Olfactory learning in the yellow striped millipede, Spirobollelus sp. with evolutionary significance for phylum Arthropoda

Abstract

Learning and memory have been widely studied in the phylum Arthropoda, but no learning studies have been done with millipedes. It has been suggested that millipedes are basal arthropods that are closely related to arachnids, in which learning has been demonstrated. Therefore, a demonstration of learning in millipedes might provide evidence that arthropod learning is evolutionary conserved. In this study, olfactory learning was investigated in the yellow striped millipede, Spirobollelus sp. Animals were given a binary test with a choice between vanilla and peppermint scents, consisting of two initial preference trials and four experimental trials. If during an experimental trial, an animal chose its innate preference, ethyl alcohol (EtOH) was applied to the head capsule as negative reinforcement. To test whether the intensity of the reinforcement had an effect on learning, animals were split into two experimental groups: high alcohol (95% EtOH; n=10) and low alcohol (0.475% EtOH; n= 10). Here it is shown that 90% of animals tested in both groups initially preferred vanilla over peppermint. In the high alcohol group, there was a significant decrease in visits to vanilla (p=0.00037) and a significant increase in visits to peppermint (p=0.0063). In the low alcohol group, there was a significant decrease in visits to vanilla (p=0.0177), but the trend in visits to peppermint was variable (p=0.0877). Results provide evidence for olfactory learning in Spirobollelus sp. dependent on stimulus strength. This implies that neurological complexity is not important for learning to occur, and arthropod learning is evolutionarily conserved.