Hi Jeremy. Here’s my attempt at a super-succinct explanation:
Arachnologists almost universally identify spiders by genitalia. Post photos of spider genitalia from the proper angles, and we can probably take them to species.
And we preserve spiders in alcohol, which changes their color and makes them floppy. So we can’t go through a museum taking pictures of spiders and expect the pictures to look as they do live. And sometimes multiple species and even multiple genera look identical until you examine tiny features under a microscope, so those could never be IDd from in situ photo.
We have to collect spiders live, photograph them live, and then examine their genitalia. Field arachnologists who have done this a lot have a sense of a number of spiders, particularly in their areas of specialty, and that sense isn’t documented well anywhere. There aren’t many field arachnologists on iNat, and I myself come and go in spurts as I get frustrated.
So we don’t have a reliable way to ID spiders by photo. But a bunch of us are trying to work all this out on BugGuide by posting live photos and then photos of genitalia so we can all come to agreement on the species. Once we have agreement on the species, we have a sense of just one color form that spiders of that species can take. Sometimes they take multiple color forms.
And until we have reliably identified photos of all the spiders of a genus in a particular area, we can’t know that some other spider of the genus won’t look exactly like the one we just identified. So we still can’t call spiders that look like this newly identified one the species we found it to be.
It’s mostly the highly visual animals that can be reliably identified by photo. Those include birds, butterflies, and dragonflies. It also includes jumping spiders – at least the males, which tend to be colorful for the same reason that male birds and butterflies are colorful.