Monday, June 11, 2012

The remarkable world of mimicry

One of Nature's most interesting facets is mimicry. The natural world is loaded with animals that are not what they appear to be. This deception gives the imitators an advantage, whether it be predator avoidance, ease of capturing prey, camouflage, or some other edge. Through the miraculous fits and spurts of evolution, the mimickers have developed disguises that are often incredibly similar to quite unrelated animals.

Even though mimicry is one of the greatest examples of Darwinian natural selection at work, Darwin himself missed the entire subject. It goes unmentioned in his landmark The Origin of Species. It was English scientist Henry Bates who brought mimicry to the forefront as an outstanding example of Darwinian evolution, shortly after Darwin's book was published in 1859. Far from being jealous or bent out of shape by Bates' elucidation of mimicry, Darwin became a great admirer and went so far as to say "I have just finished, after several reads, your paper. In my opinion it is one of the most remarkable and admirable papers I ever read in my life.... I rejoice that I passed over the whole subject in the Origin, for I should have made a precious mess of it".

Last Saturday, David Wagner, myself and a few others were able to make a two hour foray through an eastern Pennsylvania park. It was a nice place, but certainly nothing out of the ordinary. Meadows buffered by a corn field, some woods and a stream, and brushy edge habitats. We still managed to find some interesting animals, including the mimics that follow.

A small ant forages along the surface of a leaf. But wait - something looks amiss.

This is no ant! Count the legs. It's a spider, doing a remarkably good job of looking like an ant. This is one of a group of jumping spiders that are ant mimics, and very good ones at that. I believe this one is Synemosyna formica, but I will stand to be corrected if someone knows better.

The ant mimic jumper is about the size of an average ant, and typically moves along the ground, often with ants. It's resemblance to the totally unrelated insects (spiders aren't even insects, they're arachnids) is startling. It may be that the spider gains an advantage by looking like something that potential prey might tend to ignore. Or it may be that potential predators of the spider often find ants distasteful and shun them.

A spotted thyris, Thyris maculata, forages on the flowers of common milkweed. Were this little day-flying moth not actively feeding, it would have been quite hard to see.

This moth and many of its ilk are fabulous leaf mimics, or have otherwise evolved patterns and shapes that resemble plant parts. When this thyris is at rest on leaves or other plant parts that match it, your chances of spotting it are slim to none, and presumably that holds true for beasts that would like to eat it.

We were quite pleased to encounter this insect, which is an awesome example of mimicry and one to watch for in your own local patch. To the casual eye, it would appear to just be a large fuzzy bumblebee at rest on a leaf. But the trained observer will hone in on this bug right away, as there are several things amiss. For one, bumblebees very seldom stop and bask on leaves like this. If the weird behavior stimulates a closer inspection, you'll quickly see that the insect has huge eyes and only two wings - the hallmarks of a fly.

This is one of the robber flies in the genus Laphria (thanks to Benjamin Coulter for the species specific ID of Laphria thoracica) and they are consummate bee mimics. Robber flies are barbaric predators that typically hunt from perches such as this leaf. They wait and watch, and when a suitable victim happens by, the robber fly darts out, grabs it, and stabs the prey with that syringelike proboscis that can be seen projecting from the mouthparts. The fly then injects chemicals that cripple the victim, and dilute its innards. After suitably stewing the prey, the robber fly sucks it dry via the proboscis.

Note how the hairs of the upper legs and proboscis tip are shaded in yellow-orange, making them look just like pollen-dusted bumbebee hairs. Incredible. The selective advantage gained by resembling a bumblebee may be predator avoidance. Birds, which might be the most likely animals to bag a robber fly, may well avoid bumblebees in most cases, as they'll know they could receive a painful sting. Thus, the fly is free to loaf conspicuously on perches and scan for prey.


OpposableChums said...

"... the miraculous fits and spurts of evolution" indeed. Amazing. Thanks.

Janet Creamer said...

Very cool! I have always wanted to see one of the ant mimic spiders!

Sharkbytes said...

I featured the robber fly last summer, but I've sure never seen that spider!