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Gender is - contra general believe - not determined by DNA but by hormones. And of course, hormonalic levels are determined by DNA, .... if nothing goes wrong ! But quite often something goes wrong, spontaneously or under outdoor influences (in insects, for instance, heavy metals).

In vertebrates, humans included, due to blood circulation, hormonalic levels are even all over the body. ' Intersexes' here are generally caused by intermediate levels. In insects, there is no blood and therefore hormon levels might differ hugely from bodypart to bodypart, and sometimes they do. Put bluntly but not incorrectly, in an insect gender is a notion on cellular level. So you can have (and I do !) a Tachinid with female head, male claws and split genitalia (left female, right male).
Given this complexity, I am not sure if a general answer is possible to your question. If there is, undoubtedly it will be ' infertile'

This is an interesting example. In Dexia the sexual dimorphy is strong, which gives the opportunity to detect abberations. In, for instance, Peribaea tibialis, the sexual dimorphy is so low, that having a male with female vertex will practically be undetectable, unless you know what to look for. Therefore, my ' 1 out of 1.000 or so' might very well be an underestimation.

If you ever find an odd Tachinid (or even an supposed spec. Nov.), the first question you should ask yourself is ' Are we sure it is not an intersex ?"


Theo