In the 1960s, San Francisco’s Fleischhacker Zoo posted a sign in front of the ape cage: “Caution, ape throws dirt.” “Dirt” turned out to be a euphemism, for the ape was throwing his own feces, of which a semi-circular splatter zone was visible with the ape at its center and the visitors at its perimeter, mostly just out of range. Tossing feces on people below is also an unwelcome habit of tropical American monkeys. These are thoughts that came to me when I discovered that aphids also throw their “feces,” but they do so for their health, not in hostility.
This situation arises for aphids because they subsist by sucking the sap directly from the phloem tissue of plants. As apparently abundant as this foodstuff appears to be, it is woefully low in nitrogen-containing compounds such as amino acids, and insanely high in sugars. This is because plants are built primarily of carbohydrates (think cellulose, a polymer of sugars) with little protein, whereas animals are built largely of protein (a polymer of amino acids of which there is precious little in plant sap). Aphids thus need to make their diet match their nutritional needs, namely, get plenty of amino acids.
So, what should a poor aphid do? How about getting rid of all that excess sugar and retaining the amino acids and other needed nutrients? Amazingly, aphids have evolved a digestive system that does just that. Here is how it works: the incoming sugary sap comes down the esophagus and into the anterior part of the midgut, which is intimately entwined in a chamber with the posterior section of the gut. Water and excess sugars are moved directly from this incoming stream into the posterior section of the midgut and the rectum, from which it is excreted as a sweet, sugary fluid called honeydew, i.e. aphid poop. The remaining incoming fluid, now enriched in amino acids, enters the midgut where it is digested and absorbed (see below).
The excretion of honeydew provides energy food for an astounding array of insects, most of whom lick the honeydew from contaminated surfaces. But the honeydew also creates a problem for the aphids, for it is sticky and viscous, threatening to gum up the aphid that excreted it. The honeydew on surfaces is soon colonized by black mold, a health threat to the aphids. Therefore, the aphids have to get rid of this crap.
Aphids (and many of their relatives) have solved this problem in several ways. First, by hiring ants to collect the honeydew before it becomes a problem. Ants, being adult insects, don’t grow and thus need little in the way of amino acids, etc. On the other hand, they need sugars to provide the energy for their high activity, and aphid poop is a great source. Some of the symbioses between hemiptera (e.g. aphids, mealybugs, scales etc.) and ants are so evolved and specific that the ants treat the hemiptera like a herd of cattle, sometimes building shelters (barns) for them and defending them against predators, parasites, and other ants. Some species of ants move their “cattle” from “pasture to pasture”, as necessary, and a few species are as totally dependent on their “cattle” as a dairy farmer in Wisconsin is dependent on his cows. One species of ants is so dependent on its mealybug “cattle” that queens on their mating flights carry one of the mealybugs in their mandibles to start their own dairy herd. Neither the ant nor the mealybug can survive without the other.
But this is not the only way that aphids have solved the honeydew problem. Many aphids toss the drops of honeydew into the void, perhaps kicking them with their hind legs. It is easy to show this because the honeydew is sticky, and the drops will bind graphite powder scattered on an acetate film, as below. The wide spatter pattern reminded me of the apes in the San Francisco Zoo.
With a bit more sophistication, it is possible to show that the droplets contain sugar by treating the paper with a reagent that reacts with sugars to produce colored spots. The outcome is shown below as both a positive and a negative image. Think about the distances these tiny creatures can throw drops of honeydew— well over fifty-fold their own size. Throwing crap is one of their top skills, and their health depends on doing it well.
Many species of aphids toss their honeydew, and their range is impressive even if they are not high up on a plant stem, well over 15 aphid-lengths. That’s a long way to toss “stuff,” and it certainly ends up where it can’t do the aphid any harm.
For animals that can move about, it is a simple matter to separate themselves from their excrement. They just walk away. But when you are an aphid with your proboscis stuck in a plant’s phloem, you have to come up with other ways of avoiding contact with your own excrement. If you can’t convince ants to do it, you make it into a sport, a sort of aphid shot-put event. And you get very good at it, tossing it 50 body-lengths with no sweat. To achieve a comparable record, a human tossing a 16 pound shot would have to toss it 300 feet, far more than the actual record of about 75 feet. Once again, scale is crucial to performance—- the larger the critter, the less impressive the relative performance. For visitors to the San Francisco zoo, it’s a good thing that the ape didn’t have the relative throwing capacity of an aphid.
Birds do it
Bees do it
Even aphids on their knees dew it…
Crap-tivating!