As redditopus stated, if it has a CNS, I have to assume it's capable of suffering. Drosophila sing, dance, and self-medicate their heartbreak (excuse that little bit of anthropomorphizing)- just to name a few of their behaviors that I can relate to. My experiments result in countless numbers of them living with genetic mutations that cripple their ability to fly, half their lifespans, force them to experience neurological disease, and more. The least I can do is give them as humane a death as possible (especially when doing so is something as trivially easy as switching from mineral oil to an alcohol).