"Nothing in life is to be feared, it is only to be understood." So said Marie Curie, a brilliant scientist who had clearly never heard of the horrifying Hawaiian caterpillars that dine on living flesh. Here, we explore how evolution came to play this cruel joke on us all.
Note: All animations feature in this post were sampled from BBC Two's brief (and, consequently, somewhat informationally lacking) introduction to carnivorous caterpillars, which I have here distilled down to its purest, nightmarish, eternally looping form.
This, dear reader, is a carnivorous caterpillar. It is one of the more than 20 species belonging to the genus Eupethecia believed to reside exclusively on the Hawaiian islands. As you may have already gathered from the GIF up top, members of this insular clade tend to sport some rather scary-looking appendages:
Now, scary-looking body parts, in and of themselves, aren't especially rare among caterpillars. But unlike, say, the poisonous bristles of the nettle caterpillar Darna pallivitta, or the don't-eat-me-or-you'll-regret-it coloration of Forbestra olivencia larvae, which are predominantly passive forms of predator-deterrence, the pincers on Hawaii's caterpillars serve a decidedly active purpose, viz. ensnaring unsuspecting prey. They also play a vice-like role once the caterpillar has acquired its victim; unwilling meals, after all, tend to wriggle.
That these caterpillars make active use of their scary-bits calls attention to their transition from a prey-species to a predatory one. And, in fact, the conversion to carnivorousness appears to have been a very successful evolutionary path for Hawaiian members of Eupithecia; of all the species identified on the archipelago, only two of them are herbivorous. The rest carry out their murderous larval lives making meals of everything from flies and moths to crickets and cockroaches. They've even been known to prey on other caterpillars.
The carnivorous caterpillar's predatory technique is rather straightforward. First, the larva secures itself inconspicuously along a leaf or twig, using its rear set of appendages. When an unsuspecting meal treads across its posterior, the caterpillar springs forth, snatches its prey, and evanesces the futilely twitching body to the other side of its chosen perch (above); or, alternatively, whips its prize right into the air, with the gleeful alacrity of a child who's just grabbed hold of something she's probably not supposed to (below).
Most of Hawaii's carnivorous caterpillars capture prey via this hair-trigger mechanism, a rare exception being Hyposmocoma molluscivora, a species – belonging, you'll notice, not to the genus Eupithecia, but Hyposmocoma – first reported by University of Hawaii entomologist Daniel Rubinoff in 2005, that weaves spider-like webs to capture and restrain snails. (Like members of Eupithecia, H. molluscivorae consume their prey while it's alive.)
It bears mentioning that the meat-eating caterpillars of Hawaii are not omnivores. They're carnivores through-and-through. To quote Rubinoff, these caterpillars "wouldn't sample foliage if they were starving." How or why they first set out on this evolutionary course is unclear, but researchers agree the caterpillars' path was almost certainly cleared by the isolation provided by Hawaii's islands.
Since Darwin's paradigm-shifting observations on the Galapagos, scientists have recognized islands as hotbeds of evolutionary activity. Unchecked by natural predators and enabled by vacant ecological niches, mutations can spread quickly and more easily across such isolated ecosystems. It's why island populations often look so different, and evolve so much faster, than their most closely related mainland counterparts. As Rubinoff so eloquently put it in his 2005 paper: "Hawaii's isolation and consequently disharmonic biota likely promote evolutionary experiments that occur nowhere else."