Male jumping spider (Servaea vestita) gauging a drop.
Credit: Daniel B. Zurek, Macquarie University
A jumping spider captures prey.
Credit: Ajay Narendra
LONDON: Jumping spiders have an unusual visual mechanism that allows them to accurately pounce on their prey, a new study has revealed.
A reliable three-dimensional perception of the world is crucial for an animal's successful hunting and mating behaviour, and ultimately its survival. The most common way an animal gauges an object’s distance is by either adjusting the focal length of the lenses in its eyes or moving its head to shift images on the retina. A new study published in Science today suggests that jumping spiders use a third option to achieve the perfect jump.
"Jumping spiders particularly rely on their depth perception for catching prey - they are excellent jumpers with outstanding vision. But in the end they need to get the jump just right," said Marie Herberstein, a behavioural ecologist and spider expert at Macquarie University in Sydney who wrote a "Perspective" article in Science today about the study conducted by scientists at Osaka City University in Japan.
Ways to the third dimension
Most animals, including humans, use a mechanism called binocular stereoscopic vision to obtain 3-D images. Due to their different position, each of our eyes is fed a marginally different image of our surroundings. Based on this dual input, our brain is then capable of reconstructing the third dimension.
Another way of perceiving depth in many vertebrates is the need to adjust the thickness of the eye lens - a process called accommodation - in order to achieve sharp images.
Some insects instead exploit what's called motion parallax, a mechanism by which a regular side-to-side movement of the head causes a shift of images on the retina and hence reveals the distance to an object.
A staircase to depth
The new study has revealed that the Adanson's House Jumper Hasarius adansoni, found in warm climates such as in Japan, Taiwan and Australia, makes use of yet another depth perception technique called depth defocus.
'Most jumping spiders possess three to four pairs of eyes, of which the large principal eyes face forward. The principal eyes' 'staircase' retina features four layers of photoreceptor cells, which detect light. Visible, 'white' light around us is a composition of rays of different wavelengths, which individually evoke the perception of a particular colour.
The two deepest layers of the jumping spider's retina, L1 and L2, both detect green light, but objects only ever appear in perfect focus on the deepest layer L1 and therefore must remain blurry on the layer above, L2.
The scientists hypothesised that this lack of focus, or defocus, of green light may hold the clue as to how jumping spiders perceive depth, because in principle, the distance to objects can be determined from how fuzzy the image appears in L2.
See green or go hungry
To test this rationale, the scientists examined the spiders' capability of jumping at prey under different light conditions. If defocus in the green channel on L2 is crucial, the animals should lack accurate depth perception in the absence of green light and miss their target.
The experiments indeed showed that spiders flooded in red light consistently misjudged the distance to their prey and jumped too short of target. Meanwhile, the control group under green light remained spot-on in judging how far away their prey was.
"The impact of this research is really in highlighting the combination of techniques and approaches that are necessary to understand animal perception," said Herberstein. She emphasised the elegance and importance of the exceptional experimental design. "The molecular and physiological methods were needed to propose a hypothesis of how spiders perceive depth, but the behavioural experiments were really the test of this hypothesis."
A nifty freak of nature
The revealed visual mechanism, depth of defocus, is a technique used by developers of computer vision, but the discovery that jumping spiders exploit it for depth perception is its first real-life example.
Vision expert and evolutionary zooligist Dan-Eric Nilsson from the University of Lund in Sweden, who was not involved in the study, acknowledged the impact of the findings. "This is good evidence for a new mechanism for depth perception, and as such, it is an important discovery," he said.
The news may, however, only have a limited scope in the field of vision research. "The mechanism is only found in a small group of spiders, and not likely to be a common principle. It can thus be described as an interesting curiosity."
