The spread of the trident

Most of us are used to finding spiders indoors, but harvestmen (which are arachnids but not spiders) tend not to enter buildings quite so readily. However, for the last few days, one has taken up residence in the toilet-roll stack in our bathroom, so (as is often the case when an invertebrate grabs my attention), I decided to look a little more closely.

Harvestman on a toilet roll; body length approx 8mm

As there are several similar species of harvestman in Britain and this wasn't one of the more obviously distinctive ones, I headed to my bookshelves to consult Hillyard (2005), the standard work on the subject. However, the start of the key involves looking closely at the pedipalps (shorter leg-like appendages at the front - they have mainly sensory, and partly feeding-related functions) but I didn't want to capture the specimen on this occasion. Instead I decided that a photo would have to do and that I would see if an identification could be made on the balance of features. This is what I noted (see below for photos illustrating some of these points):

• The central stripe or 'saddle' is clearly outlines in black and paler in the middle, and its rear margin is cut off in a straight line.
• The legs are relatively short and robust (for a harvestman) unlike the longer and more spindly legs of genera such as Leiobunum. The femora are more or less cylindrical, while the tibiae and patellae are more angular in cross-section.
• The ocularium (eye-bearing structure, indicated by a blue arrow) is relatively small, has small bumps or 'tubercles' on top and is about twice its own length from the from edge of the cephalothorax.
• The trident (red arrow) is titled slightly upwards with the prongs diverging slightly and not especially different in length (the central one is a little longer, but starts further forward so the actual lengths are very similar).
• There is a paler line widening forward from the ocularium to the trident (outlined by green dashes).
• The sides of the cephalothorax bear small, variable prongs and protuberances (green circles).

Along with the indoor/building-related location, these features combine to indicate that this is Odiellus spinosus. There are a number of superficially similar species, but these do not have the same combination of spines/tubercles and leg features etc.while Mitopus morio does not have a trident.

Features associated with the cepahlothorax of Odiellus spinosus

Leg segments of Odiellus spinosus. (1) femur, (2) patella, (3) tibia, (4) metatarsus, (5) tarsus

Although known from Britain since the middle of the 19th century (it is found from northern Spain to the Balkans in continental Europe), this is a species that has recently been spreading northwards through Britain, having reached at least as far as Cumbria. It is probably that this is another of the many invertebrates whose range is expanding with the warming due to climate change - as it avoids high ground in its continental range, it is likely that temperature is a key controlling factor. So, if you do see one of these, and you are sure what it is, why not help map its changing distribution by sending the details to your local Biological Records Centre.

References

For a cheaper option than Hillyard's 2005 work, Richard's 2010 fold-out guide is also a very useful starting point for a few pounds.

Hillyard, P.D. (2005). Harvestmen (3rd ed.). FSC, Shrewsbury.
Richards, P. (2010). Guide to Harvestmen of the British Isles. FSC, Shrewsbury. [laminated fold-out card]