September 2023

Harvestmen

Last month I talked about crab spiders and featured the most common of the species encountered in the UK (Xysticus cristatus). At a subsequent gardening session, I spotted another spider-like creature, but this one had extremely long, gangly legs: It was a harvestman.

Although they superficially resemble a spider, and occur within the wider group (Class) of Arachnids, they are a totally separate Order called Opiliones. Interestingly, the Swedish zoologist, Karl Sundevall, who proposed the name was referencing the Roman term for a shepherd; opilio. Apparently these ancient sheep guardians once used stilts, to give them a better vantage point from which to count their flocks. Sundevall clearly felt that harvestman’s long legs were suggestive of this, and at one time they were actually called shepherd spiders. But the common name by which they’re best known today, derives from the fact that these creatures reach maturity and become most visible in the late summer, around harvest time.

Harvestmen differ from spiders in many significant respects. Spiders have two distinct body parts; the head (cephalothorax) and body (abdomen), but in harvestmen these are fused into one round, compact body. Spiders have six, or more generally eight, eyes, whereas harvestmen only have 2. These are set in a knob like structure on top of the head which is called an ocular tubercle. It isn’t known how effective their vision is, but the second pair of legs which are longer in most species, are used as feelers. Given that harvestmen are usually active at night, these are probably the main means of navigating their environment.

Unlike spiders that are totally carnivorous, harvestmen don’t have silk glands to make insect trapping webs, nor do they possess fangs that produce venom. They are omnivores. They catch small insects by using the hooks on the ends of their legs and consuming them with their pincer like jaws. Otherwise, they will eat decaying plant material, bird droppings, fungi etc.

Spiders and harvestmen also reproduce differently. Whereas male spiders hand sperm packages to females using their pedipalps (segmented appendages beside mouthparts), the male harvestman do actually mate with the females. She then goes on to lay hundreds of eggs in various damp locations. They hatch in the spring as small versions of the adult and undergo several moults to reach maturity. Harvestmen generally only live for about a year

But perhaps the most impressive thing about the harvestmen, are the defence strategies they deploy. As well as the more common defences such as camouflage, fleeing and fighting (using the spines on their legs,) they also adopt techniques such as bobbing, ‘freezing,’ clustering together and ‘playing dead’ (thanatosis). Harvestmen also have impressive chemical defences. There are a pair of glands located near the base of the second pair of legs, and when threatened these release noxious chemicals that deter predators. However, perhaps the most drastic means of self-defence is the sacrifice of one, or even more, of their legs. If a harvestman feels under threat, it can voluntarily shed one of its legs, enabling it to distract the would-be predator and escape. The discarded limb can continue to twitch for anything up to an hour….and if that doesn’t divert you away from malicious intent, I don’t know what will! Unfortunately for the harvestman, it can’t regenerate the lost limbs and like the one shown here, 

which is missing its second left leg, it has to adapt.

There are about 25 species of Harvestman in the UK and I think this one is Opilio saxitilis. They sometimes find their way into our homes at this time of year, but contrary to popular belief they are completely harmless, so please give them house room…with or without their 8 legs!!

Words and photos by Denise Long