Here’s a weird case of mimicry—or potential mimicry—involving an onycophoran and a caterpillar in Ecuador. It’s in American Entomologist, and is free (click on screenshot below, or see the pdf here).
First, in case you don’t know what an onycophoran is, it’s a group of 177 known species that occupy a single phylum, the Onycophora, which means “claw bearer.” Also called “velvet worms,” they are weird-looking, like worms with claw-bearing legs on a segmented body, big antennae and small eyes. They look like this:
And they have a bizarre behavior, described in the paper like this:
Although closely related to the arthropods, onychophorans are so unusual in their form, differing significantly from all other extant animals, that the taxon has been ranked at the level of phylum (Nielsen 2012, Grimaldi and Engel 2005). The Onychophora is unique due to the possession, by all species, of an elastohydrodynamic squirt-system in the mouth region. Remarkable structures, including glands that open on oral papillae, constitute a mechanism that enables onychophorans to squirt oscillating jets of proteinaceous liquid “slime” using slow muscular contraction (Concha et al. 2015). The slime rapidly fires from each papilla on either side of the mouth, intersects, and forms a disordered net (Morera-Brenes and Monge-Nájera 2010, Concha et al. 2015). The slime solidifies on contact with air and entangles small invertebrate animals; prey items or potential predators are quickly immobilized. Onychophorans then use their large-toothed, sclerotized jaws (Mayer et al. 2015a) to penetrate the victim, introduce saliva containing digestive enzymes, and partially digest the soft tissue prior to ingestion by sucking. Prey tissue may also be sliced up by the jaws and then swallowed (Lawrence 1953, Newlands and Ruhberg 1979, Hamer et al. 1997).
Here’s a National Geographic video showing an onycophoran sliming and then nomming a beetle:
Their appearance once made people think that onycophorans were a “missing link” (or rather something close to the common ancestor) between arthropods and annelids (segmented worms), and they were described as “living fossils”. But now they’re thought to be a sister group to arthropods + tardigrades, and not that closely related to annelids. Here’s their current phylogenetic position.
The new paper describes what may be a case of Batesian mimicy between a caterpillar and an onycophoran. The hypothesis is that a caterpillar which is tasty and preyed upon (birds, small mammals, reptiles, etc.) has evolved by natural selection to physically resemble an onycophoran, which is presumably avoided by predators because it has that nasty slime as well as teeth. If the predator has learned (or evolved) to avoid eating onycophorans, it may also avoid eating caterpillars who resemble them, thus giving a selective advantage to any caterpillar that closely resembles an onycophoran and thus can fool predators.
The authors found both onycophorans and caterpillars that resemble them in a collection of vegetation from trees in Ecuador (onycophorans are susceptible to desiccation and thus live in humid places in the tropics and wet subtropics). Here’s one of the tree-dwelling onycophorans and the presumably mimetic caterpillar. The paper’s caption is below, and be sure to watch the video at the link (I think they got “a” and “b” reversed in the figure caption, so I switched them).
Posterior view of the head of the caterpillar (B) found together with an onychophoran (A) in a sample of arboreal bryosphere. Note that the surface texture of the tubercles on the head of the caterpillar resembles the surface papillae of the onychophoran (see supplemental video 2 showing the entire caterpillar and onychophoran moving about in a lab set-up inside the field station).
That seems to be a pretty close resemblance, as the caterpillar has that pair of protruding bits that look like onycophoran antennae, as well as the body papillae that look like those on onycophorans. The caterpillars have no obvious defense that the authors could detect, and so they could be edible but avoided Batesian mimics of the onycophorans.
Now this is only a possible case of Batesian mimicry, though it looks suggestive to me. What we’d need to know to reach stronger conclusions is this information:
a. The caterpillar and onycophoran are attacked by a common species of predator (or more than one species of predator).
b. The predator has learned to avoid killing the onycophoran when it gets close to it, since the beast is nasty.
c. The predator will eat the caterpillar if it is “naive”, that is, if it hasn’t encountered an onycophoran and so hasn’t learned to avoid them. (This wouldn’t work if the aversion to onycophorans is evolved rather than learned.)
This could easily be tested in the lab (well maybe not that easily!). Just give a naive, hand-reared predator an onycophoran. See if it tries to kill it but then is repulsed and avoids them afterwards. At the same time, give other naive predators the caterpillars. They should be nommed readily. Then give a predator who has learned to avoid an onycophoran a “mimetic” caterpillar. If this is a case of mimicry based on predator learning, the predator should now avoid the caterpillar.
As it is, this is a suggestive but not well-documented case of mimicry, but it could be strengthened with some lab work.