Ants are one of the most abundant groups on earth, but, curiously, not a lot of things eat them. Yes, there are anteaters (who also eat a lot of termites), and some lizards specialize on ants, but the little critters are full of noxious chemicals and pheromones that put them way down on the list of predators’ preferred foodstuffs.
Because of this, many other insects and arthropods have evolved to mimic ants, taking advantage of the aversion of predators to anything antlike. These mimics are called myrmecomorphs, and they’re the subject of a really nice eponymous feature in this week’s Current Biology (access is free, too). The name comes from the Greek “myrmecos”, for ant, and “morph” for form. The authors are Florian Maderspacher, the journal’s senior reviews editor, and Marcus Stensmyr, a researcher at the Max Planck Institute for Chemical Ecology at Jena.
I won’t summarize the text, which talks about the history of work on these beasts; you should read that for yourself. But I do want to show some of the amazing photographs of ant mimics.
When a perfectly edible species evolves to resemble a noxious one that is avoided by predators, thereby gaining protection from being eaten, it’s called Batesian mimicry, after the English naturalist and explorer Henry Walter Bates, who described the phenomenon.
We’ve read a lot lately about the amazing shapes of treehoppers (membracids). Here are some photographs of the treehopper Cyphonia clavata, whose helmet (pronotum) has evolved to resemble an ant.
The picture below shows the hopper with a sympatric (living in the same place) noxious ant, Cephalotes atratus.
As the authors note:
Notably, the ant-mimicking structure seems to be inverted, with the imitated head facing towards the back of the treehopper. That way, as the treehopper moves forward, it probably creates a rather good impression of a reversing and agitated ant in erect defensive posture, deterring any would-be predators. To complete the illusion, the terminal segments of the treehopper’s hindlegs, coloured like the ‘ant’, most likely serve as the ‘ant’s’ forelegs, which provides the static protrusion with the illusion of movement. Too bad our specimen was dead.
Of course, for this mimicry to evolve (and work), the noxious ant “model” and its edible mimic have to live in the same area, and be encountered by the same potential predators.
Some mimics imitate the ants only during part of the life cycle. Here’s a nymph of the Texas bow-legged bug (a true bug), Hyalymenus tarsatus (left) imitating an ant of the genus Ectatomma (right).
Some of the most remarkable cases of ant mimicry involve spiders. To pull off the trick, the spiders have to make their extra pair of legs look like antennae, put a constriction in their cephalothorax to resemble the separate head and thorax of ants, thin out their body, and, often, evolve fake eyespots to look like the large eyes of ants. Here’s a spider Sphecotypus niger (left) looking like the ant Pachychondela villosa (right), which the authors describe as a an “aggressive and predatory ant.” Note how the spider extends its first pair of legs forward to look like antennae:
There are several types of ant mimicry. Besides Batesian mimicry, we have “aggressive mimicry,” in which an animal will evolve to resemble another animal so that it can deceive it into thinking it’s one of its fellows, who then unwittingly allows it to approach. (There are other types of aggressive mimicry as well: some mantids imitate orchids, hanging from trees and waiting to eat the hapless insects who come to pollinate it.)
Here’s a remarkable case of aggressive ant mimicy. The animal on the left is actually the crab spider Aphantochilus rogersi, which resembles ants of the genus Cephalotes (right). It’s hard to tell them apart! The spider’s head is biting the ant’s neck, so the ant’s head is bent down. It’s a goner.
This type of mimicry implies that the “model” ants must have pretty good vision, for otherwise there would be no selection on the spider to resemble an ant so closely. And indeed, ants of this group do see pretty well: you can see that its eyes are quite large. From this you’d predict that mymecomorphs who prey on ants that don’t see very well might be less perfect mimics.
I love cases of mimicry, for they truly show the power of natural selection. The degree to which mimics resemble models—and it’s often spot-on—shows that there is lots of genetic variation in the model that can be used by natural selection, and that the selection is strong enough to affect many features of the mimic. It’s one of the few cases—sex ratio is another—in which biologists know a priori what the optimum result of selection should be, and how closely selection can achieve that target. As you see from the photos above, it comes damn close!
The power of selection acting on pervasive genetic variation is, of course, also responsible for the power of artificial selection, something that Darwin highlighted in The Origin:
“Breeders frequently speak of an animal’s organization as something plastic, which they can model almost as they please.”
I won’t use the word “spiritual” to describe my feelings when I see the remarkable forms that have resulted from blind, materialistic processes acting on DNA molecules, but they certainly evoke considerable wonder.