The process by which solitary animals evolve to live in groups is a central question in biology. Conspecifics that compete for patchy resources may evolve incipient social behaviours such as group formation and conspecific tolerance when persistent aggressive interactions reduce resource access. In this investigation, a facultative group living species of whip spider was studied to understand the microhabitat resources that support group formation. Although most species of whip spider are solitary and oftentimes cannibalistic, Heterophrynus batesii sometimes aggregate in small groups at the bases of tree trunks. Twenty-five groups of whip spiders and associated tree buttress microhabitats were surveyed at Tiputini Biodiversity Station bordering Yasuní National Park in Eastern Amazonian Ecuador. Tree buttress complexity, surface area and other microhabitat variables were recorded. Heterophrynus batesii aggregated in groups of 2–8 animals and used large, buttressing, complex trees with more leaf litter relative to those available in the environment. This study showed that large groups of whip spiders require more complex microhabitats than smaller groups that were associated with more variable microhabitat parameters. These microhabitats act as patches of limited resources important for the species, and may have led to the evolution of tolerance and facultative group living.

Plesiosiro madeleyi Pocock, 1911, the monotypic representative of the fossil order Haptopoda Pocock, 1911 from the British Middle Coal Measures (Upper Carboniferous, Westphalian B) of Coseley, Staffordshire, is redescribed. A specimen from Sparth Bottoms, Lancashire is not a Plesiosiro and is referred to Arthropoda incertae sedis. Plesiosiro was probably a predator, having tooth-like projections on the femora for prey capture, and shows adaptations of the limbs for crawling in narrow spaces. The uncertain nature of the chelicerae, eyes and respiratory organs makes resolving the phylogenetic position of Plesiosiro difficult, though Haptopoda deserves its status as a separate order. Plesiosiro resembles both cythophthalmid and trogulid opilionids. Its broad prosoma–opisthosoma junction could be interpreted as a synapomorphy for Plesiosiro + Opiliones, but this character is not unique to these orders. Alternatively, Plesiosiro has a large, ventral opisthosomal sclerite. This is interpreted here as a genital operculum, a synapomorphy for Tetrapulmonata, and Plesiosiro is tentatively referred to this clade. Based on the synapomorphies of a divided sternum and subdivided telotarsi a relationship of the form: (Plesiosiro (Amblypygi (Thelyphonida + Schizomida))) is proposed.