I didn't see this the first time but I can confirm foreleg (one) vs. hind leg (two) for AE as well. 'Fore-' is a prefix, 'hind' is an adjective, I think that's the only reason.
But it's not a huge deal; if I saw 'hindlegs' in print, I'm not sure it would leap out at me as wrong, which is probably why it sneaks by occasionally. Also, if you don't pronounce 'legs' with a secondary accent, you would be more inclined to write it as one word: 'hindlegs (one accent) vs. 'hind 'legs (two accents).
Whether that happens often enough to include it as a variant, I couldn't say. NOAD doesn't list it, but it does give 'hind limb (also hindlimb).'
'Mid' is sort of in between being an adjective and a prefix. AE tends to prefer it as a prefix, BE tends to use it more often as a freestanding word.
I've never had occasion to talk about more than four legs very much, and obviously I don't know what's in among entomologists, but just from the point of view of language, 'middle leg(s)' sounds the safest to me for insect purposes.
The catch with 'midleg' (or 'mid leg' more BE-ish) is that it looks like an adjective, say for describing pants: clam-diggers or capri pants are a midleg style. Even as a noun it could suggest the middle part of one leg (calf, knee, thigh, etc.): a wound in the midleg. 'Middle leg' at least is clearly a leg in the middle of the body, as opposed to something in the middle of a leg. If that makes any sense at all.