Harvestman

It's a sign of the high summer when harvestmen appear in our garden. Initially I though this was Leiobunum rotundum, until Richard Burkmar (see comment below) pointed out that it's an invasive, non-native species called Opilio canestrinii which was first documented in Britain in 1999.This species has remarkably long black legs, like L.rotundum, but there is an orange ring around the trochanter ('knee') that I hadn't notice. This male, sunbathing on a leaf in the late afternoon sunshine, is in pristine condition, probably having just reached maturity. Females have a darker saddle-like marking on their dorsal side.

The longest legs are the second pair and key senses of taste, smell and touch are located on these. If you watch them walking through the undergrowth you can see them using these to explore their surroundings.

Harvestmen often lose a leg of two, shedding them if they are grabbed by a bird or caught in a spider's web, and this isn't too much of a problem unless it's the second sensory pair - then they are in more serious difficulties.

For more harvestmen (including pictures of a female Leiobunum rotundum) on this blog, click here.

Opilio canestrinii has spread northwards quite widely since it was first found in Essex eighteen years ago. The Spider and Harvestman Recording Scheme web site (click here) shows that it has reached as far north as Inverness. 

My guess is that it arrived in our garden via a plant from a garden centre. The wholesale and retail garden plant trade provides perfect distribution network for newly arrived arthropod species like this.

Lovely Legs

Today's Guardian Country Diary is about encounters with harvestmen while clearing an overgrown garden.

Although they're often referred to as harvestmen 'spiders' they belong to a quite different taxonomic order from true spiders and have none of their menace, mainly because they lack large poisonous fangs but also because of the tentative, non-threatening, painstaking way that they move. Those long legs are not just for walking - the second pair also carry their senses of touch and smell, which are far more sensitive than their single pair of tiny eyes that are mounted on a turret on their topside. 

When you watch a harvestman on the move it gently feels its way over new territory by extending that extra-long second pair of legs, constantly touching the ground ahead and picking up scents. If you put two harvestmen together in a confined space they'll each extend their second pair of legs towards each other, picking up each others' scent.

Harvestmen scavenge dead animals and will eat fungi and drink juice of ripe fruit, but they are also hunters, catching and eating animals smaller than themselves. There is one intriguing account of a harvestman penning up its prey within a circle of those long legs, drawn close-in to its body, then dropping on its prey like a pile-driver. 

A variety of animals prey on harvestmen, including frogs, toads and birds. On a couple of occasions I've seen robins with their beaks full of harvestmen in autumn. 

They have two forms of defence. One is the secretion of a noxious fluid, said to smell of walnuts, which will deter small predators like ants. Their last line of defence is autotomy - the shedding of legs to allow then to escape a predator's grip (or the sticky threads of  spider's web). It's common to see harvestmen with fewer than the full complement of legs at this time of year but if they lose the second pair - and with them their vital senses of smell and touch - they stop feeding, essentially losing the will to live.

This is the pose adopted when they're faced with a threat, with that second pair of legs extended as wide apart as possible - assessing the threat by detecting its scent.

Although they tend to move slowly and precisely they can, if the need arises, run with considerable speed over uneven surfaces - like long grass, for example - thanks to those amazingly long legs that allow them to simply step over objects that would slow shorter-legged animals down.

At this time of year harvestmen will be laying eggs, which will overwinter and then hatch  next spring as nymphs, which will then slowly grow larger as the summer progresses.

.

When Jurassic harvestmen walked the earth.

The raspberry bushes in our garden seem to be a favourite haunt of harvestman, including this one which I think is a male Leiobunum rotundatum. They're often there early in the morning, when they drink dew on the leaves, but later in the day I suspect that they come to drink the juice leaking from damaged fruit.

Fossils from the Rhynie chert in Scotland show that harvestmen have existed for at least 400 million years, more or less unchanged, and you can download a scientific paper with pictures of an exquisite 165 million year old Jurassic harvestman fossil by clicking here.

Since these animals first evolved there have been five great, catastrophic mass extinction events that wiped our many life forms, including the dinosaurs; amazing, isn't it, that these lanky, fragile animals that will shed legs if you touch them have survived while the dinosaurs didn't............

You can find more information about these intriguing animals by clicking here and some more pictures of harvestmen by clicking here

Harvestman

It isn't until you watch a harvestman walking aross a rough surface - like this pebbledashed wall of a house - that you realise how practical those lanky legs are. Anything with short legs with a body close to the ground would have to negotiate every pebble, but a harvestman can stride aross this rocky terrain with no impediment.

It's interesting to contemplate what the view must be like from those two eyes, mounted on the turret on the animal's back, that give all-round vision. It must be much like the view of a tower crane driver I guess, exception that eight knees substitute for the single crane jib.

I think this might be Leiobunum rotundum........(?)

You can find more on harvestmen here