The thick lines are bugs. They duplicate after living long enough, the thin ones are the spiders. They duplicate after killing a bug and die if they're alive for too long. There are hunting regulations in place to prevent each population from exterminating the other (they have their own clone caps and spiders cannot attack if there are too few bugs)
thanks to wisco11 for the original project. I think that with a larger scale simulation you might be able to keep it stable for longer without law enforcement.