When you think of the Amazon Rainforest – what do you think of? Usually, the rainforest comes to us in images of color, biodiversity and an exoticness which echoes in our imaginations. We often think of fauna or birds – colorful macaws and mysterious jaguars.
But when one steps off the boat or plane, one might be surprised to find that the Amazonian ambassador is not the regal jaguar, but rather the treehopper. The little creepy crawlies that dresses to impress and though they may be little, a finer representative of biodiversity is not to be found.
To many, the treehoppers' entire appeal rests on their pronotal ornament (a plate-like modified structure covering the dorsal surface of the thorax). These appendages are bizarre, and their function is still being debated amongst biologists. However, what is known, is that some species use it as a form of physical protection (cage-like pronotum), others as a form of mimicry (ant/wasp-like), and still others as camouflage (thorns, spines, trichomes-like).
An ant approaches a treehopper with a bulbous pronotal ornament. The black and white coloration warns of potential toxicity whilst the hollow ornament may serve as either a shield or as a diversion to potential predators.
To stop and watch the rich lives of the treehoppers unfold is one of the many pleasures the rainforest has to offer and the more one looks, the greater the complexity of this small world - they engage in animal husbandry (ant farms), economies of scale (treehoppers and their young form large groups catered to by ants), commodity market dynamics (exchange of nutrients for protection) - in a word, mutualism.
While some species of treehopper remain solitary, many have formed a special relationship with a bodyguard that serves and protects their interests and is paid in a kind of gold - honeydew. Understanding where this gold-honeydew comes from and its importance is key to understanding the mutualism that has evolved.
An ant gently palpates and crawls over a teehopper providing the stimulus for a physical feedback which stimulates the release of honeydew.
The story begins at the bottom of the food chain, where we find the real workhorses of any ecosystem: the plants. The lifeblood of the plant – sap - holds all the nutrients and sugars necessary for survival and growth. It represents a bounty for those daring and evolutionarily ambitious enough to take advantage of it. But this is no easy task because the leaves and stems are often armed with mechanical (spines or trichomes) as well as chemical defenses (tannins, agglutinating proteins, etc. …). So, like the safe-cracker, the treehopper uses specialized tools for the job. It probes the surface of the plant before using its needle-like modified mouth, the proboscis, to break in and steal sugars. However, the sap is under pressure, and thus puncturing the phloem (which is responsible for nutrient transport) results in an uncontrolled, high-pressure stream. In order to deal with this, the insects must discharge huge volumes of the sap so that they don’t explode. Thus, they filter out the nutrients they need and extrude the rest in the form of a sugary exudate - honeydew, which can either be flicked off or else consumed more directly by attendant insects. And no organism is better poised to take advantage of this supply than the super-organism, the ant.
The sun sets, and still the ants are busy farming their honeydew. It's a 24hr job protection detail.
Over time, a mutually beneficial relationship has evolved, whereby ants, possessing defensive bites and stings, protect the treehoppers which have taken on the biological equivalent of living spigots. The ants will palpate them, move them from place to another to maximize their productivity, protect them from parasitization and predators, and all to ensure a continuous supply of their precious resource - honeydew.
So, next time you’re in the Amazon, stop a moment, lean in and smell the flowers. And you just might see that it’s not the jungle rose where sweetest lies, but rather the inconspicuous treehopper, the thorn that's not a thorn, whose sweetness lies.