What is it like to be a fly? It's impossible to even imagine what it's like to see out of their compound eyes, which are formed out of thousands of mini-eyes, or to be able to *feel* magnetic field lines as they do.
But, in other respects, the texture of flies' sensory world may not be so different from ours. Like humans, flies are attracted to sweet food and avoid bitter food, so I think it's reasonable to assume that, in some sense, they can taste sweetness and bitterness just as we can. But the internal neural circuitry that processes "sweet information" or "bitter information" in the fly brain is completely different from ours.
What you're seeing here is part of that circuitry: a "Hugin neuron." It was recently discovered that this neuron sends "bitter information" from the fly's taste receptors to its brain. We have nothing like the Hugin neuron in our brain, nor do we have anything like the taste receptors that flies have. Our taste-related systems evolved independently from the corresponding systems in the fly.
And that makes you wonder: if the information carried by taste-related brain circuits in human brains and fly brains is the same, is the *perception* that emerges from that information the same - even if those circuits evolved independently of one another, and look nothing alike? In other words, is the *sense* of bitterness the same in all animals that can *detect* bitterness, regardless of *how* they can detect it? I don't think we can ever know. We can never step into the mind of a fly and try out its sensations like slipping into a new pair of shoes. The internal world of every corner of the animal kingdom might forever be beyond our reach.
Image credit: Sebastian Hückesfeld, via @zeiss_microscopy