An insect with just one ear
Most insects have a pair of eardrum organs for hearing, one on each side. In 1986 the Americans David Yager and Ronald Hoy showed that a mantis has a single ear in the middle of the underside of the metathorax, between the hind legs. Because there's only one, on the body's midline, they called it the "cyclops ear," after the one-eyed giants of myth.
This ear responded mainly to ultrasound of 25 to 45 kilohertz. That's higher than the highest sound people can hear (about 20 kilohertz) and overlaps the range of the ultrasound bats make when looking for food.
They dodge only when flying
In a 1990 study, when ultrasound was played to flying male mantises (Parasphendale agrionina), they swerved slightly if the sound was far away and fell in a hurried spiral if it was close. When sitting, they didn't react to the same sound. In five cases in the field where a hunting bat attacked and the mantis did this evasive move, none was caught.
In 2008, real bats (big brown bats) and mantises were made to fly together in a lab flight room. 76% of the mantises that could hear got away, while only 34% of the mantises with blocked ears did. None of the mantises that began a dive was caught.
Not every mantis can hear
More than half of all mantis species are said to hear ultrasound, but not every species does. Mantises with well-developed wings that can fly generally hear well, and short-winged species don't hear well.
In species where the sexes differ in wing length, the female's hearing is always weaker, and in some species females can't hear at all. Females are heavy and short-winged and rarely fly, while males fly around at night searching for females, so they run into bats a lot.
Mantises with a second ear
Some mantises, such as the genus Creobroter in the flower mantis family, have another ear on the mesothorax. The metathorax ear hears high sounds of 25–50 kilohertz and the mesothorax ear hears low sounds of 2–4 kilohertz, and the two ears work almost independently.
An ear that came before bats
According to Yager's phylogenetic analysis, the mantis ear is estimated to have evolved before bats appeared. It's thought not to have arisen to avoid bats, but to have come to be used for bat avoidance after bats appeared.
Things people get wrong
- "All mantises dodge bats with the ear on the chest" — short-winged species and the females of many species can hardly hear.
- "They hear 25–100 kHz" — the figure in the original 1986 paper is 25–45 kHz.
- "The ear evolved because of bats" — the ear is estimated to have evolved before bats.
- "They hide at bat sounds even when sitting" — the evasive behavior appeared only in flight.
Research in this article
Yager & Hoy (1986) The cyclopean ear: a new sense for the praying mantis. Science 231: 727–729
- What they did
- Recorded nerve activity in mantises while playing sounds, and examined the structure of the ear.
- What they found
- A single ear in the middle of the underside of the metathorax. It responds to 25–45 kHz ultrasound.
Yager, May & Fenton (1990) Journal of Experimental Biology 152: 17–39
- What they did
- Played ultrasound to flying males and observed real bat attacks in the field.
- What they found
- They react only in flight. If far, they change direction; if close, they dive in a spiral. All 5 who dodged escaped.
Triblehorn et al. (2008) Journal of Experimental Biology 211: 555–562
- What they did
- In a flight room, mantises that could hear and mantises with blocked ears were made to meet bats.
- What they found
- Escape rate 76% vs. 34%. All mantises that started a dive escaped.
Yager (1996) Journal of Comparative Physiology A 178: 463–475
- What they did
- Nerve recordings from Creobroter mantises and ear-removal experiments.
- What they found
- The mesothorax ear handles 2–4 kHz and the metathorax ear 25–50 kHz, separately.