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Dear Louis,

I have found similar aberrations in several groups (Drosophilidae, Formicidae) but do not yet know anything about the cause. It must be different form the addition of a finger in vertebrates, since that is something wrong in the ontogeny. In these insects it is something going wrong during metamorphosis, since the tarsus per se did not exist when metamorphosis began. I assume it has to do with a disturbance of the pupa at some point during the pupal stage. Similarly, there may be aberrations in wing veneation and abdominal segmentation.

Paul