Aggregation, extended mother-offspring-sibling interactions, and complex social behaviors are extremely rare among arachnids. We report and quantify for the first time in Amblypygi prolonged mother-offspring-sibling associations, active aggregation, and frequent “amicable” (tolerant, nonaggressive) tactile interactions in two species: Phrynus marginemaculatus C.L. Koch 1840 (Phrynidae) and Damon diadema (Simon, 1876) (Phrynichidae). Sociality is characterized by frequent contact and tolerance, and infrequent agonism until sexual maturity in D. diadema and into adulthood in P. marginemaculatus. We experimentally examined potential benefits and costs affecting aggregation: risk of predation, preferred habitats and prey availability. Only increased predation risk decreased nearest-neighbor distances and increased maternal vigilance. Individuals aggregated on a variety of surface textures and locations that varied daily, rather than aggregating only on preferred microhabitats. Manipulation of prey abundance had no affect on the tendency to aggregate.

Patterns of parental care, duration of association, and the presence of social traits found in the most social taxa of non-spider arachnids are reviewed. Species in most arachnid orders have transient parental care with defense of eggs, a brief period of association with newly emerged young prior to independent foraging and explosive dispersal from the natal nest. More prolonged sociality, with long-term associations among mothers-offspring-siblings is rare and is only described in a few species in the Amblypygi, Scorpionida, Pseudoscorpionida, and Acari. All such species have subsocial origins, but current use of the term subsocial is overly broad and we propose a more restricted terminology for clarity.