Is a bee sting or scorpion sting one of the things that invalidate the fast?

We do not know of any
scholar who says that a bee sting or scorpion sting is one of the things
that invalidate the fast. Bees and scorpions inject poison into the body of
the one who is stung and he tries to avoid their sting, whereas in the case
of all the things that invalidate the fast, it is stipulated that in order
for the fast to be invalidated, the fasting person does them voluntarily. 

But things that are done involuntarily do not invalidate the
fast, as in the case of a person into whose mouth a fly flies and he
swallows it – the fast is not invalidated by that. 

There is no known difference of opinion among the
contemporary scholars that injections to the skin or muscles do not
invalidate the fast. The basic principle is that the fast remains valid
unless there is proof that it has been invalidated. These injections are not
food or drink, and they do not take the place of food or drink, so if the
injection is medicine, it does not invalidate the fast, so it is more likely
that the sting of a scorpion or bee does not invalidate the fast. 

In the answer to question no.
38023 we have listed in
detail the things that invalidate the fast.

In the answer to question no.
2299 there is a discussion on
the effect of using medicines and medical treatments on the fast. 

And Allaah knows best.