It looks like the general pattern of the broader Meloidae family of blister beetles is for their larva to feed on other insect larva, mainly bees. Strategies vary among different species in the family: some lay eggs near bee colonies and have larva that seek out bee (or other insect) larva, some lay eggs wherever and eat whatever's handy, and some go through varying degrees of trickery to actively attract the target species and hitch a ride.
The Meloe genus is mostly specialized on bee larva, and particularly on hitching a ride on adult bees to the bee larva. These three species
in particular all appear to hang out near flowers, then hitch a ride on a solitary bee back to its nest, where it attacks the larva.
I imagine the evolutionary path looks something like:
0. Generalist ancestral blister beetle lays eggs near bee nests, and its larva try to crawl in and attack the larva.
1. Over many generations, the generations of beetles evolve to hang out near flowers and attack worker or solitary bees rather than going looking for bee nests, as with the other species currently in the Meloe genus.
2. Some unlucky larva will get picked up by male bees instead of females, but some have mutations that lets them transfer from male to female bees. The larva with that mutation are more likely (twice as likely?) to survive and reproduce, so the mutation spreads.
3. Mutations arise that make some of the beetles emit pheremones that attract male bees, increasing the likelihood of bees vising their flowers in particular. Since they already know how to transfer from male bees to females, and from females to larva, this is a big evolutionary advantage.
4. This works so well as a survival strategy that they don't even need to look for flowers anymore, just hang out at the tops of any old stalk and smell like sexy bees.
5. The more they *look* like bees in addition to smelling like bees, the more likely they are to attract bees and survive to reproduce. So mutations that make them start hanging out together in a vaguely bee-like lump are advantaged. Then more mutations accumulate, doing a better and better bee impression over the generations.