Spider-Man: Ranking Every Comic Costume Worst To Best
38. Six-Armed Spider-Man (1971)
Peter Parker's blood was infected with radiated spider-venom, which is how he was able to transform into Spider-Man, but the process wasn't exactly complete.
There was a time when he spontaneously grew four extra arms making him a true arachnid — this required a new costume, which looks much like his other costume, but allowed for the inclusion of the new appendages.