I don't know much about evolutionary biology so I may be missing something obvious.
It seems that vertebrate evolution somewhat frequently re-invents the fin out of the leg or wing and the wing out of the leg or fin.
If this pattern was extended, one would expect to see gliding spiders with leg-wings (dear God); insects whose ancestors had wings that now use their wings as a 4th pair of limbs for grabbing, sensing, or fighting; crustaceans or sea-spiders that swim with fin-like legs, etc. But as far as I know, for all their incredible number of species, we never see leg-fins, leg-wings, wing-legs, or wing-fins in arthropods. The occasional swimming bug (shrimp, etc) just uses its body segments and tail, and if it uses its legs at all it's just for stabilization or steering. Insects may or may not have wings, but they never have back-mounted limbs for grabbing, sensing, or fighting.
Is there a straightforward reason for this?