The membracid treehopper I posted recently is weird enough, but it is not the strangest insect I’ve seen this year. Those treehoppers evolved complex structures on their backs, sometimes imitating ants, showing that under some conditions this kind of imitation gives the treehopper a fitness advantage. Now I’ve found another insect with a fake insect on its back, but this time the insect itself builds the fake insect, out of its own droppings, and without being able to see what it is doing!
Evolution works with what’s available in small steps, and not all groups of insects have complex structures growing out of their backs that can later be molded by natural selection into the shape of a nasty insect. Herbivorous beetle larvae, for example, lack such structures. They’re just shaped like boring caterpillars (and often mistaken for them, though they have no extra “prolegs”, just the regular six legs of most insects) without much fancy ornamentation. Even if models of scary insects would be useful for these larvae, there is not enough random structural variation to get the evolutionary process started in that direction.
However, several groups of these beetle larvae have evolved an unusual defense that might provide material for further natural selection: they build a shield on their backs, made out of their own droppings, often mixed with their own shed skins. The poop-shield, usually called a “fecal shield” in the literature, is not only a physical protection but also a chemical one, which has been shown to repel some predatory insects. It can have considerable structural complexity, and lots of variability.
Some of the most elaborate fecal shields are made by the chrysomelid beetle larvae which turn into “tortoise beetles”. Some species make coils of rope-like structures made of poop, while others make flat solid shields. These insect sculptors use a prehensile anus called the “anal turret” to accurately place each piece of poop on the ever-growing structure. The structure is connected to their bodies by some mobile hooks that the insect can move, deploying the shield as needed, though the range of movement is limited.
These complex yet variable structures, like the pronotum of a treehopper, provide natural selection with raw material for more elaborate constructions. Some of these structures, by accident, might vaguely resemble scary insects. If there is a selective advantage for structures that resemble scary insects (in other words, if the benefits outweigh the cost, so that bearers of such structures leave more offspring than larvae whose structures do not resemble scary insects) then eventually the members of a species may all end up building models of scary insects on their backs, and the accuracy of such models will increase over time in the population.
I recently found the chrysomelid beetle species shown here, whose larvae appears to be following this route, building a crude statue of a scary insect or spider on its back. [Note added April 22: expert Caroline Chaboo, U of Kansas, confirms this belongs to the tribe Cassidini.] I found two of these larvae on a small cloud forest tree called “morochillo” (in the tomato family, Solanaceae). When the larvae were young, they made fairly boring “legless” shields not much different from the kinds that many other chrysomelid larvae construct. They would use their anal turret, which is a flexible sort of hose, to place their feces carefully on the shield. When they shed their skins, as all insects do, the head capsule and hollow spines and other skin debris would be added to the shield. I have no idea how they knew where to put the droppings and skin pieces, since their eyes can’t see the top of the shield, where all the interesting stuff is.
As the larvae got older, they began to build long artificial “legs” on their shields. The shields were not both equally convincing; one larva had more realistic “legs” than the other. Eventually one of them disappeared, while the other made a pupa on its leaf, with the shield still attached.
I took the pupa into my house so I could see the beetle that would emerge. Eventually it did emerge, and I was surprised to see that the adult beetle ALSO had a fake insect on its back! But this one was just a flat silhouette of a big fly, in black, on a transparent carapace.
The beetle was fascinating not only for its fake fly but also for its strange feet. Under a microscope they looked like janitors’ mops, with divided, flattened hairs dripping a clear liquid from their ends. This clear liquid stuck the flat hairs to any smooth surface. My beetle easily held tight to perfectly smooth glass with these feet. Its two long claws between the flattened hairs probably act as levers to unstick the hairs when the beetle wants to leave. This group of beetles is famous for its ability to stick to leaves; they can hold on to a leaf even against a force 60-100 times their own weight. For more detailed analyses of the remarkable mechanisms involved, see here and here.
A note on the photos: The photos of the live larvae were made using normal macro techniques. The photos of the live adult were stacked composites of a few photos in the case of the views from the top. The live adult on the camera filter, however, was made from a stack of several hundred photos. It didn’t sit perfectly still and I had to manually edit out the wandering antennae.
The high-magnification images of the feet were made immediately after the beetle died, and each consist of about seven hundred photos. I probably shot 5000 photos in total. It was hard!!! I used a 10x Mitutoyo microscope objective attached to various long-focal-length lenses on an old Nikon D90, moving on a StackShot rail. I get roughly similar results mounting the microscope objective directly on a Panasonic bridge camera, the FZ300, or on a Sony HX400V, and these can be managed by wireless smartphones with no need for a moving rail (the lens changes focus internally to image the different planes of the subject). In some cases, cross-polarized light was used to limit reflections. Small stacks were processed in Photoshop while those with hundreds of images were stacked with Zerene.