Sounds Trigger Unusual Evasive Maneuvers

white, black and yellow praying mantis on brown log with a green background

Biological Strategy

Mantids

Mary Hoff

Image: Cathy Keifer / Shutterstock / Some rights reserved

Respond to Signals

To interact with its environment, a living system must not only sense a variety of signals, but also respond to them. To be energy- and material-efficient, those responses must be appropriate to the signal. This generally requires sensing thresholds to trigger an appropriate level of response (for example, hiding under a shrub versus running away to avoid a predator). Response strategies are tied to a specific signal and often have a response threshold, which determines how strong a signal must be to warrant expending energy to respond. One example is a plant that lives in arid regions in South Africa. Its seed capsules remain closed until rainfall triggers them to open to release the seeds. But the plant only responds to a second rainfall, thus protecting against releasing its seeds before there is enough sustained water for them to grow.

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Navigate Through Air

Although free from many of the physical obstructions found on land and in water, organisms that move through the air must still avoid hitting each other and objects in their flight path, such as trees and mountains. They also need to navigate from one place to another, which presents a different challenge. They use one strategy to navigate around obstructions and another to move towards their destination. For example, bats use echolocation to detect both obstacles and prey. To navigate during migration, bats also use vision, sun orientation, and likely other strategies.

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Sense Sound and Other Vibrations From the Environment

For living systems, sensing sound and other vibrations is important for communicating and detecting conditions within their environment. Living systems must locate a signal’s source so that they can move toward it (such as when it is food or a potential mate) or away from it (such as when it is a predator). To prompt an appropriate response, living systems must sense these signals, recognize their amplitude or volume (which is sometimes very low), and determine their direction. Living systems must be attuned to signals relevant to them and able to distinguish these from irrelevant sounds to avoid expending unnecessary energy. For example, owls’ ears are asymmetrically placed. This enables them to detect sounds more accurately, which helps them locate small prey at night and avoid wasting energy chasing down irrelevant sounds.

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Protect From Animals

Animals–organisms that range from microscopic to larger than a bus–embody a wide variety of harms to living systems, including other animals. They threaten through predation, herbivory, defense, and parasitism, and they compete for resources such as water, nutrients, and space. Any given living organism commonly faces threats from a variety of animals, requiring strategies that effectively defend from each. Trout and other bony fish, for example, escape predators by having scales made of very thin, flake-like pieces of bone covered with slippery mucus. They also have behavioral strategies such as camouflage, fast swimming, and twisting and turning to achieve release from a predator’s grip.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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This mantis detects ultrasound and responds with erratic elusive behavior.

Introduction

It’s an eat-and-be-eaten world, and the budwing mantis (Parasphendale agrionina) does its part by foraging on butterflies, grasshoppers, and other insects roaming the savannas and dry forests of East Africa. Its habitat overlaps with that of the big brown bat (Eptesicus fuscus), which finds these gangly creatures decidedly appetizing. But budwing mantises are far from sitting ducks when it comes to becoming bat food, thanks to a combination of strategic hearing and startling aerial acrobatics.

Image: Steve Smith / Flickr / CC BY NC - Creative Commons Attribution + Noncommercial

The budwing mantis has an ear in the middle of the underside of its upper body that senses the ultrasound hunting signals of insect-eating bats.

The Strategy

The strategic hearing starts with a single “ear” located on the upper part of the insect’s underside. Evolution has tuned it to preferentially pick up sound waves of 25–50 kilohertz—which is roughly the range of their bat-predator’s vocalizations.

What to do with that heads-up? Many insects that can hear ultrasound, including other mantis species, respond by rapidly descending in a “power dive.” The budwing mantis does this, too. But when researchers tethered a mantis so they could observe the details of its maneuver, they discovered that it not only dives, but also rolls its head, flings out its front legs, arches its back, and alters its wingbeat.

This suite of actions is similar to what the budwing mantis does when it’s threatened on the ground. It’s a type of behavior, known as a startle response, that disrupts the focus of the predator long enough to give the prey a chance to escape.

The researchers speculate that the wild moves may retain the original function of startling and confusing the predator, or they might contribute to the effectiveness of the dive, or both.

The Potential

Possible applications for the ability to detect and respond to particular frequencies of sound include designing systems to avoid midair collisions, improving medical diagnostics, and navigating without visual cues.

The dive and startle responses, in turn, offer insights into eluding adversaries: A rapid change in direction is intuitively a good choice. And by studying the aerodynamics of the startle response, researchers may be able to develop flight patterns for planes, satellites, and other engineered devices to make them more energy efficient, better able to elude space junk, or more.

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References

“Recently, we have learned that some praying mantises have sensitive ultrasonic hearing mediated by a unique and independently evolved auditory system. Physiological and behavioral evidence points to a function in bat avoidance in this insect as well, and a preliminary study showed that a tethered, flying mantis Creobroter gemmatus (Stoll) responds to pulses of ultrasound with a short-latency suite of behaviors including full foreleg extension and abdomen dorsiflexion. In free flight in the laboratory, these mantises respond with a sharp turn and dive. Here we present the results of free-flight experiments in the laboratory and in the field that define the ultrasound-induced flight maneuvers of the mantis Parasphendale agrionina (Gerst.). We show that the response occurs only during flight and, most importantly, our observations with wild bats in the field establish the responses of the mantis to ultrasound as true bat-evasion maneuvers.” (Yager, May & Fenton, 1990:18).

Journal article

Ultrasound-triggered, flight-gated evasive maneuvers in the praying mantis Parasphendale agrionina. I. Free flight

Journal of Experimental Biology | September 1, 1990 | D.D. Yager, M.L. May, M.B. Fenton

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Reference

“Foreleg extension plays a prominent role in all of P. agrionina’s defensive behaviors, particularly the deimatic response, and it is not surprising to find that it is a component of the ultrasound-triggered evasion response. Further, the in-flight response includes abdomen curling, another major component of the secondary defensive display, and the increased forewing swing angle may correspond to wing raising. We suggest that these components of the airborne behavior may have their origin in the earthbound defensive display. They may retain their startle function, suddenly changing the shape of the animal and the nature of the bat’s returning echo, or they may have secondarily taken on aerodynamic significance, e.g. by shifting the center of mass forward and possibly facilitating a dive, or both.” (Yager & May 1990:55)

Journal article

Ultrasound-triggered, flight-gated evasive maneuvers in the praying mantis Parasphendale agrionina. II. Tethered flight

Journal of Experimental Biology | September 1, 1990 | D.D. Yager, M.L. May

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