Darwin wasps

  Ichneumon wasps in the garden, menacing with their twitchy antennae and jerky movements, taking a break from parasitising insect larvae to satisfy their own energy needs.

Black Darwin wasp Ichneumon delicatorius

Yellow-striped Darwin wasp Ichneumon xanthorius feeding on nectar from lovage flowers. Its larvae develop parasitically inside the caterpillars of moths and butterflies.

 They are known as Darwin wasps because their gruesome life cycles played a role in undermining the great naturalist’s belief in a benevolent creator. “I cannot persuade myself,” he wrote, “that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their [larval stages] feeding within the living bodies of caterpillars.”

The Cats' Pee Orchid's Life of Deceit

We found a fine display of early purple orchids blooming on the cliff tops south of Seaham, along the Durham coast, today. They're beautiful flowers to look at but less attractive when you smell them, as they emit a strong aroma of cat's urine which is generally repugnant to people but attractive to bees........ which are in for a disappointment when they visit these flowers.

Charles Darwin spent many years studying orchids and was intrigued by their elaborate flowers and complicated pollination arrangements. In his seminal book in the subject, The Various Contrivances by which Orchids are Fertilised by Insects, first published in 1862 and which you can download here, he noted that although the flowers have a well developed nectar spur they don't produce any nectar. Bees that visit are duped into exploring one or two flowers in an inflorescence but soon leave when they find no sweet reward - but by then they've performed the essential pollination service that the plant requires.

This is Darwin's detailed dissection of an early purple orchid flower. The two important features to note are the hollow nectar spur in the uppermost illustration, which shows a longitudinal section of the flower, and the club-shaped structure in the centre, which is the pollinium containing the pollen grains. 


Try as he might, Darwin could find no trace of nectar in the nectar spur and was reduced to suggesting that if visiting bees did extract any reward from the flower it could only have been by piercing the floral tissues with their proboscis, which has never been proved to be the case. He concluded that the basis of pollinator visits was deceit on the part of the plant, that relied on naive, newly-emerged bees in spring making exploratory visits to any flower that might contain a food source.  


The other part of the flower that intrigued Darwin was the pollinium - the male stamen that is carried away in its entirety when a bee probes for nectar and the pollinium sticks to its tongue (click here for a picture of a pollinium-encumbered bee visiting an orchid). 

Each flower has a pair of pollinia - they're the two dark, club-shaped structures under the hooded petal in the flower above. When they're carried away by a bee they bend forward within about a minute, so they're well positioned to make contact with the stigma of the next orchid flower it visits.

In his book Darwin demonstrated rather neatly how you could mimic the whole process by poking a sharpened pencil into the flower. Try this - it really works. When you withdraw the pencil with the pollinia attached their stalks dry out and they slowly bend forwards. Incidentally, the ever-meticulous Darwin dissected, counted and estimated the number of pollen grains in a single pollinium - a staggering 122,400.

This orchid's deceitful pollination technique is only partially successful because bees are smart and after visiting a couple of flowers look elsewhere for an energy reward. Darwin's work was followed up in the 1990s by Dr. L.A. Nilsson at the University of Uppsala who showed that bees would visit more flowers on an inflorescence if those empty nectar spurs we filled with artificial nectar. 

Nilsson noted that early purple orchids only produce a few seed pods, developing from the first few flowers to open at the bottom of the inflorescence. The rest almost always remain unfertilised, although 80% of them could set seed if they were hand-pollinated. The first bees of spring may be naive but they learn fast and if they don't find the reward they are looking for they soon ignore that attractive scent of cats' pee and take their pollination services elsewhere, leaving many early purple orchid flowers unpollinated. 

Fortunately the seed production  from a single  pollinated flower is quite large  ..........Darwin counted 6200 seeds in a single capsule of spotted orchid and this species' seed production would be similar.... so early purple orchids manage to survive on deceit.

Earthworms- Nature's Ploughs

This is a rather alarming view of a very familiar animal - an earthworm. That's its mouth at the tip. As Charles Darwin observed in his book The Formation of Vegetable Mould, through the Action of Worms, published 1881, 'the whole country has passed many times through, and will again pass many times through, the intestinal canals of worms'. 

This was one of many that I disturbed when I was digging our vegetable garden yesterday. 

Darwin's book - the last that he wrote - remains one of the most fascinating sources of information about these useful animals. In his day there were still many people who treated them as pests, but he confirmed their essential role in maintaining soil fertility. Darwin was an inveterate experimentalist, demonstrating that earthworms swallowed earth 'for the nutritious matter it contains' and finding tiny smooth  stones in their gizzards 'used as millstones' for grinding up buried leaf material. His treatise contains information about their food preferences (he showed that they preferred onion, celery, carrot and cherry leaves above all others) and reports experiments that showed that up to 53,767 worms can live in a single acre of healthy garden soil, that they add fine soil to the surface (via worm casts) at the  rate of between 7.56 and 16.1 tons per acre per year (depending on soil type) and that their constant 'ploughing' of the soil, via tunnelling and feeding, buries objects like large stones at the rate of about 0.2 inches per year.

Worms' bodies are beautifully adapted for tunnelling. The inside of that spindle-shaped body is a fluid-filled cavity which forms a hydraulic skeleton, so that contractions of the circular muscles that run around the segments or the longitudinal muscles between them, acting on the incompressible fluid within, makes sections of the body shorter and fatter or longer and thinner.

Here, contraction of the circular muscles elongates the front segments of the body. You can also see the 'upper lip' (prostomium) of the mouth which, with the aid of powerful pharygeal muscles within, can act as a kind of sucker and allow the animal to grip and drag leaves - and surprisingly large stones used to block its burrow entrance. Those little pores on each segment are the site of retracted bristles.

When the longitudinal muscles contract and the circular muscles relax the body becomes shorter and fatter, filling the tunnel and forcing out short, curved bristles that give the animal purchase on the tunnel walls, allowing it to drag the hind segments forward or resist being pulled out by a predatory bird if caught dangling its tail out of the tunnel - as earthworms are prone to do. 

The bristles are too small to see with the naked eye but you can hear them - put a worm on a sheet of paper and you'll hear them scratching the surface as the worm wriggles. 

Darwin investigated the senses of earthworms in considerable detail, demonstrating that they could detect light but had no sense of hearing, although they were 'extremely sensitive to vibrations in any solid objects' - like soil, for example. His famous description of these experiments in his book are worth quoting in full.

'Worms do not possess any sense of hearing' he wrote. 'They took not the least notice of the shrill notes from a metal whistle, which was repeatedly sounded near them; nor did they of the deepest and loudest notes of a bassoon. They were indifferent to shouts, if care were taken that the breath did not strike them. When placed on a table close to the keys of a piano, which was played as loudly as possible, they remained perfectly quiet'.

He then went on to show that if he moved his pot of worms from the adjacent table onto the piano, they immediately retreated into their tunnels when notes were played, because the vibrations travelled through the piano, pot and soil.

You can download the whole of The Formation of Vegetable Mould, through the Action of Worms from Darwin on-line, here - it's well worth reading.

Tales From the Herbarium

There's a delightful Country Diary in the Guardian today by Derek Niemann about Gamlingay Great Heath in Cambridgeshire, that reminded me of a fascinating historical artifact linked to this location that I came across a few years ago.

Once or twice each year, I delve into the darkened corners of our herbarium in the basement of Durham University’s School of Biological and Biomedical Sciences to dig out a few plant specimens for an undergraduate tutorial. The herbarium is permeated by the faint smell of mothballs and lined from floor to ceiling with cupboards crammed with foolscap manila folders containing thousands of pressed flowers.


It’s a relic of a more romantic age in science, when botanists spent long summer afternoons roaming woods, heaths and hillsides in search of rare or unusual plants, trying to make sense of their relationships and unpredictable distributions. Whenever I go down to the herbarium I can’t resist taking a peek at one particular specimen of bracken fern labelled, in elegant, flowing handwriting, Pteris aquilina Gamlingay Cambridgeshire 17th. August 1831 J.S. Henslow.

It’s the signature and date that’s interesting, not the specimen. John Stevens Henslow, Reverend Professor of Botany at Cambridge University, amassed a personal herbarium of thousands of sheets of pressed plants. Somehow this single stray specimen found its way into the Durham University herbarium. And on the day that he collected it Henslow might have been wrestling with an urgent decision that - unbeknown to him - would dictate the course of history.

Just a few days before, on August 13th. 1831, he’d received a letter asking him to recommend a suitable gentleman ship’s naturalist to accompany Captain Robert Fitzroy, captain of HMS Beagle, on a circumnavigation of the world. Henslow, the finest field botanist of his day, would have been the prime candidate but family responsibilities stood in his way. His next thought was to nominate his brother-in-law, Reverend Leonard Jenyns, but he declined in favour of tending to the spiritual needs of his parishioners. So by the end of August Henslow had offered the job to Charles Darwin, his student protégé at Cambridge. The rest is (natural) history.

I like to imagine Henslow mulling over the problem of who to nominate while he botanised through Gamlingay Woods on the summer day when he collected the bracken specimen. If Henslow, a creationist until the day he died in 1861, or his cleric brother-in-law, had sailed with Fitzroy it’s unlikely that they would have come up with a theory of evolution. Charles Darwin’s date with destiny would never have arrived. It would have been left to some other scientist, in another place and at another time, to provide the theory that underpins all of modern biological science.


For informatio on another historically interesting herbarium specimen click here.