insects detect sensations, like they know when they're being touched or if something is wrong with them because they can't fully function properly but they don't feel pain. I think it has something to do with the fact that they lack nociceptors? yeah hopefully that doesn't strike up some kind of crazy argument, haha
Oh yeah, I rushed to my copy of R.Chapman,
The Insects, knowing that it has a chapter on insect neurology, lots of which I don't understand, in the hope of starting a crazy argument on nociceptors, but no mention of them, nor in another basic text on insects. But yes, insects have 'em, and there are a number of scholarly articles since around 2000. Here's a good one:
http://jeb.biologists.org/cgi/reprint/204/3/457.pdf. When I was a kid in England, we used to do just this sort of "experiment" on hawk moth caterpillars
. That said, I know that it is a mistake to portray a critter's turning away from a noxious stimulus as "pain." When we smell something unpleasant, we are likely to be repelled by it, but our action is not accompanied by the emotional component that is present when we experience pain. It is fashionable to put down articles in Wikipedia, but its article on nociceptors makes a valuable point (i.e. one that I strongly agree with): "Due to historical understandings of pain, nociceptors are also called pain receptors. This usage is not consistent with the modern definition of pain as a subjective experience."
So I'm afraid that someone else will have to initiate the crazy argument, but I did find a reasonably priced copy of Blum, M. S. (1985).
Fundamentals of Insect Physiology. which I ought to have, even if I don't understand it, either.