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from several days ago, but not the fly equivalent of what they ate at an earlier meal. These different strengths and weaknesses make sense in the natural world of a fly; rovers are likely to move from one food source to another, so being able to quickly learn whether a given fruit is ripe or not is more important than remembering what happened in the more distant past. Together with Marla Sokolowski from the University of Toronto, who first discovered the rover-sitter dichotomy and has worked on its details for many years, the scientists then discovered that the differences in memory can be manipulated by increasing or decreasing the amount of an enzyme in the odor detection centers of the insects' brains. That enzyme may be the key to the trade-off between memory types, at least in flies, and suggests some interesting directions for similar studies in people.

Another set of experiments focused on a different chemical. Using a modified version of the Tennessee Williams paradigm, in which flies are placed into a chamber that heats up on one side when the flies move to it, a group of researchers from the University of Missouri recently demonstrated that serotonin, the same brain chemical that features so prominently in human depression and its treatment, is key to the tiny flies being able to learn to avoid the hot spots.

The ability to stick to a task after having been distracted—something many children with learning disabilities struggle to accomplish—is also controlled by a few nerve cells and chemicals. Flies tend to move toward a visual object, for example, a stripe on the end of their container. If you remove the goal and show them a "distracter" stripe somewhere else, they veer off for a short time but can still remember where the original stripe was located. Geneticists have bred flies with mutations in various genes that produce chemicals important in learning (as with many specialized strains of fruit flies, these have fanciful names such as dunce and ignorant), and it turns out that while some of the mutants can still perform the task of recalling their goal as well as normal flies, others cannot. The mutant amnesiac, for example, learns just fine in the first place, but forgets what it learned almost immediately. This distressing tendency can be attributed to a defect in a single neurochemical, one that is extremely similar to a chemical in the human nervous system. Being able to break down a behavior such as recovering after a distraction into components so fine that we can determine exactly which gene is responsible for which part of learning is possible only in insects, at least so far, but maybe someday we will be able to extend this kind of detailed understanding to our own learning difficulties. What's more, the prospect of altering or curing defects in memory with gene therapy in insects suggests that similar treatments may eventually substitute for drugs or surgery in humans, a solution that could have fewer side effects and be targeted more precisely than current approaches.

He Who Learns Last

WHICH came first, learning or instinct? Because humans rely so heavily on learning, we tend to think of it as an innovation, an evolutionary novelty that we alone have mastered. In effect, we like to think we invented invention. But centuries ago, naturalists believed that instincts, behaviors that are performed more or less the same way every time, arose after learning. Early animals, they claimed, had to learn things from scratch, and then after time, the repetition of a task was somehow impressed into the fiber of the organism so that eventually it became instinctive. This idea was particularly championed by Jean-Baptiste Lamarck, the French biologist whose ideas about the inheritance of acquired characteristics were first embraced by early evolutionists, including Charles Darwin, but later discredited. Knowing nothing about how genes and chromosomes could be passed from parents to offspring, Lamarck and his contemporaries reasoned that if, say, a horselike animal continually reached for its food at the