Sunday, March 30, 2014

Riverside Place: Katydids Have Ears On Their Front Legs

Photo: Jonathan Vandiveer, March 22, 2014.  Last week, we first identified this as a katydid, but issued a correction when an LA Zoo entomologist (reviewing the photo on an iPhone in bright sunlight) identified it as a juvenile grasshopper who'd just molted.

Then we received an email from Nathan Harness, biologist at the University of Missouri who studies katydids.  Harness confirmed it was a katydid with a number of scientific details, including those long antennae, but mostly that small dark oval on its front leg, which is its ear (see arrow in photo).  

From an evolutionary standpoint, what purpose does having ears on their legs serve?  It would seem to be a liability if they lose a leg in escaping a predator.

Harness replied:
"This is a good question. Insects have ears pretty much everywhere except their head. Some on their wings, abdomen, legs, lots of places. I'm not sure there is a good answer other than, "that's where they evolved." It's hard to ask why something didn't happen (why didn't they evolve on their heads?), because the answer simply could be that there wasn't a mutation or variation that let an ear evolve somewhere else- and for katydids there was some kind of mutation or variation that let an ear evolve on their foreleg (probably evolved on an ancestor of katydids...). Perhaps there was a good reason to keep the legs far apart- like directional hearing (that's what katydids use their ears for)

As far as why there isn't selection against ears on forelegs, I have never caught a katydid that didn't have both forelegs. And usually if a leg is gone, it's a hind leg. They drop their hind legs readily, like a lizard loosing it's tail." 
And for those of you who want confirmation on the ID:
"It's in the subfamily Phaneropterinea, we call them "pheneropterans" (phen sounding like "fin"). I know that because it's ear is exposed, not covered by cuticle, it's antennae are located in the right spot, and it's hindwings are longer than it's forewings."

Learn more about katydids, see perhaps the cutest little katydid, ever, by following Harness on Twitter: @nathan_harness.