The underside of Opilio parietinus, in common with that of O. saxatilis, is very pale with brown irregular spots on the coxae - feature which separates them from all other harvestmen. Females, in particular, sometimes have a dorsal patterning of dark bands and an obscure saddle which is normally missing in males. There can be a pale central stripe on Opilio parietinus, but usually not as conspicuous as in O. saxatilis. The legs are rather long and all segments of similar colour.
Size is the first clue to separate Opilio saxatilis (up to 6 mm) and Opilio parietinus (up to 9 mm). The armature of the ocularium also provides good evidence, there normally being between 2 and 4 tubercles on the ocularium of Opilio saxatilis and between 4 and 8 on that of Opilio parietinus. If viewed with a spi-pot and lens, the genital operculum of the male is rather more flared in Opilio saxatilis (and it has a slight notch in the end) compared with Opilio parietinus. Unlike in Opilio saxatilis femurs 1 and 3 are hardly (or not) swollen.
Widespread in the UK as show by the distribution map of the Spider and Harvestman Recording Scheme . That map also shows that the species has not been recorded in many areas where formerly it was. This pattern is repeated across Europe and there is some speculation that the invasive species O. canestrinii may be partly responsible for this. However O. parietinus is itself fairly new to Europe and thought to originate in Asia Minor.
Highly synanthropic and often found on walls, fences and tree trunks, O. parietinus likes warm, dry situations.
Adults occur in from late summer and well into the early part of winter.
Hillyard, P. D., & Sankey, J. H. P. 2005. Synopses of the British Fauna (New Series) No. 4: Harvestmen. Third Edition.Field Studies Council, Shrewsbury.
Richards, P. 2010. Guide to Harvestmen of the British Isles. Field Studies Council, Shrewsbury.
Richards, P. 2017. Tabular key for Identification of British Harvestmen (Opiliones). Unpublished.
Wijnhoven, H. 2009. De Nederlandse hooiwagens (Opiliones). Nederlandse Entomologische Vereniging.