YOUNGSTERS on one of the YHA's (0629 825850) Sunship Earth ecology holidays are invited to take a journey through a 'shrinking' machine to experience a bug's-eye view of the world. This is just one example of activities on these five-day holidays at the Castleton Youth Hostel, Derbyshire, designed for unaccompanied eight- to twelve-year-old children at a cost of pounds 155 each.