Stick insects, which look like walking twigs, are popular as pets and research subjects because they’re easy to keep, can’t fly away, and move at a pace that allows observation and study.  “I think they’re really, really sweet insects,” said Chris J. Dallmann, a researcher at Bielefeld University in Germany, who just completed a scientific report on the biomechanics of a stick insect’s six legs and what are, loosely speaking, its knee and hip joints.