The ant-mimicking spider Myrmarachne formicaria (Araneae: Salticidae) is a recent arrival to North America from Eurasia, but little is known about its natural history in its native or invaded range. Most published accounts of this species in North America are from domestic settings inside or near buildings. Long-term study of populations of mound-nesting ants in Geneseo, NY, revealed an increasing incidence of M. formicaria as they are often seen near ant nests, with silken shelters built on the vinyl flags marking colonies. To determine if these spiders are associated with ant density or specific habitats, grids of pin flags were employed as a sampling method at 18 sites. Grids were visited weekly over summer 2019 to check for the presence, stage, and sex of M. formicaria on the flags. Ants were sampled at each site using pitfall traps. Spiders were collected elsewhere in New York, Pennsylvania, and Ohio for preliminary analysis of genetic variation in this colonizing species. Field observations revealed intraspecific display behavior between males that was investigated in laboratory behavioral trials. Staged encounters between the ant-mimicking spiders and potential predators were used to assess the benefits that M. formicaria might gain from its ant mimicry.
Results/Conclusions
Though commonly encountered near Formica ant nests in the main study area, M. formicaria can be found in areas devoid of such nests. They were rare in older forest with a sparse understory but frequently found in younger forest, forest edges, and fields. In natural settings M. formicaria builds its silken shelters in leaf folds. Egg masses were noted in early June, while spiderlings appeared in early July. Observations of field and captive spiders indicate that females remain in shelters with eggs and spiderlings for an extended time. Females were much more common in surveyed sites than males, which were not apparent until August. Preliminary analysis suggests low genetic variation in this spider across the invaded area sampled, though additional loci are being tested. In intraspecific display contests, larger males prevailed in almost all trials. Male display behavior may play a role in encounters with potential predator spiders, which were generally more likely to attack non-mimic spiders. Continued work includes assessing the nature of M. formicaria’s interactions with ants and other spiders. These findings expand our knowledge of the natural history of this recently introduced species, which is critical to predicting its impact on the native arthropod community.