what's the difference between a legless lizard and a snake...?

You'd have to ask a herpetologist that. Perhaps your legless lizard has some internal remnants of legs invisible from the outside. I am sure that there are differences between monkeys and apes that go beyond the simple matter of tails or lack thereof; the tail issue is sufficient for a quick identification. Moths and butterflies, on the other hand, are not as easily identified; there are differences in the way the wings are held at rest and what the antennae look like but there is some overlap in the antenna thang that I know about. Not sure about the wing thing.