private static void createAndShowGUI(int nbugs) {
        JFrame f = new JFrame("Swarm Demo");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        double dt = 0.01;
        int ddt = 20;
        if (nbugs > 20) {
            dt = 0.1;
            ddt = 2;
        }
        Playfield p = new Playfield(nbugs, dt, ddt);
        f.add(p);
        f.pack();
        f.setVisible(true);
        p.start();
    }