What if the person who could engineer such a large scale dipterocide were on one branch of a trolley track..?
Apart from the immediacy issue (you would see the kitten's fate, but the flies die out of sight and you don't get to see a rare, beautiful flower whose once in a lifetime pollination you may possibly prevent) and the all too human bias towards the cute and away from the ugly (unless you're a certain type who despises the cat-massacre of garden wildlife and closely studies brachycera, etc, in which case you might swing the other way), this question leads to:
a) So, you'd save the kitten? What if it was twice as many flies? Twice as many as that? Twice as many as that?(→"all of them!")
b) So you wouldn't save the kitten? What if it was two kittens/had a little bow on it/you knew it had been named "Mr Fluffles" by its 3yo 'owner'/Who needs an operation and plenty of relaxing bed-rest? Or only half/quarter/one small matchbox-full of flies?
This question leads on to far too many questions. Could we have skipped straight to "how many flies are equal to a kitten?"? I'd also be worried about that glass (though at least you didn't go for fluoroantimonic acid, or the like).
(I don't think I'm shitposting, but I don't think the question is any better than any other trolley-pro lem one, with quite a few more loose ends. I don't think I'd just walk away, if only because ofthe fascinating horror of encountering such a setup.)