Halloween Costumes?

One of these doesn't belong:

Castianeira descripta, Waukegan Beach, Lake Co, IL 8/9/2012

Cyrtophorus verrucosus, Van Patten Woods FP, Lake Co, IL 5/28/2014

Crematogaster cerisi, Gander Mt. FP, Lake Co, IL 5/3/2014

Thanasimus dubius, Lyons Woods FP, Lake Co, IL 6/2/2014

Did you catch it? The third one is an ant, specifically Crematogaster cerisi. The others include a spider (Castianeira descripta) and 2 beetles: Cyrtophorus verrucosus and Thanasimus dubius. This resemblance isn't a coincidence -- ant mimics are quite common in the arthropods. In this case, we have three species, all of which evolved their mimicry independently. Apparently ants are fearsome enough critters that they're worth mimicking. (Or maybe they just taste bad.)

These are examples of Batesian mimicry, which is the type most people probably think of: an inoffensive little critter pretending to be a dangerous one, so that everyone will leave them alone. But there's another form of mimicry we often find, called Mullerian mimicry. In this case, a bunch of dangerous or, more often, distasteful critters will all converge on a single appearance. This is quite common among tropical butterflies, including these longwings:

Isabella Longwing (Eueides isabella), St. Louis Zoo, MO, 3/2/2012

Ismenius Longwing (Heliconius ismenius), St. Louis Zoo, MO, 3/2/2012

So what's going on here? Well, it's worth remembering that very few predators are born knowing which butterflies taste bad. They have to learn that lesson somewhere along the way. Once is often enough, but that once is awfully hard on the teacher! By mimicking another distasteful species, you spread that lesson over two species, and reduce the chances of losing your offspring to school.

This point about the cost of teaching is worth remembering for Batesian mimics as well. Batesian mimicry only works if most predators encounter the distasteful species. If a young Blue Jay (for example) runs across tasty Viceroys for a year before he finds a Monarch, he never learns that he should avoid them. This puts Batesian mimics in a rather unenviable position - if they get too common, then they run the risk of losing the benefit of their mimicry. And since many of the models are brightly colored, that means the former mimics are now extraordinarily vulnerable. In the case of the Viceroy, that spells trouble, since Monarch numbers continue to decline. (1)

(1) Texas A&M University. (2014, March 19). Monarch butterfly numbers could be at historic lows this year, study suggests. ScienceDaily. Retrieved June 2, 2014 from www.sciencedaily.com/releases/2014/03/140319114608.htm