Did two species mix to make butterflies?

What child's imagination has not been captivated by the near-magical transformation that caterpillars undergo to become butterflies? This is the result of an ancient hybridisation between an insect and a worm-like animal, according to zoologist Donald Williamson, and now he says there is enough genetic information to test the theory.
Unfortunately for Williamson, now retired from the University of Liverpool, UK, the early returns are not encouraging.
Many insect groups, such as butterflies, bees and wasps, have larval stages that look nothing like the adults. Most biologists believe these evolved gradually, perhaps because natural selection favoured juvenile stages that differed from the adults and thus would compete less with them.
Williamson offers a different explanation. At some point hundreds of millions of years ago a larva-less insect - something like a grasshopper or cockroach, say - hybridised with a velvet worm. Also known as Onychophora, velvet worms are worm-like invertebrates with stubby, leg-like appendages. According to Williamson's theory, the resulting hybrid and its descendants now develop successively through stages resembling both parents.
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