A Buzz in the Meadow Page 6
I wanted to watch exactly what happened during courtship, and to see whether cannibalism only occurred when the female was hungry. I also wanted to see whether cannibalism was reduced when mating took place in a spacious environment with lots of twigs and cover for the male, rather than in a bare cage. I spent many happy hours watching my mantises, aided by a succession of undergraduate project students.
Typically, and understandably, the male mantises tended to approach the females extremely slowly. Mantises can move almost imperceptibly by swaying gently, as if they are a frond of vegetation being wafted by the breeze. The impression is helped by their camouflage; most species are green or brown, so that they blend in well with the vegetation in which they live. A few tropical species are spectacularly beautiful, being pink or white to match the exotic orchids on which they wait to catch prey. The French species found in my meadow is a vivid, glossy green, the colour of wet summer grass; my Gambian mantises were a twiggy brown. With each tiny sway the mantis moves slightly more forward than back, and so edges towards its target: often prey, sometimes a mate.
When courting, the males proved to have two distinct strategies. Some attempted a front-on approach, edging more and more slowly towards the female, the closer they got. When they were just out of reach of her powerful forelegs, they started to display. The exact display varies from species to species; mine engaged in something akin to belly-dancing, with the male gyrating his abdomen, curling it forwards and backwards, wiggling it from side to side, and sometimes managing both at once. The female watched, inscrutable. Or at least so it seemed to me, but presumably the males were watching her to see if she was impressed. What kind of mood was she in? Did she look amorous, or peckish? If he felt the moment was right, he would pause, crouch and in a single bound would leap on to her back, pivoting in mid-air as he did so, like a vaulter at the Olympics.
Other males approached the females from behind. In this case a male would typically approach slowly at first, but – in marked contrast to males approaching from the front – he would accelerate as he got closer, sprinting the last few centimetres and then hurling himself on to the female. So far as I could tell, these females had no idea he was there until rudely mounted from behind.
Once on the female’s back via either strategy, a male is not within easy reach of the female’s forelegs, but he is not safe. She invariably struggles when he first lands on her, presumably weighing up her options. If he is lucky, she may decide to accept his attentions. If not and she continues to struggle, it is only a matter of moments before her superior strength prevails and he becomes dinner. This might occur if the female decides that the male is of inferior quality – an extreme form of rejection – or it might be just because he caught her at a bad time. Sometimes she lets him mate, but then decides that she needs a snack halfway through and that is when, typically, she bites off his head.
The results of our studies were pretty clear. At no point could we discern any sign that the male was a willing victim of cannibalism. Their approaches to females were cautious, and after mating they would either leap off and run for it or edge discreetly off her back, keeping well away from her front end. When confined with a female in a simple, undecorated cage, the males were much more likely to be consumed, usually without mating, than when there were plentiful twigs and leaves. When confined with a hungry female, their chances were not great. But when the female was well fed and they were confined together in a twig-filled cage, rates of cannibalism were very low, with males surviving more than 90 per cent of matings. I have since repeated this with French mantises from my meadow, and these Gallic lovers show no more signs of wishing to be eaten by their partners than did their Gambian counterparts. None of this tells us for sure how common sexual cannibalism is in nature, but it suggests that it may be nowhere near as common as once supposed.
Sex in insects is fraught with conflict. A meadow in summer is a seething mass of sexual adverts, courting couples, brutal rejections, conquests and copulation, a mad rush to reproduce and ensure there are offspring to carry on the line when the season ends. The song of the birds and crickets, the flash of a butterfly’s wing, the swarming of dance flies on a summer’s eve – all are driven by the evolutionary imperative of reproduction. However, as we shall see in chapter twelve, sexual intrigue is not confined just to animals.
CHAPTER FIVE
Filthy Flies
17 July 2008. Run: 38 mins 17 secs. People: one friendly old lady on her bicycle. Dogs: 5. Butterfly species: 12, the highlight of which was a silver-washed fritillary restlessly nectaring on brambles, a powerful creature with black-spotted rusty wings, the hue of dry, late-summer bracken. A cloudless blue sky once more, and it is already warm at 8 a.m. As I am recovering from the run, gasping and damp, I can see a female green lizard burrowing into my heap of building sand, preparing a nest for her eggs. In the distance, the wack-wack bird is calling plaintively from the south-east.
God in His wisdom made the fly
And then forgot to tell us why.
Ogden Nash, ‘The Fly’
The hand is quicker than the eye is,
But somewhat slower than the fly is.
Richard Armour, ‘Inscription for a Fly Swatter’
Flies are a group of insects that has few fans, which is a great shame. God may not have told us why he made them, so I shall give it a stab. Flies are a huge and diverse group, comprising about 240,000 known species, roughly one-fifth of all known life on Earth. Following the logic of J.B.S. Haldane’s quip that God must have ‘an inordinate fondness for beetles’, I would add that He must also have a penchant for flies. They are found in more or less every terrestrial and fresh-water environment on Earth, from the deserts to the Arctic, from the sludge at the bottom of garden ponds to all but the top of the highest mountains. The larvae of one species can survive temperatures above boiling in the trunks of the desert cacti in which they live. Flies are a staple food for many thousands of animals – at Chez Nauche they are eagerly consumed by swallows and martins, pipistrelle bats, lizards and toads, mantises and many other fascinating creatures. Millions of birds migrate each year to the Arctic Circle to breed, so they can feed their young on the countless midges that emerge in the brief Arctic summer. The larvae of many flies are vital in helping to break down rotting organic matter, recycling the nutrients so that they become reavailable for plants to use in growth. Some flies are strikingly beautiful, clothed in metallic green, blue or gold; the large furry hoverflies that mimic bumblebees are almost cuddly. Picture-wing flies have intricately patterned and coloured wings, like miniature stained-glass windows. On the muddy edges of my pond, bright-green dolichopodid flies perform elaborate courtship dances, the red-eyed males tempting the females by swirling and twisting their wings in a manner reminiscent of a dancing geisha girl with her fan. Others are weirdly outlandish and mesmerising in their strangeness – for example, male stalk-eyed flies have their eyes on the end of very long stalks that protrude from opposite sides of their heads. Most flies never bother us to the slightest extent, living out their diverse lives beneath the human radar, and most of those 240,000 species have not been studied at all. For example, the larvae of most of the European species of dance fly have never been seen, even though they are common insects. We have no idea what they look like, where they live or what they eat – the adults just appear as if from nowhere every summer. What fascinating discoveries must await future dipterists.1
A small number of species are more noxious and have given flies a bad name that most do not deserve. Some bite, which is irritating; try having a picnic in western Scotland in August and you will be consumed alive by midges. Some of those that bite spread diseases – mosquitoes, blackfly, tsetse flies, and so on. Much as I love biodiversity, I’m pleased to say that, apart from the odd mosquito and horse fly, we have few biting flies at Chez Nauche. However, there is one fly species that sometimes appears in abundance, and of which I am not fond.
I have probably given the
impression that Chez Nauche is a rural idyll, paradise on Earth, and in my eyes this is not a million miles from the truth. However, there is one small creature that does its level best to spoil the peaceful atmosphere – one literal fly in the ointment – and that is the house fly. Periodically during the summer months plagues of house flies arrive, having bred in piles of manure on nearby farms. They gather in swarms around the picnic table and around the doors and windows, finding their way inside before long. My wife Lara and I try to keep the doors and windows shut, and from the beams we hang dozens of fly papers, which quickly become covered in a revolting mass of struggling, sticky flies. We arm the children with fly swatters and set them to work, but their most enthusiastic efforts seemingly make no dent in the multitude. Like the Spartans at Thermopylae, the bodies mount up in front of them, but countless more flies pour in through gaps in the door frame and under the old clay tiles in the roof, and eventually the boys are overwhelmed and give up.
This is not the first time I have fought a futile and losing battle against house-fly invasions. In 2006 I inherited a PhD student named Jason Chapman from a retiring academic. Jason is from Cardiff, a proud and vocal supporter of all things Welsh, and one of a select group of people I have come across who is truly passionate about flies. Jason’s PhD was on trying to develop novel, environmentally friendly means of controlling house flies. The house fly, properly known as Musca domestica, is a greyish-black, smallish, bristly fly that is found wherever in the world there are people, animals and their dung, which is to say virtually everywhere these days. They have been around for perhaps sixty-five million years in something close to their current form, but really began to thrive as human populations grew and spread around the world. House flies probably evolved in the Middle East, but from there they have followed people around the globe. They can survive in the coldest climates by living in cattle sheds and chicken houses, helped by the warmth created from lots of animal bodies and fermenting dung.
The adult flies lay their eggs in any damp, rotting vegetable matter where there will be plentiful bacteria for the maggots to eat. Since the dung of most herbivores consists of damp, half-digested plant material mixed with bacteria, this is perfect. The maggots grow quickly and, once fully grown, crawl away to a drier spot to pupate. Under ideal conditions the whole life cycle takes about fourteen days, and a single female can lay 500 eggs in her life. It is easy, if a little pointless, to calculate the potential for population growth. One female fly leads to 500 flies in two weeks, 125,000 flies in four weeks, thirty-one million flies in six weeks, seven billion flies in eight weeks, and so on. Thankfully this never actually happens, as cold weather, outbreaks of disease or attacks from predators tend to curb population growth eventually, but nonetheless this explains how very large populations of flies can quickly appear when the conditions are favourable.
Of course rotting vegetable matter isn’t very nice, and neither is animal dung, so something that thrives on eating it ought perhaps to be encouraged, welcomed, even admired. Dung beetles perform a similar function and were so valued by the ancient Egyptians that they carved huge stone statues of them. The problem with flies is not the maggots that eat the dung, but the adults. They have the most intensely annoying and deeply unhygienic habits. Their mouthparts are rather like a sponge on a stalk, with which they mop up more or less any soft, damp, digestible organic matter. If the food is too dry they vomit on it, puddle the vomit around a little with their mouthparts and then suck up the resultant mess. Their favourite comestibles seem to belong to one of two categories – animal faeces and human food. If they can’t find either of those, flies will happily land on animals or people, preferring the lips and the corners of the eyes, or any cut or running sore, where they mop up any juices they can find. Failing that, they will happily drink a little sweat, vomiting occasionally as they go. The persistence with which they repeatedly land on ourselves and on our food is incredibly vexatious2. They have lightning reactions, making them hard to swat, and they return over and over again, as if taunting us. A swarm of flies can turn a relaxing picnic into a nightmare.
If only they were simply annoying, it wouldn’t be quite so bad. The real problem is that their behaviour makes them the most incredibly efficient vectors of an appalling range of diseases. Their stomach juices are chock-full of bacteria and viruses from previous meals on faeces, rotting food, dead animals, and so on. So when they vomit on my Saint Agur,3 they are contaminating it with all manner of potentially dangerous disease organisms. Amongst other diseases, house flies are known to spread cholera, typhoid, salmonella, polio, hepatitis, E. coli, tuberculosis, anthrax, parasitic worms and conjunctivitis. More than 100 human diseases are spread by the little fiends, including many that can be fatal.
Now I strongly object to the modern obsession with sterility. Antibacterial sprays and hand-wipes may be vital in hospitals, but in my view they have no place in domestic homes. I’d much rather my children ate food with a bit of dirt on it, and grubbed around in ditches and ponds, than that they were kept in a sterile bubble. But I’d really prefer that they do not catch cholera, so flies go too far, even for me.
To return to Jason’s PhD, his remit was specifically focused on trying to find a way to control house flies on battery chicken farms. These are among the most unpleasant places that man has thus far managed to create. If you’ve never been in one, count yourself lucky; continue to avoid the experience, and stop buying cheap battery-farm eggs. Battery farms are vast, dusty, foetid places. The chickens sit in tiny cages ranked three or four high, one above another. The floors are mesh, so that the excreta from the topmost chickens dribble down on to those below – if ever you are reincarnated as a battery-farm chicken, pray you don’t end up on the bottom row. The floor of the building is slatted, so that the combined excretions of all the chickens eventually make their way through the gaps in the slats into a vast chamber beneath the floor, which slowly fills with rotting chicken faeces. This is bulldozed out every few months when it is full. The whole building is heated, perhaps because many of the chickens tend to be more or less bald and would die otherwise, but it is kept in sepulchral gloom to save electricity. The stench inside a chicken house is initially horrendous, but disturbingly it goes away if you are unfortunate enough to spend a long time inside.
You will have worked out by now that I am not a fan of battery farms. I speak from long experience, as my first paid job as a teenager was as a weekend egg-collector on the local battery farm. Modern farms have conveyor belts to automatically gather the eggs as they are laid, but in those days they were all collected by hand. It is a very odd feeling being the only person in a huge dimly lit shed with several thousand chickens all staring at you – of course they don’t have much else to look at, since your appearance is the only remotely interesting thing that has happened to them all day. The job involved pushing a trolley loaded with egg cartons along each aisle and scooping up the eggs from both sides. It was almost too dark to see the eggs along the bottom row, so the easiest method was just to run your fingers blindly along the rack, picking up the eggs by touch, but occasionally you would accidentally grab the head of a dead chicken, which had slumped down on to the bottom of its cage. Almost worse, some of the eggs were deformed, having no shells or having strange bulges. The deformed eggs were placed in a special bucket and were sent off to make shampoo. Thanks to these early experiences, I was not especially delighted to take over supervising Jason and renew my acquaintance with battery farms.
The problem he was trying to solve is easily explained, but exceedingly difficult to address. The accumulating mass of chicken faeces under the floor provides a perfect environment for house flies to breed. The fermenting mound remains warm throughout the year, and there is a constant supply of new food falling down from above. Flies can breed in their millions in these conditions. The adult flies are drawn upwards by the dim light filtering down through the slats above, and so they gather in countless numbers in the chamber above with
the chickens. As if their life wasn’t miserable enough, the poor chickens have to put up with the constant bother of flies landing on them. The flies can rapidly spread diseases amongst the chickens, and they make life intolerable for the humans who have to venture inside. Jason’s job was to find a way of controlling these flies.
You might think this would be easy enough. There are plenty of chemical insecticides on the market – cans of fly spray can be bought from any supermarket. There are two problems with this approach. First, many insecticides are pretty unpleasant chemicals that are also toxic in varying degrees to both chickens and people. Some types of insecticide were originally developed as agents of chemical warfare – nerve gases – during the Second World War. Many have been banned, particularly in situations where they could contaminate human food – such as eggs – and so the selection available to a farmer is limited. The second problem is that flies have evolved resistance to more or less all of the insecticides that are available. Agrochemical companies are doing their best to develop new ones, but it takes many years and investment of millions of pounds to do so, so new insecticides reach the market very infrequently.
Of course there are those who deny that evolution takes place, but the development of resistance in flies to insecticides provides one of the neatest and most incontrovertible examples of evolutionary change, happening so rapidly that we can watch it in action. The speed of evolution depends upon the size of the population, the length of their generations and the strength of the selection pressure. Large populations with short generation times can evolve very rapidly in response to a strong pressure such as regular exposure to a deadly insecticide. Imagine a population of one million flies living in a chicken shed. The farmer sprays them with the latest type of insecticide. Just as in any population, the flies are all slightly different. They vary in size, wing length, the thickness of their cuticle, their colour, and so on. In part this variation is due to their genes, the combinations of which vary from individual to individual. Most will be carrying a small number of recent mutations, genes that were altered by a mistake during copying. Because of this variation, just a tiny proportion of them – let’s say one in 1,000 – have a gene that makes them a little more resistant to the chemical than their fellows. Perhaps the gene allows them to break down the insecticide in their body, or perhaps it makes their cuticle a little less permeable to it. The vast majority of the flies are killed by the insecticide, but most of the few that survive carry one of the formerly rare genes that confer resistance. Freed from competition, these few flies breed rapidly in the abundant dung, and soon the population springs back towards one million. The farmer sprays again, but this time most of the flies are resistant, and his chemical has far less effect. Within a few months the chemical achieves nothing at all. The flies have evolved.