There are some 11,000 species of moth in North America. In your own state or province, there may be two or three thousand. Trying to learn so many species is enough of a challenge even if each one of them was as unique as a male Scarlet Tanager or American Goldfinch in summer. Unfortunately, just like the tanager and goldfinch, many species of moth come in more than one colour or pattern. What’s a moth’er to do?
There are still some species for which the colours are pretty distinctive and one should have no trouble locating it in a field guide (online or otherwise). However, if you’re not sure where to begin, are confused about the possibilities, or aren’t having any luck finding your moth, chances are it’s a variation on what’s displayed in the guide. It could be that your moth is an unusually light or dark individual. Some species are dimorphic, with different colours or patterns for the males and females. Some species vary regionally. And some species are just extremely variable such that it’s tricky to show all the possibilities in a guide.
The key is to focus on the bones of the pattern, rather than the colours. Take The Joker above as an example. In this species most individuals encountered are green, but there are also brown variations (I’ve encountered different explanations, one that it’s just a colour morph, and another that the brown individuals are the females). However the underlying black and white pattern is the same for both moths.
These are both Hemina Pinions. You can see that they both have that pinion shape, with the puffy epaulets and convex forewings. However, the colour patterns on these two are rather different. The key to their identification is in the orbicular and reniform spots, which are the same shape/pattern, the pale areas at the shoulders, and the dark patches along the bottom of the wing. (Incidentally, there is another species that looks similar has both a light and dark morph, but the different field marks for that species are common to both morphs as well.)
Unlike the Hemina Pinion, which has two reasonably distinct morphs, the Bicolored Woodgrain has a light version, a dark version, and every shade in between. Shown above are two ends of the spectrum. The common field marks across all shades of this species are the shape of the fused wing spot (that T shape is very distinctive), and the white anal dash at the rear of the wing.
The Bicolored Woodgrain at least sticks to shades of the same colour pattern. The Lunate Zale is all over the map, and it’s possible to catch half a dozen in the same evening, each of which looks slightly different from the last. Here are three examples. The first time I saw the top pattern I was convinced it was a different species, and was quite excited, only to come home and look it up and realize it was yet another Lunate. The zales can be a bit of a challenge with subtle field marks, but the thing that I notice with all of these is that small white spot in the middle of the forewing, as well as the shape of the dark lines curving over the rear of the wing. Also, behaviour can be a helpful key in identifying a species. Lunate Zales usually hold their wings spread out when at rest, while most other zales typically fold them backwards so they are triangular in shape.
And finally, the Rose Hooktip. No one would fault you for thinking these were two completely different species of hooktip, but they’re actually male and female. The Rose Hooktip is in a different genus than the others, and it shows in the shape of the wings’ hooked tip. However, even this aside, you can see the two share the same streaky markings along the top of the forewing, and the black arch under the hook.
Of course, these are tips, not hard and fast rules. Just as there are many species with very different colour morphs, there are also lots that look very similar but are actually different species. Really, the only thing that can easily help you sort things out is experience, but these suggestions should help you along the way.