Phalangium opilio is a large harvestman, the males of which are unmistakeable due to a horn-like apophysis on the distal segment of the chelicerae. Although this 'horn' actually projects backwards from the proximal end of the cheliceral segment, because of the way the harvestman folds and holds its cherilerae, the two horns project straight out in front of it giving it a distinctive appearance. The palps of the male are also extremely long.
There is no trident, but there can be many tubercles in the trident area. Colouration is very variable but most individuals are quite light with a more or less distinct - often much darker - saddle. The saddle has one or two restrictions along its length giving it a waisted, or double-waisted appearance.
A common difficulty is separating females of Palangium opilio and females of Mitopus morio. These can be similar in size with a rather similar looking saddle. In general, the ocularium and armature of Palangium opilio is more robust. The saddle area around the eyes in Palangium opilio tends to be paler than the rest of the saddle whilst in Mitopus morio this difference is not so apparent. The real clincher is the presence, on Palangium opilio, of two small tubercles on the supra-cheliceral lamella (between the edge of the cephalothorax and the top of the chelicerae), which are absent in Mitopus morio. Normally a hand lens and spi-pot are required to see these (occasionally, and with luck, they are visible in photographs).
Phalangium opilio is widespread over the whole of the UK but more so in the south. Many records from the north are in coastal areas - see the distribution map of the Spider and Harvestman Recording Scheme . It is also found throughout Europe with the exception of the far north and south.
This harvestmen loves warm dry places and in the UK is common around the coasts but scarce in wetter upland areas. Within this broad preference for dryer places, Phalangium opilio will inhabit a wide range of habitats and micro-habitats.
Adults occur from early summer and well into winter with greatest numbers in August and September (although there appears to be a sharp drop-off from October). This species is known to have two generations per year and in the south of England, some adults can be found as early as May.
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.