ARACHNIDA > OPILIONES > Phalangiidae > Phalanginae

Phalangium opilio is a large, robust, long-legged species with easily recognisable males, but with females that can be difficult to distinguish from females of Mitopus morio.

Body colour is variable, but typically quite light, with a more or less distinct, darker, yellowish-brown to reddish-brown saddle. The saddle is often obscure in males, but in females it is generally well marked with two restrictions along its length giving it a 'waisted' or 'double-waisted' appearance. It's been observed that different populations of Phalangium opilio have noticeably different overall colouration dependent on habitat, some with contrasting red-brown tints, others described as having a more even tan colour. It will be interesting to see if this bears out over time from my own sightings. 

The dorsal surface of the cephalothorax, particularly between the anterior margin and the ocularium, has many small black-tipped tubercles. The ocularium is a little longer than wide with two rows of six to ten acute, black-tipped tubercles on top. The legs are long, pale or yellowish-brown.

The males are pretty much unmistakable due to the large, prominent horn-like protuberance on the distal segments of the chelicerae, and their long and slender, almost leg-like, palps.

Females are characteristically chalky-white on the underside, but their size, general appearance and shape of the saddle looks very similar to that of the female Mitopus morio, which makes the two species rather difficult to distinguish. The visible differences are subtle. However, a close look at the ocularium will show that in Phalangium opilio there are not only more tubercles around the eyes than found in Mitopus morio, but they are also far more robust.  Also, although sometimes more apparent than others, is that the saddle area around the ocularium tends to be paler than the rest of the saddle in Phalangium opilio, whereas in Mitopus morio there's no discernable difference. And, when comparing the legs of these two species, it will be seen that in Phalangium opilio the femora have longitudinal rows of acute, black-tipped tubercles, being stronger and more defined than in Mitopus morio. But, although these features are all useful, the only positive way of confirming Phalangium opilio for recording purposes is by the presence of two tiny tubercles on the supra-cheliceral lamella (between the edge of the cephalothorax and the top of the chelicerae), which are absent in Mitopus morio. Unfortunately, this important distinguishing character is rarely visible in photos, which is why many images of the females of this species are undetermined, or at best 'likely' based on the appearance of the ocularium. That said, the feature is just about visible in a couple of my photos.

Phalangium opilio is common and widespread across much of the country, but more so through central and southern counties where it can be found in a variety of habitats. Distribution is more scattered in the north with many of those records being from coastal locations. In the most part the species prefers drier and warmer places, favouring open habitats including open woodland, grassland and gardens. It is rarely encountered on heathland, although I have found them there. As with many harvestmen, adults occur from early summer through to the winter with greater numbers being seen during August and September. In the south they may be active as early as May as there are two generations in some areas.