Are Sepsis flies Batesian mimics of ants?

Myrmecomorphy is the most prevalent type of Batesian mimicry among arthropods. Some species of the fly family Sepsidae also bear resemblance to ants. We examined whether the black scavenger fly, Sepsis thoracica, bears a phenotypic resemblance to the abundant ant Lasius niger (workers and males), and whether this resemblance protects it from predation by the syntopic wolf spider Pardosa agrestis. We compared the motion, body shape, size, and coloration traits of S. thoracica to Lasius workers and males, as well as to Drosophila hydei, a non-mimetic fly. Overall, S. thoracica resembled Lasius ant males in motion and body shape, D. hydei in size, and Lasius workers in coloration. Visual models of spider and avian predators revealed that in terms of luminance, S. thoracica was indistinguishable from ant males but different from D. hydei. In predation experiments, S. thoracica was preyed upon less frequently by spiders than D. hydei, and at a frequency similar to ant males. The low consumption frequency of S. thoracica was presumably in part due to the release of defensive chemicals. We conclude that it is an inaccurate Batesian mimic of several Lasius species. This paper brings the first empirical evidence of myrmecomorphy in flies.