Sensor of the Week: super-seeing spiders
Lauren Sumner-Rooney from Oxford University’s Museum of Natural History makes the case for the super-seers of the animal kingdom...
Lauren - I think lots of people would say that the champion has got to be the mantis shrimp. These guys have super-powerful arms for spearing and smashing their prey. So in complex and colourful habitats like reefs, they really need great vision to hit their targets.
Each eye is subdivided into parts so they can judge target distances using just one eye, where most animals need to use two. They’re also capable of detecting polarisation in light, as well as having 12 different types of colour sensitive cells, where we only have three in our eyes. Strangely, despite this difference their colour discrimination actually seems to be quite poor; much worse than ours because they seem to use each type in isolation rather than comparing signals between them.
So, for me, the other main contenders for visual super-sensor in the animal kingdom are spiders; they’ve evolved some incredible adaptations in their visual system. Most species have four pairs of eyes, each of which has three dedicated areas in the brain; they can be specialised to perform different roles and they often point in a different direction to each other.
Jumping spiders actually have two pairs of lenses inside their principle eyes, kind of like a telephoto camera giving them some of the highest resolution vision seen in any invertebrate, despite their tiny size. This ‘zoomed in’ effect restricts their field of view, but they’ve found a way around this by using specialised muscles to move the retina around the lens, and by arranging other pairs of of eyes around their heads to give exceptionally good peripheral vision. They’re also sensitive to red, green, blue, and UV light and appear to use these different colours to help them accurately judge distances, which isn’t known in any other animal.
Other groups have evolved completely different, but equally amazing abilities. Ogre faced spiders are deadly nocturnal hunters with some of the most sensitive eyes in the animal kingdom. Several other families, such as Wolf spiders, can also detect the polarisation pattern of the sunlight and use these to navigate using one or more dedicated pairs of eyes.
If any group deserves recognition for all round success, creativity, and diversity in vision, I think it’s got to be the spiders.