's caterpillars in eastern North America. Happy identifying!

And if anyone finds an error in this guide, please let me know!

Guides & Other Resources

Allen, T.J., J.P. Brock, and J. Glassberg. 2005. Caterpillars in the field and garden: Butterfly caterpillars

Marquis, R.J., S.C. Passoa, J.T. Lill, J.B. Whitfield, J. Le Corff, R. E. Forkner, and V.A. Passoa. 2019. Illustrated guide to the immature Lepidoptera on oaks in Missouri. FHAAST-2018-05.: The only publicly-available guide I'm aware of that provides diagnostic characteristics for some microlepidopteran families, although some of them are impossible to see on most iNat observations

Wagner, D.L. 2005. Caterpillars of eastern North America. Princeton University Press.: Mostly macrolepidopteran moths and butterflies. Note that the current Noctuoidea were still the Noctuidae when the book was written.

Wagner, D.L., D.C. Ferguson, T.L. McCabe, R.C. Reardon. 2001. Geometroid caterpillars of northeastern and Appalachian forests. FHTET-2001-10.: A guide to many but not all of the geometroid caterpillars in the project region.

Other References

Flaim, D. 1993. Comparative biology of the insect/plant interactions of eastern North American Limentis archippus and Limenitis arthemis astyanax and their palearctic ancestor, Ladoga camilla (Lepidoptera: Nymphalidae). Ph.D. dissertation, University of Maryland-Baltimore County.