Fruit flies are known to have nociceptors, and it is likely that other insects do, too. Bees also respond differently to electric shocks given with and without anesthetic. And insects, generally, seem capable of learning to avoid noxious stimuli. But can they suffer?
Hans Smid, who studies the brains and learning behavior of parasitic wasps at Wageningen University in the Netherlands, dismisses the possibility. "I am absolutely convinced that insects do not feel pain," he says.
Like Elwood's, Smid's interest in pain began with a simple question. A few years ago, a visiting journalist was surprised at how casually Smid squashed a wasp that had escaped from its cage. Why hurt an animal you are so enthusiastic about, the journalist wanted to know.
Yet Smid is confident that insect behavior is best understood in terms of a relatively simple series of reflexes and innate responses. Unlike crustaceans, insects seem to have no pain-related behaviors. If an insect's leg is damaged, for example, it does not groom or try to protect the limb afterward. Even in extreme cases, insects show no evidence of pain. Imagine a praying mantis eating a locust, Smid says. With its abdomen opened up, the locust will still feed even while being eaten.
In terms of relative brain size, fruit flies and the parasitic wasps that Smid studies are the masterminds of the insect world. But neurons consume a lot of energy, and there is evolutionary pressure to keep brains as compact as possible. In short, there need to be good reasons to have enough brain for pain. Smid thinks that insects simply do not have the need. "I don't see the evolutionary advantage for insects to sustain such a complex system as emotion, of which pain would be just one component," he says.