Nov. 11, 2014

Visual cues control hunting beetles' jaws

Zurek, Perkins and Gilbert
A predatory tiger beetle, Cicindela tranquebarica, chases a high contrast prey dummy moved by hand on a nylon monofilament in an experimental arena. During the chase the beetle opens its jaws in anticipation of catching the prey and closes them again when the prey moves away. Opening and closing of the jaws are triggered by visual expansion and contraction, respectively, of the target image.

When Cornell researchers observed tiger beetles opening their jaws as they neared the prey they chased, the scientists wondered how they did it, since insects have notoriously poor depth perception and tiger beetles run very fast.

Through experiments published Nov. 5 in Biology Letters, the researchers discovered the beetle’s behaviors are triggered by shifts in the prey’s angular size in their field of vision. In other words, when a prey target starts to look bigger, the beetle knows it is gaining ground, and it opens its jaws 16 milliseconds later.

Similarly, when the prey image shifts from expanding or getting bigger to receding and becoming smaller, the beetle recognizes the prey is escaping and its jaws close 35 milliseconds later.

Why the two different reaction times?

“We think that the beetle keeps its jaws open longer [when prey is receding] as an adaptation to compensate for errors in aiming [itself],” partly because tiger beetles run at blinding speeds requiring them to stop periodically to see and relocate their prey, said Cole Gilbert, Cornell professor of entomology and the paper’s senior author. Daniel Zurek, a postdoctoral researcher in Gilbert’s lab who is now at the University of Pittsburg, is the paper’s first author.

“If it leaves its jaws open and its aim is a little off because its vision gets blurry, it could still bite the target when it’s a little bit off,” Gilbert said, noting that other studies have described great white sharks keeping their jaws open when closing in on prey. Similarly, larval fish engage in ram feeding, where they swim with open mouths to engulf prey, with increases in mouth radius making up for aiming errors.

In the study, three weeks prior to experimentation, beetles were only fed in an arena until they hunted regularly and pursued an artificial prey item, a black sphere attached to a line the researchers could then drag around the arena. The beetles were then videotaped from above at high speeds, and the film was analyzed.

The researchers ruled out a number of possible explanations documented in other animals, including insects, that also could have explained how the beetles recognize their distance from prey as a cue to open their jaws. The researchers wondered whether the beetles always open their jaws when they are a certain distance from prey, or when prey was in a certain position in the visual field, such as straight ahead, or did they open their jaws according to a measure of time rather than distance, a calculation made by animals when approaching an object at a constant velocity. All of these potential explanations were proved wrong.

The study demonstrates a new mechanism to determine impending collision, a field that looks at how creatures know when something will hit them, said Gilbert. In previous studies either the viewer or the target was fixed in space, but this experiment is the first to test with the observer and the target in motion, he added.

Madeleine Perkins ’16, a Cornell undergraduate biology major, is also a co-author on the paper.

The study was supported by the National Science Foundation.