Researchers fitted African praying mantises (Sphodromantis lineola) with 3-D glasses to study how the insects perceive motion, according to a paper published in PNAS yesterday (December 9).
Animals detect movement in two ways. One is through first-order motion perception, the detection of changes in light over space and time. This can be seen “when a dark bug moves against a bright background,” the researchers write in the paper. The other method, second-order motion perception, is more complicated. Instead of changes in light, it involves more-subtle alterations in contrast across a static background, such as when “wind creates waves of movement over a grassy field,” according to another study.
Because most movement involves changes in light from first-order motion, the authors of the new paper wanted to understand why second-order perception may have evolved. Animal behavior researcher Vivek Nityananda at Newcastle University in the United Kingdom and colleagues looked at how mantises detect prey. The team made small, 3-D glasses with orange and purple lenses for mantises to wear while the insects were shown a pattern of dots on a computer screen. The glasses were necessary for the insects to use stereoscopic, or 3-D, vision to see the computer-generated dots on the screen as lifelike prey objects.
Based on the mantises’ responses to scenes of moving dots and moving backgrounds, the researchers determined that the insects actually depend more on detecting subtle cues from second-order motion than they do on changes in light from first-order. The authors suggest that this may be an adaptation that allows insects to better see camouflaged prey that matches a similarly-shaded background.
V. Nityananda et al., “Second-order cues to figure motion enable object detection during prey capture by praying mantises,” PNAS, doi:10.1073/pnas.1912310116, 2019.
Emily Makowski is an intern at The Scientist. Email her at firstname.lastname@example.org.