Caenorhabditis elegans is an approximately 1mm long nematode (worm) that lives in the soil and eats bacteria such as Pseudomonas. Scientists have determined that when C. elegans is hunting for food, the worms have different patterns of movement that depend on the location and quality of the food source. Scientists hypothesized that C. elegans might be attracted to certain chemicals produced by Pseudomonas. They tested this hypothesis by placing 100 C. elegans in the center of each of a series of petri dishes containing a layer of agar over which the worms could move. A drop of a test chemical diluted in ethanol was placed at one edge of each dish and an equal-sized drop of ethanol was placed at the opposite edge of the dish. The scientists determined the number of worms to reach each spot over a period of time. (a) Describe why the location and quality of food sources in the soil are sometimes likely to stimulate random movements by C. elegans, but at other times are more likely to stimulate directed movements. (b) Justify the scientists’ use of two different spots on each petri dish and the counting of worms at each of the two spots. (c) Under harsh environmental conditions, C. elegans produces a pheromone, a complex of molecules that affects other C. elegans. One effect of the pheromone is to cause developing larvae to temporarily arrest as larvae rather than mature to reproductive adults. Predict the relative amount of pheromone most likely produced by C. elegans that are only weakly attracted to one of the test chemicals. (d) Provide reasoning to justify your prediction with respect to how the predicted amount of pheromone is beneficial to the worms that are only weakly attracted to the chemical.

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