Wood nymphs (Cercyonis) seem to give a lot of people trouble so I thought I'd share some tips using illustrations from my Pocket Guide to the Butterflies of Washington (still available for $20 if you contact me privately via my email in the side bar). In addition to the three species we have in Washington (pegala, sthenele and oetus), silvestris occurs in Oregon and California, and meadii occurs in the Rockies down into Mexico. Cercyonis silvestris remains a bit of an enigma to me, it's sort of an indistinct version of sthenele with a bit of a pegala vibe, that's about all I can say about it at the moment. C. meadii on the other hand is quite distinct with its reddish coloration, and doesn't need much of an explanation. So let's look at the remaining three.
There are two key factors I look at first: the relative size of the two large eyespots on the forewing (FW), and the shape and definition of the ventral hindwing (VHW) median line.
First compare the eyespots: if the lower eyespot is larger than the upper one, it is almost always pegala, but in rare instances you may encounter a female sthenele with a slightly larger lower eyespot. If the lower eyespot is significantly smaller than the upper, it is usually oetus. If the eyespots are the same size, you're usually looking at either a male pegala or a female sthenele. If the lower eyespot is only slightly smaller than the upper, you may be looking at sthenele or a female oetus.
Now compare the VHW markings: the median line on oetus is very jagged (indicated with a blue line in my examples below) compared to the other species. However, it is not uncommon to find oetus without a defined median line, in which case the overall fine mottling, especially of the VFW, with no apparent two-toned coloration like we usually see in pegala and sthenele, makes it relatively easy to identify oetus in most cases. It is usually easy to identify pegala simply by its large size, but in photographs where that is hard to judge, one must compare the eyespots and wing markings. They often have somewhat of a flat affect similar to indistinct oetus, but unlike oetus, usually maintain a bit of a two-toned appearance between the outer and inner halves of the wings. Meanwhile, sthenele has a much less jagged median line compared to oetus, and usually a two-toned appearance between the outer and inner halves of the wings, similar to pegala but usually with more frosty appearance.
There are other slight differences, and of course regional variation across North America that isn't fully illustrated here with these WA examples, but I'll let the photos do the talking from here on. Nothing beats studying lots of photos to train your brain!
