Can an ant turn into a berry?

03/04/2015 |

A tropical ant (Cephalotes atratus) infected with a parasitic nematode (Image: Steve Yanoviak via Wikimedia Commons)

No, don’t be ridiculous

“Ha! Ha! Always like a puzzle. Now then, where’s my Exeter Book of Riddles? When it comes to puns and word-smithing one can always trust those Anglo-Saxons, although that fellow Gollum had his moments.” Hum, hum, flick, flick, finger down the index, “Maybe an anagram? Hand me the thesaurus? Funny, drawing a bit of a blank. What did you say ants? berries? Sorry, old boy, completely stumped. Drop more tonic …?”

Yes, of course

Have another swig, while I explain. It concerns an ant from the American tropics, living high in the forest, and amongst other curiosities it is wingless but can glide. In the canopy there are also birds, and it can be a serious mistake if the ant has a nibble on their droppings. Not because of the taste, but because they may contain parasitic nematodes. Once swallowed the worms make their way to the end of the abdomen (the gaster), mate and then the female produces its eggs. So far, so normal, but by absorbing nourishment from the exoskeleton the cuticle thins (but not too much to fatally weaken it) and changes colour, from black to an attractive red. But that is not all the parasite achieves. The ant’s behaviour goes off-piste. Not only is it much less aggressive than usual, but it holds the “berry” up in the air. Fruit-eating birds are attracted to the berry mimic, down goes the ant and life goes on – at least for the nematode.

It all depends on the question

Nobody doubts humans have free-will, or do they? See what happens when a parasite first invades and then grabs the steering wheel and accelerator. Volition goes out of the window and the host is turned into a zombie. The key step is to hi-jack the command centres, perhaps by subverting the immune system, releasing neurotransmitters or even taking up residence in the brain itself. With the parasite at the controls, a variety of gruesome options present themselves. The infected host may swell alarmingly, engage in manic activity, or alternatively freeze into immobility. Sooner or later, however, the parasite must move on if it is going to complete its life cycle and thereby make somebody else’s life miserable.

Exit strategies vary. To gain freedom the host may see its head drop off, burst through the body like an alien, or simply deliquesce into a sticky goo. Or what about just drowning your host? This happens when parasitic hairworms (nematomorphs, relatives of the nematodes) give their marching orders to crickets to wander around erratically until sooner or later they approach a stream where they jump in and commit “suicide”. Tough on the cricket, but when hairworms have to complete their life-cycle in water that’s the way it must be. A similar fate awaits an ant from Papua New Guinea. This time it is a nematode that provokes the mischief, compelling its victim to strike out from the shore (well, a small pool). If rescued, the victim promptly turns back and before long it, but not the nematode, drowns. And even if the parasite has left the host it can still meddle, instructing the host to serve as a bodyguard for their vulnerable young. Parasitized caterpillars turn into aggressive assailants, sending would-be attackers flying, while spiders are induced to spin protective cocoons of silk for their “guests”.

However macabre every story is an adaptive tour-de-force. If, for example, tropical ants manage to avoid being turned into pseudo-berries, then at least one fungus (Ophiocordyceps) should make them shudder. Once infected, the ant is directed to a precise location (25 cm up, north-west facing, please) on the plant. This is not to enjoy the view, but because this fungal parasite is pernickety, demanding an optimal temperature and humidity. Once checked-in, the mandibles of the dying ant grip the leaf, so powerfully that equivalent scars can be identified in 50 Ma-old fossils. An elongate stalk then protrudes from the head and develops a capsule that splitting open wafts the spores towards the next victim. In another species (a carpenter ant) zombification results when a so-called “brain worm” (parasitologists know all about flukes) lodges itself conveniently adjacent to the root of the mandibular nerve. Once again the ant is transformed into an automaton, climbing the plant, engaging in a death-grip before a passing herbivore sweeps grass, ant and parasite into its gut. Putting your host in danger’s way is par for the course. When fluke-worms (trematodes) infest marine cockles, their digging foot shrivels. Unable to burrow, shore-birds and other hungry animals move in.

In this Darwinian chamber of horrors, who takes the biscuit, or rather the cockroach? Fly forward the jewel wasp. Relatives of the ichneumons, this wasp uses its ovipositor to deliver a short shot of venom. The wasp then strolls off to find a nest, while in her absence the cockroach engages in furious grooming, maybe to remove potentially dangerous microbes. Returning the wasp plunges the ovipositor into the brain of her host, using on-board monitors to precisely target particular ganglia. Heavy work and feeling peckish the wasp nips off an antenna and imitates Count Dracula. The cockroach can freely walk but having surrendered all self-motivation the zombified victim is led to the sacrificial site. Here an egg is attached, and no prizes what happens next. There is, however, another twist because as the larva munches its way through its home it secretes anti-microbial droplets to keep pathogens at bay.

One might take solace that at least the contest is between animals with nervous systems, even though fungi can also seize the reins. But surely not a virus? Unfortunately so. Sometimes the parasite uses a virus for its own devious ends. Other times the virus manages solo. When a corn earworm moth is infected, the virus heads for the reproductive tissue. The only way forward is for boy to meet girl. With the latter pumping out sex pheromones to staggeringly high levels and radiating other come-hither signals it would be churlish for a chap to decline the invitation. Alas, it is all a wasted journey. The female is sterile and anycase access is blocked by a viral plug.

Still the male doesn’t leave entirely empty-handed. In crickets an iridovirus serves as an enticing but dangerous aphrodisiac, while caterpillars infected with a baculovirus actively scurry to the top of plants, before turning into infective puddles. Should we see these hijackings as anything more than exquisite adaptive stories or at a subterranean level is our belief in free-will beginning to crumble? Tricky, so let’s conclude with a salutary tale. Did you know that roughly one in three humans are infected with a little horror known as Toxoplasma (the malarial Plasmodium is related). Cats are the main vector because Toxoplasma finds their guts an ideal place to have sex. When not devouring song-birds, in their spare time cats hunt rats. The latter strongly disapprove of this, but should their brains be full of Toxoplasma then a strong aversion to the smell of cat pee is subverted and caution replaced by an entirely misplaced fearlessness. In humans Toxoplasma lurks in various tissues, especially the brain, and there are intriguing similarities between toxoplasmosis and symptoms of schizophrenia. The link is suggestive, not proven, but maybe this way madness lies?

Is the ichneumon beautiful?

03/04/2015 |

Ichneumonid wasp
(Image: USGS Native Bee Inventory and Monitoring Laboratory/Wikimedia Commons)

No, don’t be ridiculous

The old man was deeply troubled. Beyond his book-lined study the apparently peaceful scene masked a world of unrestricted savagery. Once, on a sunny morning, he had written how in the struggle for existence “no fear is felt … death is generally prompt [while] the vigorous, the healthy, and the happy survive and multiply”. Now he knew this consolatory passage to be bogus. A benevolent Universe? Not in the grave-yard of Malvern Priory, nor for that matter in the remorseless activity of that little horror, the ichneumon-fly. Actually a wasp, the female employs the ovipositor first to paralyze the host (typically grub or caterpillar) and then to lay her eggs. Hatching they proceed to consume their living home from the inside, ultimately to erupt to freedom and so complete a life cycle of imposed agony. Is this insect a totem of existentialist despair or actually a thing of austere beauty? From stroking his beard, the man let his fingers drum on the desk, but eyes once glittering with scientific curiosity had long since faded. The small gravestone, inscribed Anne Elizabeth Darwin, was the metaphysical ichneumon that had drilled her father’s heart.

Yes, of course

Blind the watchmaker may be, but the ichneumon remains a near-miracle of biological order as she seeks a deeply buried prey. Immobile and invisible how can the ichneumon ever locate the concealed grub? In order to “see”, the female ichneumon runs across the tree, tapping its surface with a hammer-like antenna. It is a sort of sonar; as the returning vibrations travel up the ichneumon’s front legs they are channelled to a complex array of mechano-sensors that dispatch the news to the brain. Target identified, drilling commences. Even if much of the ovipositor is coiled inside the body or when free guided along a groove, its extreme length (up to 14 cm) and thinness (hence the risk of buckling, but essential to minimize friction) represent a major challenge. But the optimal vertical position of the ovipositor is facilitated by elongate legs and an arching of the body using the “wasp-waist” (it was this invention that then gave the wasps a decisive advantage in many other evolutionary spheres). With predators hovering in attendance, drilling needs to be as speedy as possible, but hours of effort are typical. If, however, the tip of the ovipositor was not equipped with cutting teeth and more astonishingly reinforced with substantial quantities of metal (often zinc, sometimes manganese) the process would be much slower.

The ovipositor is also cunningly constructed, consisting of three inter-locking units that channel first paralytic toxins and subsequently the eggs. During the cutting process they neatly slide past each other, but successful interception with the grub demands precise navigation. The controlling muscles are housed in the insect body, but the ovipositor is flexible and is also equipped with mechanical sensors that detect the smallest displacements and chemical “sniffers” to seek out the prey. An exquisite system, but surely only of interest to biologists? Not in the future. Clinical investigations have revealed a brain tumour, inaccessible to orthodox surgery. The surgeon opens the patient’s skull and places the needle-thin probe in position. It is a wonder of engineering, but its direct inspiration is the ichneumon ovipositor. Robotically it navigates precisely to the centre of the cancer and sets about its destruction. Millions of miles from the operating theatre an unmanned vehicle arrives on a remote moon. Deep beneath the surface life may lurk, but how to find out? Flaps open, a robotic arm extends, and the ichneumon mimic drills deep into the ice. This moon lacks an atmosphere and meanwhile other instruments survey the glittering stars.

It all depends on the question

Composed of stardust, our first birth was in the cataclysms of supernovae. Much is made of the strange coincidences that guarantee a universe rich in carbon, but about a quarter of the Periodic Table is essential for life. Some elements are obvious (nitrogen, oxygen, phosphorous, potassium and so on), but what about silicon? Employed by single-celled organisms (such as the photosynthetic diatoms) and in plants like grasses (as phytoliths) at first sight animals eschew this element. In fact, sponges construct exquisite siliceous skeletons, and curiously silicon plays a key role in the first stages of bone formation. But this element finds more unexpected employment.

Take a moment to consider the fate of diatoms as they encounter copepods in the freezing waters near Antarctica, or for that matter deep in the waters of Lake Baikal where their counterparts in the form of amphipods roam. Each have mandibles equipped with a thin but extremely tough capping of silica (a sort of quartz known as α-cristobalite). The diatoms may possess an ingeniously designed crush-resistant skeleton, but it is out-classed by these crustacean teeth. Teeth come in all shapes and sizes but they are united in their ability to combine strength with flexibility. How do the quartz jaws of copepods manage? Their ingenious solution is to incorporate pads of a natural elastic (the protein resilin, also widely used by insects). While these animals play the Rock-Scissor-Paper game with silica, other animals turn to metals. Seldom as crystalline masses, but usually cross-linked with amino-acids (histidine is a favourite) and a halogen (often chlorine). If ovipositors can brandish zinc tips, so scorpion stings and spider fangs are similarly hardened. As leaf-cutters ants gather their harvest, termites chew their way through dry wood, and beetles infest stores full of beans, they too employ razor-sharp, zinc-impregnated mandibles. Out to sea voracious bristle-worms and hunting arrow-worms deliver death with either zinc-tipped teeth or as alternatives manganese and even copper.

And what about that de rigeur accompaniment to any decent horror film, the brute with steel teeth? Can nature oblige? In vertebrates red coloured teeth do not point to appalling dental hygiene but the weaving of substantial quantities of iron into the enamel. Enhancing hardness (and perhaps resistance to acids) animals as diverse as fish (are you surprised these include piranhas?) and salamanders, as well as shrews (famous for their voracious appetites) and other mammals (including gnawing rodents and their extinct avatars, the multituberculates) have taken the ferrous option. But perhaps the nearest thing to a natural iron file is found in the ribbon of teeth (biologists cheer at the name radula, but Caesar meant a scraper) that enables molluscs like the limpet and chiton to rasp rocks draped with nutritious algae and bacteria. Using a template of chitin each tooth forms an exceptionally strong cap of iron oxides (goethite in limpets, magnetite in chitons) situated above a less tough but still mineralized core that includes phosphates and silica. It sounds simple, until you look at the nano-structures. Exquisite arrangements of rods and fibres confer immense strength with resilience. Given the greater hardness of the leading edge as against the trailing margin, each tooth is also conveniently self-sharpening. Across the planet ovipositors and teeth deliver death and impose destruction, but take a moment to admire their exquisite engineering. Very dangerous, but also beautiful.

Can an ant turn into a berry?

No, don’t be ridiculous

Yes, of course

It all depends on the question

Further reading

Is the ichneumon beautiful?

No, don’t be ridiculous

Yes, of course

It all depends on the question

Further reading