Rainforest Ants 4 – The Bodyguards, Part 2: Azteca ants protecting Cecropia trees

Last week we met fierce Pseudomyrmex ants protecting their tropical myrmecophyte home, the Acacia tree. Myrmecophytes are plants that form a symbiotic relationship with feisty ants which serve as bodyguards. Another group of trees common to tropical forests are Cecropias, a collection of several dozen species many of which are pioneer trees found in disturbed areas along roadways, riverbanks, or light gaps bordering dense tropical forests. Eons ago many species of Cecropia formed partnerships with ants in the genus Azteca. This mutualistic “deal” has two parts. Cecropia trees provide several sources of food including glycogen (a carbohydrate storage molecule), proteins, and lipids in structures called Müllerian bodies, found at the base of a leaf’s petiole, and in Food Bodies scattered on the petioles and blades of young leaves.

Undisturbed, Azteca ants patrol their host at a leisurely pace, but with a shake of a leaf workers swarm from within the stem to defend their colony. I could not resist the chance to assess the potency of Cecropia’s defenders. Compared to last week’s attack by ants guarding Acacia, the sting of Azteca was mild. However, to a caterpillar or beetle attempting to eat leaves of Cecropia, a mass attack by dozens of tiny stinging bodyguards might convince an intruder to seek a meal elsewhere.

Cecropia also satisfies the “gimme shelter” requirement for Azteca ants. The hollow stems of Cecropia are the perfect redoubt to house the queen and raise the brood. And what does Cecropia get from the “deal?” Protection. Clever studies demonstrated that Cecropia devoid of their Azteca bodyguards received more damage from marauding chewing herbivores such as beetles and caterpillars. An additional benefit of ant occupation was the removal of aggressive vines capable of smothering young Cecropia trees. The net effect of ant bodyguards revealed in these studies was more rapid growth in trees protected by tiny but fierce Azteca defenders. For a pioneer species like Cecropia, any advantage you can get on your competitors is an important one and Azteca ant bodyguards may provide just the advantage you need to survive in the wild tropical forest.

On the surface of the trunk, small holes allow Azteca workers to enter an exit the colony. Within the hollow stem, workers tend hundreds of eggs and larvae produced by the queen.

Acknowledgements

We thank Costa Rica Vacations, and the intrepid guides Mono and Kenneth at Rafiki Lodge and Ale and Gera at Playa Cativo Lodge, for providing the inspiration for this episode. The wonderful books “The Insect Societies” by Edward O. Wilson, “The Ants” by Bert Hölldobler and Edward O. Wilson, and the fascinating articles “Azteca protection of Cecropia: ant occupation benefits juvenile trees” by Eugene W. Sehupp and “Food bodies of Cecropia pachystachya (Cecropiaceae) leaves: structural and functional features suggesting complementary role to Müllerian bodies” by Patricia Gonçalves-Souza and Elder Antonio Sousa Paiva, were used as references for this episode.

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Adorning leaves are nutrient rich, orange Beltian bodies. Acacia ants harvest these morsels to sustain the queen and colony.

No, this week’s episode is not a revival of the classic Costner- meets-Houston romance, but it is a story about a different type of symbiotic relationship as curious as a previous episode where we met leafcutter ants and their fungus gardens. This week we explore the fascinating relationship between tropical myrmecophytes, plants that partner with ants, and the feisty ants which serve as their bodyguards. Whether it’s in the rainforest of Belize or those in Costa Rica, one common sight at the forest’s edge is a brilliant green acacia tree. At one such site a local guide noted that this remarkable tree was completely unmolested by any type of leaf-munching caterpillar, sucking insect, or large mammal such as the horses or cows that grazed in a pasture nearby. A closer inspection revealed fearsome looking thorns arising from nodes of the branches. Surely these thorns, locally known as cockspurs or bullhorns, helped explain why large grazing mammals avoided the otherwise delectable looking leaves of the acacia. A mouthful of thorns would be a painful experience indeed. However, as I fondled the foliage of the acacia, I was instantly attacked by a furious band of ants that bit and stung my hand with extreme prejudice. Their sting was memorable and my swollen hand throbbed and itched for hours after the encounter. 

Under normal conditions acacia ants patrol leaves and stems at a languid pace. But the pace quickens as workers begin their search-and-attack mode after I rustle the acacia’s branches. Along the trunk, a frenzied mob seeks the source of the disturbance. A momentary touch of a branch is long enough for the bodyguards to attack the nosy human and deliver fierce bites and stings.

Small holes in the thorn allow ants to enter and exit.

The secret weapons of the bullhorn acacia are ant bodyguards, part of a symbiotic deal struck eons ago by acacias and ants in the genus Pseudomyrmex. The deal works like this: A newly mated Pseudomyrmex queen lands on the bullhorn acacia and locates a large thorn. Either by chewing a new hole or by using an existing one, she enters the hollow thorn and lays eggs. Eggs hatch and develop into sterile workers. Workers, the queen, and subsequent broods subsist on carbohydrate rich nectar produced by specialized glands called extrafloral nectaries found near the bases of many leaves. But ants, like humans, cannot live by sugar alone. At the tips of some leaves, specialized structures called Beltian bodies form. These detachable tidbits are rich in nutrients including proteins and lipids. Ants harvest and consume Beltian bodies to round out their diet. By providing room and board, acacia plays the generous host for Pseudomyrmex. In return, fearless worker ants provide maniacal protection of the acacia from caterpillars, sap-sucking insects, probably large herbivores, and, certainly, nosey entomologists.

Nectaries at the base of the leaf’s petiole provide a rich source of energy for busy ants.

One additional benefit of the bodyguards is their role as vegetation managers. In addition to being devoid of leaf-eating insects, choking vines that ascended and engulfed other rainforest plants nearby were notably missing from the acacia. Why were so many other plants cloaked in vegetation while the acacia remained free of clinging vines? In a clever series of studies, rainforest guru Dan Janzen demonstrated that in addition to protecting acacias from herbivores, Pseudomyrmex also aggressively remove tender tips of encroaching plants that might compete with acacia for sunlight. Room and board in exchange for protection from herbivores and competing plants, all deals should be so good!

Acknowledgements

We thank the hearty crew of ‘BSCI 339M, Mayan Culture and the Interface Between Tropical Rainforests and Coral Reefs, Costa Rica Vacations, and the intrepid guides Mono and Kenneth at Rafiki Lodge and Ale and Gera at Playa Cativo Lodge, for providing the inspiration for this episode.  The wonderful book “The Insect Societies” by Edward O. Wilson and the fascinating articles “Interaction of the bull’s-horn acacia (Acacia cornigera L.) with an ant inhabitant (Pseudomyrmex ferruginea F. Smith) in Eastern Mexico” and “Coevolution of mutualism between ants and acacias in Central America” by Dan Janzen were used as references for this episode.

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Ferocious soldiers protect the flank of the raiding column of workers.

Giant jaws deliver lethal bites to predators and teach a lesson to a nosy bug geek.

Last week we met the consummate farmers of the rainforest, leafcutter ants. Once again, we head back to Costa Rica to warm up and visit an insect that rocks in at 9.5 out of 10 on the ferocity scale – army ants, arguably the most rapacious insect predator in the jungle realm. While walking along a forest edge near the Savegre River, my path was blocked by a streaming column of furious army ants. This bustling brigade was only a small portion of a pillaging horde hunting food in one small corner of the forest. Single colonies of army ants contain hundreds of thousands to more than a million workers capable of capturing and eating thousands of assorted arthropods each day. The column consisted of large and small workers busily transporting food to a temporary food cache or colony site called a bivouac. In lesser numbers within and alongside the column were imposing soldiers. These grotesque giants sported huge, sickle-shaped jaws used to defend workers and colony from attack. The jaws of the soldiers are so large and highly modified for grappling and pinching that they are of little use for eating. In a reenactment of a scene in Mel Gibson’s “Apocalypto”, I tested the ability of a soldier’s jaws to act as a wound suture. They worked just fine. 

At the raiding end of the ant column, a chaotic melee of murder and mayhem ensued as swarms of stinging and biting workers capture other arthropods, primarily insects and spiders. Other unfortunate small animals that fail to escape the approaching horde may also succumb. After subduing victims, workers dismember their prey and transport them in large and small pieces back to a food cache or bivouac to feed developing larval ants, their attendants, and the hungry queen. The bivouac is usually in a protected location beneath a log or between the buttress roots of a large tree. It is formed by thousands of ants linked leg to leg in a protective living cover for the queen and young. However, army ants may set up bivouacs in man-made structures. I have witnessed this event firsthand on a recent study abroad trip to Belize where army ants raided a research station and established a bivouac in an outhouse. This provided quite a surprise when a sleepy student undertook a nocturnal visit to the privy.

Watch and learn why army ants are among the most awesome predators of tropical rainforests.

A night time trip to the outhouse can be especially exciting when army ants set up a bivouac inside.

The life of army ant colonies is characterized by two distinct phases. When the colony is in the nomadic phase, workers hunt by day and bring food back to the colony, but they stop carrying food to the bivouac as night approaches. Bivouacs are relocated periodically when excited workers transport food and ant larvae away from an old bivouac to a new one along one of the outward leading trails. As the old bivouac disintegrates, the queen and her entourage follow a chemical trail through the forest and establish a new bivouac at a different location. The regular relocation of the bivouac in the nomadic phase enables legions of workers to pillage untapped areas of the forest for food each day. Several times a year, the colony enters a stationary phase. During this phase, the colony hunkers down in one location for several weeks. Larvae begin to pupate and the queen lays as many as 30,000 eggs each day. During the stationary phase, raids continue but are less frequent and intense. In a fascinating study, Nigel Franks discovered that during the stationary phase, army ants changed direction of successive foraging bouts. The direction of each new foray differed from the previous one by approximately 123 degrees in a clockwise direction. By searching different quadrants of the habitat sequentially, ants avoid hunting in the same area twice. Foraging in different areas may also allow new victims to repopulate recently searched areas. Over the span of a few weeks, thousands of eggs hatch and hungry young larvae place enormous demands for food on the colony. By now, ants produced during the previous stationary phase have completed development and matured into new workers. With thousands of new workers to forage and the demand for food high, the colony resumes its nomadic phase and it’s time for many small insects and other animals to run for their lives or die.

Acknowledgements

Bug of the Week thanks Costa Rica Vacations and the intrepid guides Mono and Kenneth at Rafiki Lodge and Ale and Gera at Playa Cativo Lodge for providing the inspiration for this episode. The wonderful book “The Ants” by Bert Hölldobler and Edward O. Wilson, “The Insect Societies” by Edward O. Wilson, and the interesting article “Army Ants: A Collective Intelligence” by Nigel Franks, were used as references for this episode.

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Leaves and buds of Brazilian fire tree are transported from the treetop to the colony by major workers.

Last week we traveled to Costa Rica and met a captivating caterpillar in the tropical rainforest. Over the next several weeks, we will visit several species of ants, so called superorganisms of the tropical rainforest, creatures with social structure and specialized roles that, as individuals, cooperate and contribute to the success and survival of their colony. It has been said that on planet earth only two creatures cultivate crops: humans and ants. So, let’s return to the tropical forests of Costa Rica to visit these farmers who are the most important group of herbivores found in tropical forests in the New World, leafcutter ants.

Leaves of a small shrub are disassembled by powerful jaws of major workers. Figuring out just the right way to carry the leaf is tricky business, but once lifted overhead, it’s off to the races and back to the colony.

Night and day members of the worker caste search for nutritious leaves on trees, vines, and shrubs. When scouts find a suitable food source, they direct other workers to the bounty by releasing trail-marking chemicals called pheromones. The amazing jaws of major workers clip small sections of leaves and flowers and carry them to the ground, where they join a rambunctious procession of nest mates. In this parade, intermediate sized workers busily transport leaf sections while smaller workers sometimes hitchhike on leaves and help defend their sisters from marauding predators and parasitic flies. Nearby, large imposing soldiers also defend their sisters and the colony with powerful jaws. As leafcutters remove foliage from a tree, a parade of ants may extend for distances of more than 200 yards as workers carry leafy cargo back to a subterranean nest.

On a nearby tree, flowers and flower buds travel to the ground. Transporting flowers seems relatively easy, but carrying a bud seems to be a challenge. Watch a worker as she finally figures out how to haul a flower bud. Amidst the hustle and bustle of the ant trail, blossoms disappear underground to fuel the nutritious fungus garden.

Ventilation shafts cool the underground ant colony and provide for the exchange of gasses.

A leafcutter nest is a marvelous structure that may contain several million ants and occupy 600 square meters of forest floor. Sophisticated ventilation systems cool the bustling nest and allow carbon dioxide to escape while drawing in oxygen. Once inside the nest, leaves are delivered to other workers that take the leaf sections and clip them into ever smaller fragments. These fragments are carefully inserted into a garden of living fungus maintained by the ants. Leaves serve as a substrate for fungi, which is harvested as the source of food for the entire ant colony. The fungus garden is meticulously tended by workers. Destructive alien fungi are detected and removed. Secretions produced by the queen and workers facilitate the growth of the cultivated fungus. Fungal strands produce specialized structures called gongylidia. Gongylidia are fed to the developing larvae and distributed throughout the colony to feed workers and the queen. Due to their agrarian life style, leafcutter ants are also commonly called fungus growing ants.

Sharp jaws of the major worker are used not only for cutting leaves but also for defending the colony from vertebrate predators and foolish bug geeks.

Leafcutters don’t leave much behind when defoliating favored plants.

To support their enormous colonies, leafcutters remove vast amounts of vegetation each day. It is estimated that large colonies may remove more than 500 dry weight pounds of vegetation annually. When nests are established near orchards or crops, leafcutters can strip trees and vegetables overnight, causing significant crop loss. Often, irate farmers destroy leafcutter colonies. One humorous account related by Hölldobler and Wilson of a westerner’s attempt to grow a European style vegetable garden in Belize reported that the gardener “… arose one morning and found our garden defoliated: every cabbage leaf was stripped…of the carrots nothing was seen…into a hole in the mound, ants, moving in quickened step, were carrying bits of our cabbage, tops of carrots, the beans – in fact our entire garden was going down that hole.” However, leafcutter ants play a vital role in recycling plant material and enriching and cultivating tropical soils. For millennia in tropical jungles throughout the New World, legions of leafcutters have been the consummate farmers in the rainforest.

Acknowledgements

Bug of the Week thanks Costa Rica Vacations and the intrepid guides Mono and Kenneth at Rafiki Lodge and Ale at Playa Cativo Lodge for providing the inspiration for this episode. The wonderful book “The Ants” by Bert Hölldobler and Edward O. Wilson was used as a reference.

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Silk moth caterpillars like this Automeris species are spectacular denizens of the rainforest.

With the return of warm weather, leaves, and insects still weeks away here in the DMV, it’s time to travel to tropical rainforests in Costa Rica to see what’s up with some of our southern neighbors. First stop is the rainforest near the village of Santo Domingo bordering the Savegre River in Costa Rica. Scrambling across the ground near the base of a tree was a magnificent caterpillar. At first glance the identity of this beauty had me stumped, but after picking it up fiery stings to my finger and palm refreshed my memory of its true identity. This caterpillar is a member of the silk moth clan in the genus Automeris. The remarkable color pattern of this extraordinary larva leads me to believe its identity is Automeris metzli, a creature found from Mexico to Ecuador and also on the island of Trinidad, where it munches leaves of oak and less commonly Erythrinaand coconut.

Don’t let these amazing spines and beautiful colors fool you. Handling this lovely caterpillar could result in a spicy and memorable surprise.

False eyespots on the hind wing may help beautiful male (top) and female (bottom) Automeris moths gain protection from hungry predators. These two are Io moths, Automeris io.

You might think that a very large (this one was several inches long) tasty caterpillar would attract the attention of hungry predators. But Automeris caterpillars have a clever defense. Lining their sides and backs are spines loaded with venom. These spikey armaments are called urticating spines. Upon contact by a predator or overcurious human these spines release venom, causing a painful and relatively long lasting sting. For most people this sting resolves without complication but for some it may cause a serious allergic reaction. For me, well, getting up close and personal with this creature was worth some minor discomfort. Urticating spines are employed for defense by several families of moths, including flannel moths and saddleback caterpillars we met in previous episodes. While stinging shock and awe are the defensive syndrome employed by these caterpillars, adult moths use a different strategy. When resting on vegetation or on the ground, the brownish dappled forewings of the moth help it blend with background vegetation, effectively camouflaging the moth. If a predator draws too near, the moth spreads its forewings revealing large vertebrate eyespots on the hindwings. This ruse is thought to startle and frighten would-be predators, allowing the moth to escape or break-off the attack. False eyespots have evolved many times in the insect world and are found on caterpillars including swallowtail caterpillars, Promethea moths, and owl butterflies we met in previous episodes.

A cousin of Automeris metzli called the Io moth is relatively common in much of North America. including the DMV. This beautiful species of moth was once very abundant from New England to the Gulf States and west to the Great Plains. However, in New England and throughout much of its range, Io moths have declined. In addition to loss of natural habitat, the introduction of the parasitic fly Compsilura concinnata to control gypsy moth caterpillars has been implicated in local and regional declines in populations of Io moths and several other moth species attacked and killed by the fly. Habitat destruction and invasive species are but two threats to these charismatic and beautiful creatures throughout their ranges.

Acknowledgements

Bug of the Week thanks Hugh and Bridgette for discovering the gorgeous caterpillar, Costa Rica Vacations, and the intrepid guides Mono and Kenneth at Rafiki Lodge, for providing the inspiration for this episode.  “Automeris metzli Sallé (Lepidoptera: Saturniidae) in Trinidad, West Indies” by Matthew J.W. Cock, “Moth decline in the Northeastern United States” by David Wagner, and “Insect Defenses” by T. Eisner, M. Eisner, and M. Siegler, were used as references for this episode.

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On hot days with brilliant sunshine a dragonfly may point its abdomen upward to reduce the surface area exposed to the sun.

This week we continue our adventures in the aquatic realm where we recently visited whirligig beetles and water striders. Now let’s meet some tiny dragons. Dragons are mythological beings common to many cultures. These fanciful and fearsome predators are often chimeras of several creatures with wings of a bat, head of a reptile, scales of a fish, feet of an eagle, or tail of a serpent. With enormous compound eyes, reticulate wings, long legs, and a snaky tail, dragon seems a fitting name for the insects we call dragonflies.

Dragonflies belong to a group of flying insects called the Anisoptera. The name Anisoptera comes from the Latin roots aniso- (meaning unequal) and –ptera (referring to wing). Anisoptera is a reference to the fact that the forewings of dragonflies are not nearly as wide as the hindwings. Dragonflies are an ancient clan. Those patrolling the skies 250 million years ago had wingspans of two and a half feet. Modern dragonflies are smaller but no less magnificent. They are active predators hunting and capturing prey while on the wing. Spiny legs held beneath the body in a basket-like arrangement trap victims. Small insects such as mosquitoes or crane flies are common snacks, but larger insects including bees, butterflies, and other dragonflies or damselflies may be captured. There are reports of large dragonflies catching hummingbirds.

On a chilly day in February it’s nice to think about a warm summer day when a dragonfly might choose a dragonfly ornament to perch upon amongst zinnias in my flowerbed. But dragonflies are fierce predators. Watch as a dragonfly devours a damsel in extreme distress – damselfly that is.

An empty shed skin attached to a stem is all that remains of a dragonfly’s life underwater.

Male dragonflies often patrol specific territories along ponds or streams on the lookout for food, potential mates, or other males. Other males entering a territory are pursued and driven away. Females are wooed and mating pairs of dragonflies are often seen flying in tandem. The male sometimes guards his mate while she deposits eggs in the water or on aquatic vegetation. Eggs hatch into fascinating submariners called nymphs. These beautiful swimmers live in ponds or streams obtaining oxygen from the water through gills found inside their anus. Muscular contractions allow the nymph to pump water in and out of its rear-end to breath. How curious!

Dragonfly nymphs capture many kinds of insects such as the larvae of aquatic beetles, midges, and mosquitoes. Yes, dragonfly nymphs are important predators that help hold mosquitoes at bay. Crustaceans, worms, tadpoles, and even small fish are fair game for these stealthy hunters. Dragonfly nymphs sit and wait for a potential meal to come near. When in range, a remarkable, hinged mouthpart snaps forward like a bullfrog’s tongue to ensnare the victim. Nymphs usually move about by crawling slowly. However, when startled or under attack, they expel a blast of water from their rear ends and are propelled forward like a jet. After molting several times and completing development, a nymph will climb out of the water and attach itself to a plant or stone. The nymphal skin splits and the adult dragonfly emerges. Once the exoskeleton has hardened, the daring aerialist will take wing to hunt, mate, and entertain lucky humans on a warm summer day.

The insect on the left with gills on its tail is a damselfly nymph. The insect more centered is a dragonfly nymph. Watch mosquito larvae disappear into the maw of the dragonfly nymph, captured by the lightning strike of the nymph’s hook-bearing mouth-parts. Clips at normal speed have been slowed by 85% to observe prey captures.

Acknowledgements  

We thank Bill Lamp for providing the gorgeous dragonfly nymphs featured in this bug of the Week. Information was gleaned from “An Introduction to the Study of Insects” by D.J. Borer, D.M. De Long, and C.A. Triplehorn, “Beginner’s Guide to Dragonflies” by B. Nikula, J. Sones, D. Stokes and L. Stokes, and “Eco-Friendly Control of Mosquito Larvae by Brachytron pratense Nymph” by S.N. Chatterjee, A. Ghosh, and G. Chandra. 

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