Harvestmen, or harvest-spiders, are invertebrates in the order Opiliones. The very long-legged ones are sometimes called Daddy longlegs, but this common term is also used for crane flies, so its probably best to call then harvestmen or opilionids. They are an ancient order; the oldest opilionid fossil (Eophalangium sheari gen. et sp. nov.) dates from the Early Devonian (Pragian) Rhynie cherts, Aberdeenshire, Scotland (Dunlop, 2003). Clearly they are doing something right to have lasted for over 410 million years! Those long loping legs might look a bit ungainly to us, but they are a versatile and adaptable ‘design’ (i.e. evolved feature) which has served them well.
Harvestmen are of course arachnids (Class Arachnida, subphyllum Chelicerata), related to spiders, mites and scorpions (their closest relatives). Two main characteristics of opilionids are i) that they have long legs – sometimes very long legs! – and ii) they have rounded bodies where the abdomen is broadly joined to the cephalothorax. Like spiders they have four pairs of legs (2 x 4 = 8 legs!); that is, unless they have lost some! It is common to come across opilionids with one or more legs missing; perhaps because they have had to shed them to escape from a predator. For more details of opilionid anatomy, see here. They are also often spiny, sometimes with a prominent spike sticking upwards from their bodies.
They are also often spiny, sometimes with a prominent spike sticking upwards from their bodies, like this amazing specimen from the island of Borneo (Sarawak, Malaysia), below.
Harvestmen are a remarkably diverse group, spanning a wide range of sizes; some of them look very different, as I will show below. The group is divided into four suborders, with over over 6,650 species, but there must be many more out there – particularly in the tropics – which we do not know about. Some newly described species are shown here, on the Species new to Science website.
One of the largest harvestmen, Trogulus tricarinatus, occurs in Europe – including the UK (rarely) – and grows to between 7 and 9 mm in both sexes. There are some with much longer legs, like a recently discovered specimen found lurking in the caves of Laos (in SE Asia), with a leg span over 33 centimeters. There are also some very large species from the tropics, see here.
Unlike spiders, they only have one pair of eyes, mounted on top of the cephalothorax, and sometimes raised above the body (an ocular tower) of the animal, like the amazing Megabunus diadema. Some opilionids look remarkably endearing, I think! (See below). Others are downright terrifying!
In close up, the body looks like some sort of strange craft balanced in the middle of huge legs.
Two of the commonest families, i.e. those with the most species, are the Sclerosomatidae (with about 1,300 known species) and the Phalangiidae (with about 380 known species), both in the suborder Eupnoi ,which contains many north temperate species, including the ‘daddy-longlegs’ type spiders (see below).
This species (below) has a very impressive, club-like spike.
One of the commonest and most widespread opilionids in the world is Phalangium opilio (Opiliones: Phalangiidae) shown below. It is quite widespread in the British Isles and in Europe, but does not appear to have a common name. Opilionids like P. opilio are highly beneficial invertebrates, generalist predators which feed on a wide range of insects and mites, including many pest species as they are common in anthropogenic habitats (e.g. agricultural fields, urban areas). For example, they will feed on the eggs and early instars of Colorado potato beetles (Drummond et al., 1990).
The males and females of this large (~3.5–3.9 mm) opilionid can usually be separated by colour. The males are somewhat smaller and have a black colouration (below) and very long palps.
The suborder Laniatores include many large (up to more than 2 cm) and colourful opilionids, common in tropical regions and the Southern Hemisphere. I came across this strange looking creature (below) whilst walking along a forest path in northern Argentina. The image is very slightly blurred, but it does capture the very spiky back legs. I did not realise at the time that it was an opilionid and not a spider.
These spiny tropical species are mainly nocturnal, like this beautiful (?) individual from Brazil (below).
Links on opilionids
Harvestmen of North America: https://get.google.com/albumarchive/105903507984958890819/album/AF1QipNV82BGYc6gB6vcPiP-EW0T4tnd66OW0GXM6_rT
Dunlop, J. A., Anderson, L. I., Kerp, H., & Hass, H. (2003). A harvestman (Arachnida: Opiliones) from the Early Devonian Rhynie cherts, Aberdeenshire, Scotland. Earth and Environmental Science Transactions of The Royal Society of Edinburgh, 94(4), 341-354.
Drummond, F., Suhaya, Y., & Groden, E. (1990). Predation on the Colorado potato beetle (Coleoptera: Chrysomelidae) by Phalangium opilio (Opiliones: Phalangidae). Journal of economic entomology, 83(3), 772-778.
Hillyard, P. D., & Sankey, J. H. (1989). Harvestmen: keys and notes for the identification of the species (Vol. 4). Brill Archive.