Eye of the Spider
By Stephen Luntz
he visual systems of jumping spiders are even more extraordinary than previously realised, according to a new study that has extracted remarkable precision from very small eyes.
Jumping spiders have four pairs of eyes, but one is vestigial. Another pair is very large relative to the spider, and provides remarkable vision while the two smaller pairs were thought to be simply used as motion sensors.
However, Macquarie University PhD student Daniel Zurek has overthrown this idea by placing removable dental silicone over the main and rear eyes of 52 jumping spiders (Servaea vestita) while leaving the forward-facing anterior lateral eyes uncovered. Zurek then showed the spiders tethered live house flies and dots moving on a screen.
“Even when the spiders were confined to visual input from this secondary pair of eyes, they could respond to targets that are very hard for other animals to see. They were able to detect, stalk and attack flies, which was unexpected,” says Zurek.
Dr Ximena Nelson, Zurek’s supervisor, explains that many of the dots shown to the spiders were either very small or had low contrast with the background, yet the spiders were able to follow them when most other animals could not. Considering the tiny size of the anterior lateral eyes, this demonstrates remarkable efficiency. Nelson says they take advantage of better depth perception provided by being widely spaced.
Zurek demonstrated that the eyesight is well-tuned for spotting the spiders’ preferred prey, with “high acuity and colour vision over a very narrow field of view”. The findings are particularly remarkable because S. vestita is estimated to have 500,000 neurons, half that of bees. Many of these are devoted to their eyes, including surprising numbers to the vestigial pair, yet the spiders can also hear, smell and sense vibrations. Nelson says her “working hypothesis is that many of the decisions must be made closer to the level of the retina rather than in the central brain”.
Lessons from invertebrate vision have been put to use designing more efficient robot guidance systems. Nelson says: “It would be helpful if we could get underneath the neural wiring because it is very efficient and the best eyesight of any animal their size by a long way”.
However, research is difficult because spiders don’t have muscles in their eyes, instead relying on several atmospheres of pressure. This allows the spider to go longer without the food energy that muscles require, but means that attempts to study the eye’s internal structure cause depressurisation. “Their neurons are also much smaller than those of insects, and are therefore harder to study,” Nelson adds.