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Girl or Boy Butterfly?

It's not difficult to tell a boy butterfly from a girl butterfly. Having learned to tell the difference, one generalization that can be made (allowing for exceptions, of course) is that here in Berkeley CA the female Anise Swallowtails are generally larger than the males.

Below is a pair of butterflies getting ready for their maiden voyages. The male is the smaller of the two, on the left.

Since wingspan is not an entirely reliable way to tell the sex of a butterfly, it's better to use the more air-tight approach: take a careful look at the butterfly's rear end.

Below is a close-up showing the tail-end of the above female's abdomen.

There's a bit of a terminal point, but otherwise no fancy apparatus.

By contrast, the male tail is elaborate---a pair of claspers to aid in the delivery of sperm to the female, and unfortunately not very clearly shown below due to their dark color and the dark background:

They are easy to see in the real live butterfly, however, if you wait for the butterfly to spread its wings out horizontally. And bright light, such as sunlight, also helps make anatomical details clearer.

There is a better picture of claspers in the section called Emergence.