Depth perception takes on a whole new meaning when you have eight eyes.
The spider in the picture belongs to the Salticidae family, which contains over 5000 described species. The Salticidae’s ability to jump is not unique among spiders, but they are set apart from other families by their capability to make precise, vision-guided leaps towards their prey.
The spider’s cognitive capacity has puzzled scientists for many years. Imagine sitting on top of the London Eye, you see someone standing on the other side of the Thames. Without any help or reference to a map you have to go and find that same person. Salticid spiders manage such feats regularly. The spiders’ pinhead-sized brains severely restrict the ability to retain large amounts of information, making this feat even more remarkable. The spider may sit for half an hour watching its prey; it systematically scans small areas of the environment building up a larger image. This process often takes over an hour due to the limited processing power of its brain; almost like downloading a very large file on a slow internet connection.
After mapping their environment, spiders then have to accurately judge the distance to their prey. Salticids hunt in the daytime, taking advantage of their highly developed visual system. In total, eight eyes are used; two ‘principal’ large forward facing eyes and six ‘secondary’ eyes, to extend their visual acuity. Interestingly, these spiders have four layers of photoreceptor (light-sensitive) cells on their retina. Recent evidence from electrophysiological experiments suggest green light is focused on the deepest ‘L1’ layer, but a blurry image is created on the ‘L2’ layer. So why have the extra photoreceptive layers if they only produce blurred images?
Takashi Nagata and colleagues from Osaka University, Japan, have found a likely answer in the species Hasarius adansoni. Behavioural experiments show a novel method of depth perception termed ‘image defocus’. Comparing the degree of image blurriness on the L2 layer with the sharp image on the L1 layer allows the spiders to accurately perceive depth and effectively target their prey.
No wonder, then, that in China Salticids are known as the ‘tigers’ of the invertebrate world.
Nagata, T., et al. Science 2012, 335, 469-471
IMAGE: Gerdina Mehrabi