Many travelers to the land of Hoenn encounter the sight of a Wurmple rearing up to shoot poison at a Swellow, and presume it to be no different than a Weedle doing likewise against a Pidgeotto: a desperate attempt to scare off a predator, or at least use its poison to avenge its own death. This understanding is completely backwards: although poison does slowly damage a Swellow, it makes it twice as strong in combat. Were a Swellow to eat a Wurmple, they would find that although their digestive system can handle them, they give so little nutrition that they are not worth the effort they take to find. The reason Wurmple shoot their barbs at Swellow is not self-defense, but a tribute paid in poison: many Swellow will stay still and absorb waves of attack until they are properly poisoned, then fly off and use their newfound power to pursue larger prey.
Wurmple venom is so weak that it takes days to wound any living thing. It evolved precisely to maximize the power of Swellow by tricking their bodies into fighting like their life depended on it while doing little actual damage. Those Wurmple whose venom is most effective find Swellow warning off the local predators with threats of eating them, and gain experience each time they return for their aid. It is these Wurmple who typically survive to evolve into Silcoon or Cascoon, because practicing on Swellow is the only way for Wurmple to grow their skill without being knocked out or eaten.
When Wurmple evolve, their venom becomes too dangerous, so Swellow carry them into the safest treetops they can find as a reward for their service. Eventually, they become Beautifly or Dustox and lay countless eggs: the cycle begins anew.