[Alice]

Here is a question for Mike or any other butterfly expert.
Today two swallowtail butterfly caterpillars were spotted on a giant fennel, a big one and a much smaller one.
A few hours later only the big one was there together with what looked like its desiccated companion. Is this a simple moulting or a case of cannibalism? It seems that the zebra swallowtail is known for its cannibalism and scarce swallowtails will attack trespassers on their territory.

[MikeHardman]

Hi Alice,
Good observation. Your photo shows a fresh instar caterpillar of Papilio machaon, having shed its former skin.
Where the smaller caterpillar went, I can't say. But they are eaten by birds; Sardinian warblers know these little morsels are to be found in my Crithmum maritimum, for instance. I suspect it was not eaten by the larger caterpillar, as that would have been busy with its moulting at the time.

[Alice]

Thank you Mike.
So we don't have a cannibal after all! We certainly have lots of Sardinian warblers, so the most likely explanation is that the small caterpillar became lunch for one of them.