Illustration by Inna-Marie Strazhnik
Some flies, known as phorids, specialise in decapitating ants in a gruesome way. They lay their eggs inside their victims. When the maggots hatch, they move towards the ant’s head, where they gorge upon the brain and other tissues. The ant stumbles about in a literally mindless stupor until the connection between its head and body is dissolved by a enzyme released from the maggot. The head falls off and the adult flies burst out.
There are hundreds of species of phorid flies, each one targeting its own preferred ants. But some ants are naturally defended against these parasites because they’re incredibly small. Most phorids are a few millimetres long. If an ant is the same size, its head wouldn’t be roomy enough for a developing fly. Thailand, for example, is home to an acrobat ant (Crematogaster rogenhoferi) which can be just 2 millimetres long. Surely these workers are safe from decapitating parasites?
No, they’re not. Brian Brown from the Natural History Museum of Los Angeles County has just discovered a Thai phorid that’s just 0.4 millimetres in length. It’s the world’s tiniest fly, small enough to sit comfortably on the eye of a common housefly. It’s easily small enough to fit inside the head of even the smallest acrobat ant. It just goes to show that there is no way of truly escaping from parasites. If you evolve a miniscule body, they will shrink even further in pursuit.
Many insects eat plants, but some plants can turn the tables on their would-be diners. The pitcher plants are among several groups that can capture insects and digest their flesh. And one species – the fanged pitcher plant – goes even further. It digests insects with insects.
There are around 120 species of pitcher plants and all of them have large leaves that fold to produce fluid-filled traps. The rims of the pitchers are usually extremely slippery, and insects that wander by lose their foothold and fall into the pool of fluid within. There, they drown and are digested by the plant.
The fanged pitcher is unusual. Its rim lacks the usual waxy layer and is less slippery than those of its cousins. And it’s the only species that recruits ants. The base of each pitcher contains a swollen tendril that houses ants of the species Camponotus shcmitzi. These insects are permanent residents; they’ve never been seen in any other plant.
The Amazonian tree known Hirtella physophora looks rather unassuming, but it is the site of several grisly spectacles. Amid its leaves and branches, an animal, a plant and a fungus conspire to create a nightmarish trap where trespassers become meals, robbers get the death penalty, and assassins are assassinated.
The tree is home to ants called Allomerus decemarticulatus, which defend it from hungry insects. In return, the tree provides the ants with leaf pouches and swollen thorns as shelter, and feeds them with nectar and sugary nodules. These food sources are rich in carbohydrates but low in proteins. To supplement their diets, the ants need flesh, and they get it by shaping the tree into traps.
The ants cut hairs from the plant and weave them together into a hollow gallery, which extends down the side of the tree’s branches. Within the gallery, the ants hide inside small holes, jaws agape. From the outside, nothing can see them. If an insect lands on the trap, hundreds of lurking jaws seize its legs and pull it spread-eagled, as if on a medieval ‘torture rack’. The victim is overpowered and dismembered.
The Pheidole ants are an exceptionally diverse group with over 1,100 species. They’re also known as big-headed ants because their soldier caste has unusually large heads. Until now, we knew that a few of the Pheidole – just 8 out of 1,100 – can also produce supersoldiers, which are even larger than normal soldiers and have even more enormous heads. They use their outsized noggins to block their nest entrances against invading army ants.
Now, Ehab Abouheif has found that the supersoldiers are the result of a genetic programme that runs throughout the entire Pheidole dynasty. It’s likely that every single species in the vast group has the hidden ability to make this special caste. In fact, Abouheif managed to induce supersoldiers among species that don’t usually recruit them, with just a dab of hormone.
I wrote about this study for Nature, so head over there to read all the details. It’s a great evolutionary story.
Many insects are armed with venom, which they can inject into their enemies via a sting. The African ant Crematogaster striatula is no exception, but its arsenal has a disturbing twist – its venom goes airborne. The ant can raise its sting and release its toxins as an aerosol spray. Its targets are termites, whose nests it raids. Even without making any contact, the ants can induce seizures in the termites, eventually paralysing them.
All Crematogaster ants have a mobile sting. The sting sits on the ant’s rear-end, which connects to its torso by a flexible stalk, so the ant can aim it in virtually any direction. Aline Rifflet from the Jean-Francois Champollion University Center saw this ability in action when she watched C.striatula take on termites.
They came to America and found a nation overflowing with calories. Carbohydrate-rich fast food was available on every corner, and with little competition for it, the migrants ate their fill. Soon, they started spreading throughout this new land of opportunity. They are red imported fire ants (Solenopsis invicta) and their invasion is well underway.
The fire ant is an international pest. It devastates native ants, shorts out electrical equipment, damages crops, and inflicts painful stings. It hails from Argentina, but it was carried to the United States aboard cargo ships that docked at a port in Alabama. That was in the 1930s; since then, this invader has spread throughout the southern states, from California to Florida. The country spends over a billion dollars every year in attempts to stem the invasion.
Now, Shawn Wilder from Texas A&M University has found that their remarkable invasion has been driven by partnerships with local insects. The fire ants run a protection racket for aphids and other bugs, defending them from other attackers. In return, they get honeydew, a sweet nutritious liquid that the bugs excrete, after they suck the juices of plants. They are both farmers and bodyguards.
What happens when you dump 8,000 fire ants into a tray of water? Nathan Mlot from the Georgia Institute of Technology wanted to find out. Mlot scooped the ants into a beaker, swirled it around to roll them into a ball, and decanted them into a half-filled tray.
Over the next three minutes, the ball of ants slowly widened and flattened into a living, waterproof raft. By trapping air bubbles trapped among their interlocking bodies, the ants boosted their natural ability to repel water and kept themselves afloat. Humans build rafts by lashing together planks of wood or reeds; the fire ants do so by holding onto each other.
The experiment might seem odd, but it mirrors conditions that the fire ant (Solenopsis invicta) regularly has to cope with in its natural environment. The ant hails from the Brazilian rainforest floodplains of Argentina, where rising water regularly submerges their nests. They respond by weaving their own bodies into rafts. The ants also come together to construct bridges, ladders and walls, but the rafts are the longest-lasting of these living structures. In this form, they can float and sail for months.
It’s not a very fair fight. In one corner is a tiny ant. In the other is a large wasp, two hundred times heavier and capable of flying. If the two of them compete for the same piece of food, there ought to be no contest. But sometimes the wasp doesn’t even give the ant the honour of stepping into the ring. It picks up the smaller insect in its jaws, flies it to a distant site and drops it from a height, dazed but unharmed.
Julien Grangier and Philip Lester observed these ignominious defeats by pitting native New Zealand ants (Prolasius advenus) against the common wasp (Vespula vulgaris). The insects competed over open cans of tuna while the scientists filmed them.
Their videos revealed that ants would sometimes aggressively defend their food by rushing, biting and spraying them with acid. But typically, they were docile and tolerated the competing wasp. Generally, the wasp was similarly passive but on occasion, it picked up the offending ant and dropped it several centimetres away. In human terms, this would be like being catapulted half the length of a football field.
The wasps never tried to eat the ants, and they never left with one in their jaws. They just wanted them out of the picture. Indeed, the more ants on the food, the further away the wasps dropped them. This may seem like an odd strategy but at least half of the dropped ants never returned to the food. Perhaps they were physically disoriented from their impromptu flight, or perhaps they had lost the chemical trail. Either way, the wasps could feed with fewer chances of taking a faceful of acid.
Reference: Grangier and Lester. 2011. A novel interference behaviour: invasive wasps remove ants from resources and drop them from a height. Biology Letters http://dx.doi.org/10.1098/rsbl.2011.0165
The black crazy ant has conquered the world. From its native ranges in Asia and Africa, it has spread across six continents and lives in almost 120 nations. It is arguably the most widespread ant on the planet. It spreads readily for many reasons, but Morgan Pearcy from the University of Lausanne has found a new and important one –a sexual secret behind the crazy ant’s crazy success.
The black (or longhorn) crazy ant, so named for its distinctive, jerky movements, hitchhikes aboard human transport networks. Any vehicle from a jumbo jet to a cargo vessel can carry a queen to a new site, where she can set up a new colony. That’s fine for a few generations but after some time, you would expect the burgeoning invasion to run headlong into the problem of inbreeding. If all the ants in a new area are descended from a handful of queens, they could only mate with sisters and close cousins. That was bad news for European royalty, and it should be bad news for ant royalty.
But Pearcy has found that the crazy ant has a special way of passing on its genes that prevents it from suffering the problems of inbreeding. With this genetic trick, it can set up a strong population from just a few individuals. It’s an ant that’s pre-adapted for conquest.
Army ants have a reputation as destroyers. As they march through the jungle in battalions several thousand-strong, they supposedly kill all in their path. But this infamy is overblown. There’s no doubting their success as predators, but army ants also bring life wherever they march. They have an entourage of over 550 species that hang around their legions, of which 300 or so depend on the ants for their survival.
Carl and Marian Rettenmeyer spent much of their lives studying army ants. Carl passed away in 2009, but this year, Marian has completed the couple’s masterpiece – a comprehensive catalogue of the animals associated with a single species of army ant Eciton burchellii. Their record is incredible – a menagerie of animals running alongside the columns, tracking them by air, living within the nests and garbage dumps, and even riding on the ants themselves.