In order to improve the recognition of millipede burrows and, therefore, the presence of millipede macrodetritivores in the fossil record, this study describes the burrowing behavior and resulting burrow morphologies of two species of spirobolid millipedes: Narceus americanus and Floridobolus penneri. Millipedes were placed in terraria filled with sediment of varied compositions and controlled moisture contents for 7-94 days. Open burrows were cast with plaster, excavated, and described both qualitatively and quantitatively. Both N. americanus and F. penneri produced vertical, subvertical, helical, and O-shaped burrows. Floridobolus penneri also produced J-shaped burrows. Burrow morphologies of both species were found to be similar and unaffected by sediment properties using the Bray Curtis similarity and Spearman's rank correlation tests, respectively. Burrows of two other millipede species of the Order Spirostreptida, Orthoporus ornatus, and Archispirostreptus gigas, produced in a previous study were also compared to the spirobolid burrows. While burrows of O. ornatus were similar, burrows produced by A. gigas, the largest of the four species, were the least similar even after removing size-dependent burrow properties from the analysis. Differences and similarities in burrow morphology were instead attributed to the behavioral function of the burrow: refuge or feeding. Despite some differences, all millipede burrow shafts and tunnels had a mean width-to-height ratio of 1.0-1.14. These results show that juliform millipedes produce burrows with unique morphologies that may be used to differentiate their burrows from those produced by other soil organisms.

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