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African Tuesday: Matabele ants of Gorongosa

Matabele ants (Pachycondyla analis) returning from a successful raid on a termite colony in Gorongosa. [Canon 7D, Canon 16-35mm with an extension tube Canon EF 12 II, diffused twin flash Canon MT-24EX]

Matabele ants (Pachycondyla analis) returning from a successful raid on a termite colony in Gorongosa. [Canon 7D, Canon 16-35mm with an extension tube Canon EF 12 II, diffused twin flash Canon MT-24EX]

Shortly after arriving in the Gorongosa National Park in Mozambique I witnessed a puzzling phenomenon: while exploring the network of roads in the woodland savannas of the park our local driver would barely slow down to avoid hitting antelopes and warthogs, but immediately slammed on the brakes if he noticed a long column of large black ants that were streaming from one side of the road to the other. We couldn’t quite get the exact explanation for his reluctance to drive over the insects (although we were very happy about it), but eventually gathered that driving over them could bring great misfortune. In most places on Earth killing a bunch of ants carries the moral equivalency of blinking, and one might wonder why in Mozambique people would show such respect for these insects. But having met these particular ants before I immediately understood that, like so many seemingly irrational cultural oddities, this one also had a very rational explanation.

I first encountered Matabele ants (Pachycondyla analis) a few years earlier in Africa, and it was a memorable experience. They are named after a particularly fierce tribe of Zulu warriors, and fully deserve this designation – a single ant delivers one of the most painful stings I have ever experienced, and just a few of them can put you out of business for most of the day. So, yes, crossing paths with Matabele ants will certainly bring great misfortune.

A column of Matabele ants streaming towards a termite mound [Canon 7D, Canon EF 14mm]

A column of Matabele ants streaming towards a termite mound [Canon 7D, Canon EF 14mm]

But other than their propensity for overreacting to being crushed under one’s foot, these insects are truly amazing creatures. Matabele ants are specialist termite hunters, and do so in a very sophisticated way. It all begins with a single scout leaving her underground nest and embarking on a mission of discovery. The scout wanders, seemingly aimlessly for about an hour in all directions, searching for a nest of termites of the genera Odontotermes and Microtermes. Often she finds nothing, and returns to the nest empty handed. But if she finds a termite mound her behavior changes immediately – she runs back along the shortest possible path back to the nest, leaving behind a trail of pheromones that will guide her nestmates to the source of prey. The pheromonal trail is produced by two glands in the ant’s abdomen: the pygidial gland, the secretion of which has a powerful recruitment effect on her nestmates, and the venom gland, which paints longer lasting orientation cues on the trail. Once back in the nest, it takes as little as 60 seconds to get the entire worker cast of the nest mobilized and streaming along the chemical path left by the scout towards the termite mound, which can be as far 100 m away (in human terms, it compares to walking to a grocery store located 10 miles away.)

Matabele ant worker carrying a pupa [Canon 7D, Canon MP-E 65mm, 3 speedlights Canon 580EXII]

Matabele ant worker carrying a pupa [Canon 7D, Canon MP-E 65mm, 3 speedlights Canon 580EXII]

Upon arrival at the termite mound, the ants pour in through an opening marked by the scout, and begin to slaughter the termites. Within minutes, several thousand termites are dead. Each Matabele ant then stuffs her mandibles with as many limp bodies as she can carry, and the entire column runs back to the nest. In most cases the ants suffer no casualties during the raid, and the entire colony acts almost as a single, large, predatory organism, equivalent in its impact on the termite colony to a small pangolin.

As I watched the column streaming in a tight formation towards the termite mound during one of many night raids that I encountered in Gorongosa, I could clearly hear the constant chatter of hundreds of workers. Bert Hölldobler and his colleagues (Hölldobler et al. 1994. J. Ins. Physiol. 40: 585-593) concluded that the sound made by the ants serves only as a warning to potential predators, and has no role as a cohesion signal to help keep the column together. But when I recorded these signals with an equipment of much greater sensitivity and frequency range than what was available to these scientists in 1994, I realized that the ants produced not one, but two types of sound, one of frequencies an order of magnitude higher than the warning signals typically heard in insects. One kind of sound was produced by rubbing together segments of the gaster, whereas the other was probably made by the ant’s mandibles. You can here the sound of the marching column here; a call of a single ant, slowed down 10 times, can be heard here – notice the knocking sound made by the mandibles, and the scraping sound made with the gaster.
In the light of the recent discoveries of acoustic communication in ants, I think that I will revisit and test in Gorongosa the idea that Matabele ants use sound to locate the column if a worker becomes separated from the rest of her nestmates. Ants may have evolved a sophisticated language of chemical compounds but when you need to shout nothing beats the good old sound waves.

A budding myrmecologist Tonga Torcida watching a returning raid of Matabele ants (you can see Tonga assisting E.O. Wilson with ant research in Gorongosa in the new, fantastic BBC documentary "Africa: The Future") [Canon 7D, Canon EF 14mm, diffused twin flash Canon MT-24EX + speedlight 580EX II]

Budding myrmecologist Tonga Torcida watching a returning raid of Matabele ants (you can see Tonga assisting E.O. Wilson with ant research in Gorongosa in the new, fantastic BBC documentary “Africa”) [Canon 7D, Canon EF 14mm, diffused twin flash Canon MT-24EX + speedlight 580EX II]

Tasty silk weavers

Australian green weaver ants (Oecophylla smaragdina) guarding a lycaenid caterpillar, which repays this service with honeydew rich in amino acids. [Canon 1D MkII, Canon MP-E 65mm macro, Canon MT-24EX twin light]

I gently squeezed the little green bulb between my teeth, and a lemony flavor flooded my mouth. I savored it for a second – Hmm, not too bad, I can totally see myself adding it to rice or some other bland food.
I shook off my arm the remainder of the insects who were aggressively intent on avenging the untimely death of their sister. I was in the Northern Territory, giddily soaking in myriad of natural history facts about Australia. These tasty ants, which feature prominently in the aboriginal cuisine, were definitely one of the highlights.

African weaver ant workers pulling leaves closer together, while others bind them with larval silk. [Canon 10D, Canon MP-E 65mm macro, Canon MT-24EX twin light]

Weaver ants are exceptional, even by the standards of a group of organisms that rival humans in the complexity of their social interactions and sophistication of their engineering. Only two species of the genus Oecophylla are known: in addition to the Australasian green weaver ants (O. smaragdina), the African weaver ant (O. longinoda) is found throughout most of sub-Saharan Africa. Both live in large arboreal colonies, and both exhibit an interesting behavior that has earned them the distinction of being silk weavers.

A worker of African weaver ant (O. longinoda) using a larva as a tube of glue to stitch together leaves (Guinea). [Canon 10D, Canon MP-E 65mm macro, Canon MT-24EX twin light]

When we think of silk, the first thing that comes to mind is of course underwear. The second is silkworms, and the ancient Chinese who first employed these insects to produce sophisticated garments. But weaver ants can also produce silk, yet, rather than spinning protective cocoons like moths, they use it in a way that is remarkably similar to the way we use crazy glue.
Being arboreal animals, weaver ants cannot build expansive underground galleries to protect their brood and queen. Instead, they pull together leaves and stitch them with sticky silk threads produced by their larvae – since adult workers do not have silk glands, some carry small larvae like tubes of glue, and squeeze them gently as they move back and forth between edges of two leaves. The leaves are temporarily held together by other members of the colony, often hundreds of them. The resulting arboreal nest is usually about the size of a football, and holds everything an ant colony needs – the queen’s chamber, a nursery for the larvae, and a larder full of prey.

African weaver ant (O. longinoda) from Mozambique. [Canon 7D ,Canon MP-E 65mm, 3 speedlights 580EXII]

Weaver ants are voracious hunters – I have seen them attacking grasshoppers, beetles, snails, even snakes, and they are not opposed to partaking of carrion. I have also felt them attacking me, and this is one of the reasons why these ants are not popular among gardeners in Australia – having them rain on you from a tree and spray formic acid on your skin is not one of the most pleasant experiences. But at the same time weaver ants can be quite beneficial. It has been shown that their presence increases fruit production of some plants, and reduces the amount of pesticides needed to control pests. Unfortunately, like most ants, they have a weakness for sugar, and will protect aphids and scale insects that cause damage to plants. In the end, their impact on agriculture is probably about neutral.

An ant-mimicking katydid Polichne sp. resembles a green weaver ant worker. [Canon 1D MkII, Canon MP-E 65mm macro, Canon MT-24EX twin light]

Some insects take advantage of the aggressive nature of weaver ants and, by assuming a similar appearance and hanging around their nests, gain protection from predators who dare not to get close to the ants. Katydids of the Australian genus Polichneare some of them, and it is still unclear how they manage to convince the ants not to eat them (chemical mimicry is the most likely explanation.)

African weaver ants carrying a live snail to their nest (Mozambique). [Canon 7D, Canon 14mm, Canon MT-24EX twin light]

Since my initial encounter in Australia, weaver ants have been my favorite social insects, both off and on the plate. I have had them with rice and sweet cakes, and they always add to the experience. Sometimes ants on your picnic table are a good thing.

African weaver ants carrying a live snail to their nest (Mozambique). [Canon 7D, Canon 100mm, Canon MT-24EX twin light]

A record breaker in my neighborhood

Eastern ant cricket (Myrmecophius pergandei) in the nest of the wood ant (Formica ?fusca) in Woburn, MA [Canon 7D, Canon MP-E 65mm, 3 speedlights 580EXII]

When it comes to finding the world’s most [insert adjective] insect, one usually needs to go to some remote, usually tropical location. But it just happens that the world’s smallest cricket lives right around the corner from my house.

The Eastern ant cricket (Myrmecophilus pergandei) is a remarkable and highly adaptable little creature. These insects are completely wingless and, unlike other crickets, do not produce any sounds; in fact, they don’t even have ears, which indicates the loss of sound production early in their evolutionary history. (And thus, do not believe what the Internet tells you – you cannot find them “by listing to the quiet ‘tweeting’ of the ant piles.”) Their size ranges from about 1.7 to 5 mm, making them the smallest members of the order Orthoptera. Ant crickets show a remarkable size polymorphism, and individuals of the same species can vary wildly in their body length, depending on the size of their ant hosts.

Female of Eastern ant cricket (Myrmecophius pergandei) (note the lack of wings and highly reduced eyes) [Canon 7D, Canon MP-E 65mm, 3 speedlights 580EXII]

As their name indicates, these crickets are obligate inquilines of ants i.e., you will not find them outside of ant colonies, and they probably would not be able to survive in the open for very long. The Eastern ant cricket is not particularly host specific, and has been found in association with about a dozen of ant species; recently it has been recorded even from the nests of the invasive fire ant (Solenopsis invicta x richteri.) They are able to move among different ant colonies thanks to their ability to mask themselves by covering their bodies with cuticular hydrocarbons of their hosts, which the ants use as chemical signatures to tell apart their nest mates from enemies. But how do crickets acquire these compounds?

It turns out that upon entering a new colony the crickets must literally steal the hydrocarbons from the ants. They do so by sneaking up on their hosts and quickly rubbing their bodies against those of the ants. They have to be very careful though, because at this point they are still seen as intruders, and will be quickly killed if caught. This is when their huge jumping legs and incredible agility come in handy. They also scrape residual hydrocarbons from the walls of the colony and cover their bodies with it. Once properly camouflaged, the crickets are less likely to be molested by their hosts, and will even attempt to elicit feeding from ants by palpating the ants’ mouthparts. Interestingly, not all species of ant crickets are as flexible in their host choice as the Eastern ant cricket: some, like the Japanese M. albicinctus, require specific hosts and will perish if transferred to a different species’ colony.

Two species of crickets I found together in the wood ants’ colony in Woburn, MA (to scale) [Canon 7D, Canon MP-E 65mm, 3 speedlights 580EXII]

A few days ago I went to a small wooded area near my house in eastern Massachusetts to look for ant crickets. I flipped a bunch of rocks, and eventually located a few individuals in the nests of two species of Formica. But I was surprised to discover that the ant crickets were not the only jumping guests of the ants. Another species of cricket, the Carolina cricket (Eunemobius carolinus), was present in nearly every ant colony that I looked at, and not only those of Formica, but also Aphaenogaster and Tapinoma. Carolina crickets are normally free-living insect, very common in my garden. I wonder if we might be witnessing the first stage of the process of a species becoming an ant symbiont, and if so, whether they use a similar chemical deception as that used by the ant crickets.