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.

The first fossil opilioacariform mite (Acari: Opilioacariformes), Paracarus pristinus sp. n., is described from Baltic amber (Tertiary: Eocene). This well-preserved inclusion has six eyes and, thus, can be excluded from the derived, four-eyed genera. Based on characters such as the extremely long leg 4, it is assigned to the extant central Asian genus Paracarus Chamberlin & Mulaik, 1942. As with many Baltic amber inclusions, we lack an exact provenance for the type specimen, but the Baltic region is further north than the present day range of Recent opilioacariform species and suggests that the range of this group was once more extensive within Eurasia. The first solifuge (Solifugae) from Baltic amber is described as Palaeoblossia groehni gen. et sp. n. Based on its size and general appearance, it can be assigned to Daesiidae, and again represents an arachnid group which previously had a much wider geographical range. If adult, it may represent the smallest solifuge known.

Spiracle and tracheal structure in the extant Ixodida is revised and shown to comprise 29 distinct component characters, some of which are common to all Anactinotrichida, while others are unique to the Ixodida or one of its six component clades of ((Argasidae Nuttalliellidae) (Prostriata Metastriata)). Structural variation both between and within families is based upon combinations of minor differences in the component characters, only one of which, spiracular position, proved to be incongruent within the most parsimonious cluster- and tree-analysis solutions. Tracheal airflow in ticks is mediated via passive diffusion gradients. In the argasid spiracle, both aeropyles and ostium are functional, although the latter is only opened briefly during infrequent periods of activity. The ixodid ostium is sealed and all gas exchange takes place via an enlarged sieveplate which reduces transpiration via small aeropyles, an underlying dense array of pedicels and possibly hygroscopic sub-atrial glands. Changes in spiracular morphology from a more ‘ancestral’ argasid type to a more ‘derived’ ixodid type are correlated with changes in tick behaviour, particularly with increased activity associated with the change from nidicoly to host-seeking.

The fossil ricinuleids are described, and their systematic relationships revised, based on a study of all British and American specimens. Phylogenetic analysis, utilising a new character set, indicates a probable basal dichotomy between the fossil and extant forms. The order is divided into two new suborders: Neoricinulei subordo, nov., for the extant genera, and Palaeoricinulei subordo. nov., for the fossil forms. Two new genera are erected: Amarixys gen. nov. for Curculioides gracilis (Petrunkevitch, 1945) and A. stellaris sp. nov., and Terpsicroton gen. nov. for Poliochera alticeps Pocock, 1911. Three new species are erected in the genus Curculioides: C. mcluckiei sp. nov., C. pococki sp. nov., and C. gigas sp. nov., and one in the genus Poliochera: P. gibbsi, sp. nov.

The wide range of organs of respiration (book-gills, book-lungs, sieve- and tube-tracheae), reproduction, sensory perception, etc., among the chelicerates indicates that the major groups made the transition to land life independently. The fossil record is patchy for most chelicerate groups, certain intervals (e.g. Westphalian) being particularly rich in chelicerate bearing Lagerstatten while in others (e.g. Mesozoic) they are sparse. Due, apparently, to their unusual hyaline exocuticle, scorpions are better preserved than other arthropods, and show a fairly continuous record from fully aquatic forms in the Silurian, to both aquatic and terrestrial faunas in the Carboniferous. In particular, new and well-preserved material of the earliest demonstrably terrestrial scorpions from the Lower Carboniferous of East Kirkton, West Lothian, suggests that book-lungs, at least in the scorpions, developed directly from book-gills by suturing of the covering plate (Blattfuss of the related eurypterids) to leave stigmata for diffusion of air. This evidence supports the ideas of early authors that the scorpion mesosomal ‘sternites’ are fused plates, contra Kjellesvig-Waering (1986) who envisaged the plates being lost to reveal true sternites beneath. The fossil evidence also indicates that by the Triassic at least two scorpion lineages had evolved intra-‘sternite’ stigmata.

A lower Carboniferous limulacean Rolfeia fouldenensis gen. et sp. nov. is described. It is regarded as a sister group of Paleolimulus and distinguished from it by the presence of fixed opisthosomal spines and by the more primitive outline of the opisthosoma. Both characters could have been derived from belinuroid ancestors, and both were retained in the advanced belinurids and euproopids. The view that limulaceans are more closely related to certain primitive belinuraceans rather than descended from the euproopaceans is thus supported by the new evidence. Although Rolfeia was probably still capable of coaption, it is postulated that, like other limulaceans, it had adopted burial as a form of defence. It is suggested that the primary adaptive function of the fixed opisthosomal spines was to support the body over a soft substrate although they may also have served a defensive role.

The occurrence at Foulden of the eurypterid Cyrtoctenus peachi Størmer and Waterston is confirmed. The eoscorpiid Trachyscorpio squarrosus Kjellesvig-Waering 1984 has been described from Foulden and disjecta membra of Gigantoscorpio cf. willsi Størmer discovered in the recent excavation are described. The type of preservation and adaptive features of the chelicerates would suggest that the eurypterid and scorpion fragments are allochthonous and the xiphosurans probably autochthonous.