We uploaded 6 nice demo videos as Supplementary Material for our Scientific Reports paper. Unfortunately the links are currently broken (I have emailed) and in any case they are provided in a slightly clunky way where you have to download them. So I thought I would do a blog post explaining the videos here.
Here is a video of a mantis shown a 2D “bug” stimulus (zero disparity). A black disk spirals in towards the centre of the screen. Because the disk is black, it is visible as a dark disk in both eyes, i.e. it’s an ordinary 2D stimulus. The mantis therefore sees it, correctly, in the screen plane, 10cm in front of the insect. The mantis knows its fore-arms can’t reach that far, so it doesn’t bother to strike.
Next, here’s a video of a mantis shown the same bug stimulus in 3D. Now the disk is shown in blue for the left eye and green for the right (bearing in mind that the mantis is upside down). Because the mantis’s left eye is covered by a green filter, the green disk is invisible to it – it’s just bright on a bright background, i.e. effectively not there, whereas the blue disk appears dark on a bright background.
This is a “crossed” geometry, i.e. lines of sight from each disk to the eye that can see it cross over in front of the screen, at a distance about 2.5cm in front of the insect. This is well within the mantis’s catch range, so the insect strikes out trying to catch the bug. You can sometimes see children doing the same thing when they see 3D TV for the first time!
Here’s a slo-mo version recorded with our high-speed camera. Unfortunately the quality has taken a big hit, but at least you get to see the details of the strike…
Sceptical minds (the best kind) might wonder if this is the correct explanation. What if the filters don’t work properly and there’s lots of crosstalk? Then, the mantis is seeing a single dark disk in our “2D” condition and two dimmer disks in our “3D” condition. Maybe the two disks are the reason it strikes, nothing to do with the 3D. Or maybe there’s some other artefact. As a control, we swapped the green and blue disks over, effectively swapping the left and right eye’s images. Now the lines of sight don’t intersect at all, i.e. this image is not consistent with a single object anywhere in space. Sure enough, the mantis doesn’t strike. Obviously, in different insects we put the blue/green glasses on different eyes, so we could be sure the difference really was due to the binocular geometry, not the colours or similar confounds.
Here’s the figure from our paper which illustrates this geometry and also shows the results: