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();
 }