Introduction

For me, looking at harvestmen was a natural progression from my interest in spiders. They are in fact a lot easier to identify than their fellow arachnids and there are only just over 30 species in Britain compared to well over 600 spiders.

Harvestmen belong to the Order Opiliones in the Class Arachnida (invertebrates with eight legs). Although they resemble spiders they are not in fact closely related to them but are nearer to the other arachnids (the ticks, mites, scorpions and pseudoscorpions). Harvestmen are often mistaken for spiders but differ in several fundamental ways. Most obvious is the shape of the body. In harvestmen the head and body areas are fused into one unit unlike the obviously separate head and abdomen of a spider (a spider’s head is actually a combined head and thorax called a cephalothorax).

This sets arachnids apart from the insects which have a body form clearly divided into three – head, thorax and abdomen. Harvestmen only have two eyes compared to the six or eight of spiders. Another major difference from spiders is that harvestmen do not produce silk and do not build webs. While spiders mate by the male transferring a sperm package to the female with the aid of his palps, a male harvestman has a penis and fertilises the female internally. Eggs are generally laid somewhere moist, in soil or vegetation. Dehydration can be a problem for juvenile harvestmen so they tend to stay in the ground layer under logs and amongst leaf litter where conditions are usually quite moist.

Harvestmen go through several moults, shedding their skin to grow and develop. When they mature into adults they are better able to cope with drier conditions and many move higher up in the vegetation layer into trees and bushes or the walls of buildings, especially at night when they become more active.

Although the body shape of a harvestman is very different to that of a spider, they have a lot of other features in common. Both have a pair of sensory pedipalps (often referred to as just palps) and jaws called chelicerae (in spiders these contain the poison sack). Unlike spiders, harvestmen are not venomous. General leg structure is the same but in harvestmen the tarsus (foot) is divided into numerous segments making it very long and flexible. Although leg length varies, in all harvestmen the second pair are the longest and are used as sensory organs helping them to find food and to navigate their surroundings.

Harvestmen are nocturnal predators, feeding on a diverse range of invertebrates from tiny mites and springtails, to aphids, flies and woodlice. A couple of species specialise in eating snails. Many will also feed on decomposing vegetable matter (strictly speaking this makes them omnivores) and have been recorded on carrion. Prey records have also included bird droppings and even other harvestmen.

Finding Harvestmen

Late summer into autumn is the perfect season to take an interest in harvestmen as this is the time of year when the majority of species are adult and are therefore easier to find and identify. In fact it is their apparent abundance at this time of year that probably led to them being called harvestmen. Although the number of species in the UK is very limited, many are common and widespread and they can be found in a wide variety of habitats.

Harvestmen tend to be more active at night so a torch-lit stroll around the garden can reveal them but they can be found during the day by looking under logs and stones or other likely places offering shelter. Sometimes you can find several individuals grouped together under the same fallen branch, often of more than one species (NB when turning over logs and stones always remember to carefully put them back into position to carry on providing shelter for whatever lurks beneath).

They can often be found outside around the house where they hide under windowsills during the day. In my local cemetery I look for them on the shady sides of headstones or under overhangs and carvings on the more ornate graves. Some can be beaten from trees and bushes and others should be searched for at ground level in leaf-litter or moss.

Considering the small number of species to be found, it is surprising that harvestmen haven’t become the focus of more ‘list-orientated’ wildlife observers. Plenty of people have set out to see all the British butterflies for example while others are keen to ‘tick-off’ the dragonflies and damselflies. Likewise there don’t seem to be many with an Orthopteran list (grasshoppers and crickets, etc). There are some very interesting species of harvestmen out there and they are definitely well worth searching for.

Identification

Many harvestmen can be identified from good close-up photographs but others require the use of a hand lens or even a microscope to examine particular features to be absolutely certain. The structure of the palps is important including the presence or absence of a palpal claw, whether there are any bulges or apophosys and any armament in the form of spiked tubercles. The ocularium where the eyes are housed and the trident (a set of spines set above the chelicerae) are other important features that aid identification. Leg length also helps to determine species as can the shape and colour of the saddle (the central area of the top of the abdomen).

On the subject of legs, harvestmen can easily loose a leg and will deliberately shed one if attacked in order to escape. Mature specimens often have one or more legs missing, especially as the season progresses into winter. You should be extra careful in picking up harvestmen; if you do need to handle one then make sure you gently hold at least two legs (preferably the third and fourth together) to stop them shedding one. If a harvestman loses both its second pair of legs then it will have difficulty in finding prey.

Enemies

Even though they are not venomous like spiders, harvestmen are not entirely defenceless. They have a pair of odiferous glands which give them an unpleasant taste that deters many would-be predators. I have however watched Wrens searching nooks and crannies in the garden and picking out and eating harvestmen and Hedgehogs are said to happily snaffle them up too. They are also frequently recorded as prey to various species of spider, either by getting snared in their webs or simply straying too close to those that hunt by sight like the jumping spiders (Salticidae).

Harvestmen are often found with larval parasitic mites attached to them. Fellow arachnids, these tiny creatures latch on to their host and suck out the haemolymph (the invertebrate equivalent of blood). Harvestmen can also fall victim to fungal attack.

Species Accounts: my personal list

Although the UK has only a small number of harvestmen, there are still quite a few that I haven’t managed to find as yet. Most of my wildlife recording is done in my home county of Norfolk which has only around two thirds of the British species. As many of them can be found throughout the autumn and into winter this happily extends my invertebrate hunting season to cover most of the year.

I started with a meagre list of only eight species, mostly through chance encounters and casual observations whilst looking for other invertebrates. Now, however, I am hooked on these engaging creatures and have started actively searching for them. In a couple of seasons I have doubled my list to a fairly respectable sixteen. Here is my list so far.

Nemastoma bimaculatum

Nemastoma bimaculatum is listed on the BAS (British Arachnological Society) website as the most frequently recorded species while my personal records have Paroligolophus agrestis at the top. I think some of this may be down to the fact that Nemastoma bimaculatum is such a distinctive harvestman that is readily identifiable. It is a short legged species with a black body bearing two pale spots (described as silvery but they often look a pale peach colour to me). They are widespread and can be found throughout the year but require a bit of searching for, being invariably found under logs and stones or amongst leaf litter. It doesn’t help that they are only 3mm long. They often freeze, pulling in their legs to make a tiny scrunched up little ball but then run off rapidly which results in lots of blurred photographs!

Mitostoma chrysomelas

Mitostoma chrysomelas is a beautiful little harvestman, probably my favourite species so far (apart from the elusive and gorgeous Megabunus diadema that is). This is another species that requires careful searching for, having a preference for damp habitats where it can be found in amongst leaf mould and under logs and stones. The brownish abdomen often has golden patches and the ocularium and chelicerae are contrastingly dark brown to black. The legs are long, as are the pedipalps which are covered in knobbed hairs and are characteristically held folded upwards and forwards.

Phalangium opilio

Another easily identified harvestman is Phalangium opilio, or at least the mature male is with the horn like process on his chelicerae making them almost twice the size of that of other harvestmen. It makes them look heavily armoured and a force to be reckoned with. The female is an altogether more sedate looking creature but is still one of our larger species with long legs. Females can be mistaken for Mitopus morio which can share a similar saddle pattern and also lacks a trident but in good light the distinguishing feature of two small tubercles set above the chelicerae show up well in photographs.

Mitopus morio

Mitopus morio is found in a wide variety of habitats throughout much of Britain. This is another common species (in the top three) that I have struggled to find that often. It is quite large with long legs and is very variable in colour. Males often have a dark saddle with a pinched ‘waist’. There is no trident but a scattering of sharp tubercles and the ocularium also bears small tubercles.

Paroligolophus agrestis

Paroligolophus agrestis is also in the top three most recorded species. It has shorter legs than many species although they are still classed as medium length. They can be relatively non-descript, being variable in colour but often with a pale central line on the saddle and paired dark patches towards the rear. The trident isn’t particularly conspicuous but is usually comprised of a group of five blunt tubercles, three at the front with two smaller ones behind. The silvery ocularium is smooth compared to similar species. Females are readily identified though by the dark pincer-like notch at the front of the genital operculum on the underside of the abdomen which is easily seen with the aid of a hand lens (x10).

Oligolophus tridens

Oligolophus tridens is a common and widespread species. Having gone some while before finding my first one, I now encounter them quite regularly. A case of knowing where to look and what to look for. They inhabit the ground layer and are often found under fallen branches in woodland. Mixed woodland is pretty good, especially where there are oak trees. Males are well patterned with a dark saddle. The three-pronged trident is usually quite prominent (as the name suggests) and angled upwards; in males the tubercles are of equal length whilst in the female the middle one is longer.

Oligolophus hanseni

Oligolophus hanseni is a rather handsome creature with a dark saddle highlighted with paired light spots. It also has a dark ocularium. There is also a striking colour form where the hind end of the abdomen is white. The small trident has the central member longest and there are further tubercles to the side and behind. In Norfolk this species seems easiest to find by beating pine trees in the autumn.

Leiobunum rotundum

Leiobunum rotundum is a very long legged species and is often quite conspicuous in late summer/early autumn. I regularly find them actively out during the day, clambering amongst nettles and other low vegetation. They can also be found at rest on tree trunks. There are several species of Leiobunum in the UK including two very recent arrivals so care should be taken when identifying them. Of the two species I have found, L. rotundum seems to be the commonest, at least in my neck of the woods. Leiobunum species show striking sexual dimorphism with the males being considerably smaller than the females and much rounder looking and also differing in colour (see photo of male in Introduction).

Leiobunum blackwalli

Leiobunum blackwalli could be considered the twin of L. rotundum and the two do look very similar. They are easily told apart though by the colour of the ocularium – dark around the eyes in rotundum and light in blackwalli. This species embarrassingly eluded me for quite some time and I still have only seen a couple. They seem to prefer damper habitats than rotundum.

Opilio canestrinii

Opilio canestrinii is a relatively recent addition to the UK list, first recorded in 1999 having arrived somehow from Italy (not its native home), most probably as an accidental import via the horticultural trade. It is another very long legged species with the males in particular being distinctive with dark legs contrasting with the light body and orangey coloured trochanters.

Females also have pale trochanters but the body is more attractively patterned with paired light and dark bars on the abdomen. I have found it regularly in our garden where it habitually rests on the wall of the house. I also find it in my local cemetery on gravestones, often in the depths of winter. Its large size and rapid spread has the potential to put it into conflict with some of our native species. Observations in the Netherlands suggest that it is responsible for the decline of at least one species of harvestmen there.

Odiellus spinosus

Odiellus spinosus, a stout looking species with medium length legs, is our largest harvestman. It has a number of features that should make it instantly recognisable: a relatively small ocularium coupled with a robust forward pointing trident, heavily spined trochanters and the saddle bordered by thick black marks. It has a liking for drier habitats than many species of harvestman and is often found in association with people around houses and gardens. They are frequent on our allotment where they can be found hiding under various bits of wood around the compost heaps.

Rilaena triangularis

Rilaena triangularis (Platybunus triangularis in some publications) is sometimes called the spring harvestman for it is unusual in that it reaches maturity earlier in the year than most other species. I first encountered them in my local cemetery in early February when I found a couple of juveniles on headstones while I was looking for over-wintering ladybirds. I have since found small immatures whilst searching amongst leaf litter in October. They are characterised by the large ocularium and prominent apophyses on the palps.

Lophopilio palpinalis

Lophopilio palpinalis is a real cracker with fearsomely armed pedipalps bearing long spines. It also has a spikey trident and blunt tubercles on the ocularium. It shouldn’t really be confused with any other species. Damp woodland seems to be the preferred habitat. Happy was the day I turned over a fallen branch and found one of these! It was a very good branch in fact as it also had a couple of Oligolophus tridens on it too.

Dicranopalpus ramosus agg.

After it was first recorded in the UK, Dicranopalpus ramosus was heralded as the most instantly recognisable harvestman in the country due to its unique resting posture and forked palps. Unfortunately that status no longer applies as it has since been discovered that there is a second, almost identical species present, Dicranopalpus caudatus. To be certain of their identity a mature specimen needs to be examined carefully under a microscope. Most have to therefore be recorded as Dicranopalpus ramosus agg. I find the odd one or two each year resting on the wall of the house, usually in late summer and I have also beaten them from oak and other trees when they can be quite plentiful.

Dicranopalpus larvatus

Dicranopalpus larvatus is one of the latest species to arrive on our shores. In Europe it seems to only occur in Italy. The first UK records were from Guernsey, the Isles of Scilly and the Isle of Wight. Since then there have been a smattering of other records from the mainland (mostly Cornwall) but it was still the last thing I was expecting to find in my local cemetery when I was turning over fallen branches. It just goes to show that you never know what you might find until you start looking!

This species is smaller than the other two Dicranopalpus spp and is found at ground level. It has the same shaped palps but doesn’t hold its legs out at an angle. It matures later too, with adults being recorded in spring rather than summer/autumn. Males are readily identifiable with their pied appearance of white band across the front of the abdomen separating the dark head area and rear of the abdomen, coupled with the forked pedipalps. Females have black eyes against a dark purplish brown body that often has lighter flecks.

Megabunus diadema

And finally we have Megabunus diadema, by far the most attractive species I have ever seen. It has a beautifully patterned abdomen and an array of armature including leg spines and a seriously spikey ocularium. I have only seen it the once and that was on top of a fell whilst walking in the Lake District. Unfortunately it didn’t pose well for photos, scurrying away out of sight and I didn’t even realise what species it was at the time. Next time I come across one I hope to be better prepared and get some better shots of it.

Harvestmen are fascinating invertebrates and there is still a lot to be discovered about them. Records of prey items are scant for many species and their distribution can change, especially with changes in our climate. Some species are spreading while others may decline and new ones are ever likely to reach our shores. Finding, identifying and, most importantly, recording observations will help in our understanding of these widely recognised but often otherwise ignored arachnids.

Vanna Bartlett.

Resources

The new FSC Guide to Harvestmen of the British Isles is an invaluable starting point for looking at harvestmen, being a highly portable laminated sheet, illustrating all current UK species and detailing main features for identification. The published keys to harvestman are unfortunately currently out of print but there are some great resources online, notably the FSC Harvestmen Identikit which is an inter-active key. There is a lot of information on species of UK harvestmen on the British Arachnological Society (BAS) website and there is a Facebook group dedicated to harvestmen (UK Harvestmen (Opiliones)).