One of the more curious puzzles of pokemon evolution which has been debated throughout history – evolution in the sense of how a species changes over time, not of how an individual metamorphosizes throughout its life - is the question of a Kakuna's arms. There is ample evidence that in antiquity Kakuna were equipped with stinger arms, a weaker version of those of their evolved form of Beedrill and these stingers provided the pokemon with its main means of defense. In prehistory, the fossil record shows that their arms were as prominent as they would be after they evolved.
Modern Kakuna, however, are powerless creatures which can only harden their shell or attempt to spit poison barbs to defend themselves. If discovered by a predator, they are nearly always eaten, and the Beedrill population has been declining throughout modern times as Kakuna struggle to survive their intermediate phase.
After a century of debate, scientists have come to the consensus that Kakuna with exterior arms evolved into Beedrill with weaker drill arms than those who were enable to grow them in the cocoon, which put them at a disadvantage after they evolved. It remains a powerful example of evolutionary self-destructiveness: had it not been for this armless, vulnerable strain of Kakuna, Beedrill would have remained as numerous as Pidgey to this day.