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(First of all, apologies if it is not the right place. I ask here because it's the most expert entomology forum I know.)

My wife, a primary school teacher, raises V.cardui every couple years to show kids insect metamorphosis. They are commercial kits with larvae, net cage, artificial food. Usually everything goes OK, sometimes a caterpillar dies or a butterfly hatches with poor wings, but that's it.

This year, a malformed butterfly eclosed from the pupa. It has a poorly developed, black head, with no apparent trace of antennae, eyes or mouthparts. Yet it is alive and some trace of eyes could be present, since it apparently reacted to my smartphone flash when I tried to make a picture using it. Wings are also distorted because it fell shortly after eclosion, apparently -my wife found it upside-down on the floor of the cage and had to help her stand up. It's too weak to fly, but it can move.

I posted it on Facebook and a friend (who is a lecturer on Drosophila molecular biology at UC London) said it could be a genetic mutation, possibly Pax6. The odds seem astronomical to me, however. What else could have happened here? I tried to find descriptions of similar malformations in the literature, but to no avail.