Mechanisms for turn alternation in four invertebrate species

Free-choice behavior following one or more forced turns was observed in representatives of four invertebrate classes (earthworms, woodlice, millipedes, earwigs). While all animals alternated, species differences occurred in free turn angle and the effects of varied angle and number of forced turns. Overall, woodlice and millipedes turned at sharper angles and responded more to the forced turn conditions than earthworms and earwigs. From behavior observed following three forced turns in one direction and then one in the opposite, it was concluded that, in earlier experiments, earthworms alternated via tactile cues, woodlice mainly used kinesthetic but could also use tactile cues, millipedes mainly used tactile but could also use kinesthetic cues and earwigs may have relied on kinesthetic cues alone. Since phyletic differences did not seem appropriate, the results were discussed in terms of other characteristics such as body shape and life style.