Most bugs have stored enough food to survive a while. But then the inevitable happens, and they begin to die off in great numbers. At this point, the population settles down to a more or less constant figure. The bugs' DNA now enables them to gather enough food that they rarely die, but they also rarely build up enough strength to breed. Green bugs are old enough to reproduce but are not yet strong enough.
Yellow bugs are both old enough and strong enough to reproduce.
Grey bugs are neither old enough nor strong enough to reproduce. Also notice the scrolling graph above the bug-world. You'll see this straightness indicator rise over time as selection pressure takes effect, reflecting the value of moving straight in the bug-world. If everyone dies, the applet restarts automatically.