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.

A Moroccan harvestman in Durham in Winter

This extraordinary harvestman, with forked pedipalps, is Dicranopalpus ramosus. It is currently living behind the door of our garage, still alive and apparently still active in the depths of winter.

This species is native to Morocco and was first noted in Britain , in Bournemouth on the south coast, in 1957. Since then it has reached the Scottish border and is still heading north.

This is only the second time I've seen it here in Durham. The first was in summer, when I found one in my greenhouse.

It's a real surprise to find this one in mid-winter, alive but missing a couple of legs.

An amazing harvestman from Morocco

A lot of exotic animals tend to reach our shores these days and when they do they often make lurid newspaper headlines, with scare stories about 'alien invaders' that threaten our native flora and fauna. But there is one alien invader from Morocco that slipped ashore in Bournemouth almost 60 years ago, in 1957, and has steadily conquered the whole of Britain. This extraordinary harvestman, called Dicranopalpus ramosus, reached the Scottish border in 2000 and is still heading north. 

Here's one that lives on the door frame of our greenhouse. Its globular body is only about 4mm. long but its legs are proportionately enormous - they stretch almost from top to bottom of the image above. And because they are all held out at right angles to the body (aligned with the door frame above) the anmials is quite hard to spot unless it moves.

Here it is a little closer, seen from behind and showing less than half of the length of the second pair of legs, which are the longest.

The other distinctive feature of this amazing arthropod is the pair of long, forked pedipalps carried in front of the head .......

That look a little like crab claws and ......

..... are quite menacing, although they are primarily sensory structures - the jaws lie between them, under the front of the head.

When you give Dicranopalpus a gentle poke it abandons its cryptic behaviour and rises up on its splayed legs, much like any other harvestman. If you look carefully at the image above you can see the full extent of one of the second pair of legs, stretched out in front, towards the bottom of the picture.

Here you can see the fine covering of sensory hairs on one branch of those forked pedipalps and also the paired eyes carried on a turret on top of the animal, giving it all-round vision, although harvestmen have rather poor eyesight - their senses of taste, touch and scent, localised on the pedipalps and that second pair of extra-long legs, are far more acute.

So there you have it - the Moroccan invader that conquered Britain, almost unnoticed except by those who take pleasure in finding and watching this obscure but extraordinary group of animals. This is the first one I've seen - I'd never have noticed it if it hadn't moved when I shut the greenhouse door.

They are quite common now in most of Britain, apparently. They're probably in your garage, shed, greenhouse of garden right now.

For more about harvestmen, click here

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.

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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

Life and death in the cabbage patch .....

This year we've had record numbers of small white Pieris rapae butterflies in the garden. At one point I counted twelve feeding on a single lavender bush - which is bad news for our brassica crops but does create some wonderful opportunities for watching insect behaviour.

Small whites lay their eggs singly on the underside of cabbage leaves ....

.... and the first thing that newly-hatched caterpillars do is to eat their own egg shell. At this stage the caterpillars are yellowish and translucent but as soon as they begin feeding .....

..... you can see the cabbage leaf that they've eaten travelling through their gut.

They are really feeding machines and begin on the underside of the leaf because the waxy leaf cuticle is much thinner there than on the upper surface, where the cuticle is very thick and probably too tough for an infant caterpillar's jaws. You can see here that it has only chewed through the leaf as far as the upper cutcle and hasn't bothered to eat all the way through, except for a small hole. but there's another good reason for stayinging under the leaf .......

.... the constant threat from parasitic hymenopterans. With their jerky movements and twitchy antennae, they are always on the lookout for a caterpillar to lay their eggs in ....

..... and this one found a victim, depositing its eggs inside the tail end of a caterpillar which was on the upper surface of the leaf and is now doomed to be slowly eaten alive.

This one took violent evasive action, dropping from the leaf and swinging on a silken thread was soon as it felt the presence of the parasitic wasp.......

.......... while this much larger one is feeding really dangerously, up amongst the flower buds on the bolting brassica - but maybe it's already parasitised anyway. [Thanks to Africa Gomez at BugBlog for identifying this caterpillar and the one above as the cabbage moth)

Much is made in the press about the effects of recent extreme climatic events on butterfly numbers but parasitism of butterfly caterpillars might be at least as important in determining fluctuations in butterfly numbers from year to year.

The inflorescences produced by the bolting brassicas are a good food source for hoverflies and also home to other visitors, like this harvestman spider - a scanenging omnivore which wouldn't be averse to eating the occasional butterfly egg or newly-hatched caterpillar.

Hitchhikers

Harvestman spiders like this Mitopus morio turn up quite frequently in our garden at this time of year and quite a few of them carry these little scarlet parasitic spider mites - which apparently are mite larvae belong to a genus called Leptus - attached to their legs and body. 

I really need to catch one of these and take a look under the microscope - at this magnification they just look like little blobs of red sealing wax.


You can see another kind of parasitic mite, on bumblebees, 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

War of the Worlds

Harvestmen are animals that could have stepped straight out of a science fiction story. That globular body, slung between unfeasibly long angular legs, could easily be a Martian fighting machine, striding through hordes of fleeing, terrorised refugees in War of the Worlds....... or maybe I’ve got an overactive imagination..... but check out the CD cover for Jeff Wayne’s musical version of War of the Worlds (see http://www.coverbrowser.com/image/bestselling-music-2007/3826-1.jpg) and I think you’ll see what I mean.

You do need a pretty vivid imagination to come close to understanding how these strange animals perceive their surroundings. Those two eyes, on either side of the turret on top of their body, are not particularly effective and they mostly explore their surroundings using their senses of taste and smell, which are located on their palps, on either side of their mouth (just visible behind the front leg in the picture above - double-click on the image for a larger view)), and on their second pair of legs. Notice how, in the pictures above and below, this harvestman is lifting and extending one of its second pair of legs towards the camera – it’s smelling me. They take great care of that vital second pair of legs and if you watch one of these animals for long enough you’ll notice that it will draw each of this pair of legs through its jaws quite frequently, to keep these sensory limbs clean and in tip-top condition.

There are scores of these intriguing animals in our garden at the moment, striding through the dry grass and dead leaves in that cautious, fastidious way in which they move, searching for small insects to feed on amongst the decaying vegetation of autumn.