Thursday, September 23, 2010


I found this delightful little harvestman amongst some raspberries that I picked yesterday evening. Although they are carnivores, they do seem to have a liking for ripe fruit.
Despite their spider-like appearance and the fact that they belong to the Arachnids, harvestmen are not true spiders but belong to an order of their own known as the Opiliones, distingished from spiders by their globular bodies, unlike those of true spiders which are divided into two parts - the separate thorax and abdomen, separated by a constriction. Harvestmen always seem to have an other-worldly appearance and, scaled up to monstrous proportions, wouldn't be out of place in a science fiction movie. Many of their sensory functions are located in their legs and if you watch the way in which they use these - particularly the second pair that are often far longer than the others - it's quite clear that they are using them to feel and taste their way around their habitat. Nevertheless, when danger threatens they can shed a leg (autotomy) and leave it twitching on the ground, to deflect the attention of  a predator. Unlike true spiders, they can't regrow limbs after moulting, so this desperate measure leaves them short of a limb and sensorily deprived. Before they resort to limb-shedding, they often exude an unpleasant-smelling liquid from their leg joints which you can smell if you hold an irritated harvestman in your hand. This isn't an infallible deterrent - I've watched robins feeding their brood with harvestmen.

One of the most remarkable features of harvestmen is the arrangement of their eyes, in a kind of turret called an ocularium, high up on their back. This arrangement makes sense when you get down to a harvestman's eye-level ...

... when it's clear this this gives it 360 degree vision around and above its globular body....

..... although one can only guess at how much detail those unblinking black eyes resolve.

Nevertheless, whatever angle you approach from a harvestman always has it of those eyes always seems to have you in its sights...

There's an excellent little booklet called British Harvestmen by J.H.P. Sankey and T.H.Savory (Synopses of the British Fauna (New Series) No. 4  ISBN 0 12 619050) that not only provides fascinating detail about their biology but also contains some delightful little anecdotes. For example, some species apparently kill their prey by positioning their globular bodies over their victim that's imprisoned by their legs and then bouncing up and down on those long legs, 'pile-driving' the unfortunate prey. Others have been noted for a prediliction for marmalade sandwiches from a picnic and on one occasion ink from ink wells (although this one refused black ink and would only drink the red stuff). 

I haven't got around to identifying the species of this individual yet and if there's anyone out there who can help me out, I'd be grateful....

Monday, September 13, 2010

Moth Pointillist Colour Patterns

I found this herald moth Scoliopteryx libatrix, with these beautiful orange markings on its wings, in my garden.

The whole moth is covered with scales, of various shapes, sizes and colours over its whole body, even to the extent that its legs are clothed in this rather fetching pattern of alternating black and white rings.

The main body is covered with fine hair-like scales, but the scales on the wings are....

... much broader, although they vary in width and colour. One interesting feature is that the patches of colour that look fiery orange to the naked eye are composed of a mixture of pinkish-red scales interspersed with variable numbers of yellow scales. The whole effect is reminiscent of colours produced in Pointillist paintings, of the kind made famous by George Seurat. By Seurat's day (1859-1891) the study of colour had revealed that the close juxtaposition of points of two colours could produce the effect of a third colour when viewed from a distance and Seurat exploited this in his meticulously executed paintings. The computer monitor screen that you are viewing this blogpost on uses a similar principle, of coloured dots, to produce its vast range of colours. Butterflies and moths have been exploiting the same phenomenon for millions of years, to either make themselves conspicuous to mates or generate camouflage patterns.
The herald moth's wings also carry small clusters of distinctive white scales, like those just above the 'orange' patch here, which I suspect may be scent scales that emit pheromones recognised by other individuals of the same species.

Monday, September 6, 2010

Brittle Stars

The swaying fronds of red seaweed that fringe rockpools near the low tide level on the seashore are home to a wealth of miniature marine life, less spectacular than the inhabitants of coral reefs but every bit as intriguing. I found scores of these tiny brittle stars, the largest no larger than a centimetre across (including arms), on a visit to the Northumberland coast at the weekend. Brittle stars, or ophiuroids, are relatives of starfish and sea urchins, in the phylum Echinodermata (which means spiny skin - a feature many members of the phylum share). The view above is of the underside of one of the brttle stars, showing the mouth fringed with five teeth formed from calcareous plates.

Seen from above, five arms radiate from the pentangular body. Each arm is formed from articulated segments linked by muscles and these are very flexible, so the animal often curls the tip of an arm around a seaweed frond to stop itself from being washed away by currents. If it's alarmed the muscles between the arm segments contract and then the arms become very brittle.....

... and it doesn't take much force to snap them, as has happened here with the upper arm. This is not a problem, as....

... arms can easily be regenerated, as is happening here with the middle, lower arm. This capacity for shedding and regenerating arms is analogous to the way that lizards shed their tails (autotomy) if they are picked up by that appendage.

At higher magnification you can see the anatomy of the arms more clearly. Each calcareous segment bears spines and a pair of tube feet, that are all interconnected by a hydaulic system of radial canals that run along the arms and a ring canal that runs around the central body. Local relaxation or contraction of muscles, compressing liquid within, elongates or retracts the tube feet.
Unlike the tube feet of starfish which have suckers on their tips and are used for 'walking', those of brittle stars are primarily for sensory purposes and to assist in feeding, by secreting adhesive mucus. In this higher magnification image you can see that the tube foot is hollow.

The ring of tube feet around the mouth on the underside, where the arms converge, help to sweep food particles beyond the five calcareous teeth)....

.... into the muscular oesophagous, and then into ....

... the stomach. The tiny central body also contains gonads, that produce eggs and sperm that give rise to the planktonic ophiopluteus larvae.

When they're fully grown some brittle star species can reach 60 centimetres in diameter (not in Britain, though), but they all begin life as planktonic larvae, often settling into the shelter of seaweeds on the nursery slopes of rock pools or coastal shallow seas, which are of such importance for the health of the oceans.

You can see a YouTube vieo sequence of an adult brittlestar here.

Thursday, September 2, 2010

Plant Plumbing

Swiss cheese plant Monstera deliciosa is commonly grown as a decorative house plant but in its native Mexican rainforests it's a rampant climber, using its adventitious roots to cling to trees and climbing in much the same manner as ivy in temperate woodlands. Those holes in the Monstera leaf, whose resemblance to holes in Swiss cheese account for its name, let flecks of sunlight filter through to the layers of leaves below, all of which are transpiring water from their surface. If you cut a section through the stem, you can see the internal pipework that conducts water from the roots to the leaves.

In this transverse section of adventitious root, stained with fluorescent dyes that colour dead, woody cell walls yellow and living cellulose cell walls blue you can see the various cells that conduct liquids up and down the root. Embedded in that thick-walled strengthening tissue that gives the root (which in this case is used for clasping tree trunks and branches - this plant is a tropical climber)  rigidity and are fluorescing yellow, are large vessels that conduct water in a continuous tensile column from the roots to the leaf, pulled upwards by evaporation from the leaf surface. The smaller tubes, lined with a layer of blue-fluorescing cell walls, may be resin ducts. The outer cells on the left, part of the ring of small bundles of living cells that encircle the root, are the phloem cells that conduct sugars manufactured by photosynthesis in the leaf to other parts of the plant. Swiss cheese plants are such familiar items of interior decor that they hardly attract a second glance, but they have extraordinary hidden beauty, only visible under the microscope