I can see why you were interested in the species with more ability to think. However, animals don't really make any conscious decision to reduce reproduction, when stressed, so it won't matter in your experiment.

Fruit flies are pretty easy to breed and easy to get. Just make sure they don't get loose in your home or school. You won't be too popular. I don't know much about breeding worms, but your local bait shop could probably help you out.

The problem you must consider here is that worms (C. elegans) are predominantly hermaphroditic (having both sexes in the same worm) so they can self fertilize. This may be interesting if you take a population and stress them out or limit their food and then ask what happens to the ratio of male to hermaphrodites in the future generations.

As for flies, I do not know much about them but I think they do not eat much during adult hood and I am not sure what kinds of behaviors you want to look at in terms of mating. Take a look on line for studies regarding reproduction and behavior in these two organisms to see what is known.