I observed that during Gully bottoming, most of the swallowtail butterflies (P. nephelus & P. polyctor) release water from abdomen at regular intervals. Please explain why they show this habit and does this have an advantage.
https://www.youtube.com/watch?v=CYZy28g55dU&feature=youtu.be
It was release from the body (abdomen) of the butterfly in the form a jet. So, I don't think, it may be water alone. There may be other components also. (I have not analysed the content).
I am not sure, but this could be due to the absorption of salts. As nectar is poor in mineral salts, butterflies use to catch salts elsewhere, e.g., on soil, rocks, sweat. Perhaps a salt surplus could provide an osmotic disequilibrium, resulting in a sort of diuretic effect.
Swallowtail butterflies release abdominal dew or sap from plastron like structure found in the second terga of lower abdomen, during Gully bottoming. It happens due collection of humid drops in anti-siphoning device during day periodicity as well as after an active flight. It assist in maintaing homeostasis in terrestrial aerial insects that is an evolutionary mechanism driven and inherited from aquatic insects which possess true plastron on upper surface of abdomen and they release water from this structure after diving inside the water or during active swimming. This activity has been diminished and does not show linearity in occurrence.
I would say that the passing of water is in order to get rid of excess fluid after they have extracted nutrients and/or salts from it. After all, water is quite heavy and would impede their flight. All butterflies that are attracted to mud puddles do so, although it has only recently been regularly recorded. This is purely due to advanced digital photography and the increasing number of photographers in the field.
They are by the side of a water stream... could they have been puddling before?
I have found an article on Butterflycircle (http://butterflycircle.blogspot.com/2013/03/butterfly-bubbles.html) about butterfly releasing fluid from abdomen...
A quote from this article "Once the minerals are absorbed, the remaining fluids are ejected from the butterfly's body, so that there is no superfluous build-up of the excess fluids that the butterfly does not need."
According to this article, all the families of butterflies shows this phenomenon for various purpose.
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