A Habronattus sp. jumping spider on a twig. Photo credit: Daniela C. Rößler.

A Habronattus sp. jumping spider on a twig. Photo credit: Daniela C. Rößler.

Glance through the window and the world is full of motion, from fallen leaves blown in the wind to rushing vehicles. But all of these movements are produced by inanimate objects. As soon as a cat slinks into view, you know that you are watching a living creature. ‘Biological motion refers to the distinctive kinematics observed in many living organisms’, says Massimo De Agrò from University of Trento, Italy, adding that many species have evolved specialised visual circuitry to recognise these unique motions. And, more recently, jumping spiders have been added to the list of creatures that recognise biological motion. But with a much smaller brain and four distinct pairs of eyes – each dedicated to a specific visual role – it was not clear which eyes the arachnids depend on to distinguish biological from inanimate movements. Given that three pairs of the spider's eyes specialise in detecting movements, De Agrò, Daniela Rößler (Konstanz University, Germany) and Paul Shamble (Yale University School of Medicine, USA) decided to narrow down which of the jumping spider's specialised eyes they depend on to distinguish a skuttling spider from a shifting cloud.

Having collected ∼200 wall jumping spiders (Menemerus semilimbatus) basking on sunny walls in Florence, Italy, De Agrò temporarily covered some of their eight eyes with white paint, leaving other pairs uncovered. Then, he placed individual animals on a rolling sphere – essentially a treadmill that would allow the spiders to scamper in any direction – while he showed them movies of a circle moving from either the left or right towards the centre of their view, as he filmed their manoeuvres, to determine how far around they can see. De Agrò confirmed that the spiders have almost a 360 deg surround view, with the front-side eyes covering the front 120 deg before the view passes to the rear-most eyes as an object moves.

Next, De Agrò tested which of the spiders’ eyes are fine-tunned to pick out animal movements, offering the spiders a choice between a cloud of randomly moving dots closing in from one side and a second set of dots, which moved like specific points on a scampering spider's body and legs, closing in from the other side. Filming the spiders’ reactions when they could see with only their front-side eyes, it was clear that the front-side eyes were tuned in to the movements made by living creatures. ‘When they had to choose, the spider turned towards the spider-like stimulus, rather than the random cloud of moving dots’, De Agrò explains. In contrast, the spiders were equally interested in both sets of moving dots when they could see with only their rear pair of eyes; these eyes appear to be tuned to see anything that moves, whether alive or inanimate.

But on previous occasions, when De Agrò had investigated the arachnids’ vision, the spiders seemed to be more interested in the randomly moving dots when both pairs of eyes were uncovered. He suspects that was because the randomly moving dots suddenly seemed to vanish from sight as they moved from the rear-most eyes’ view into the front-side eyes’ view, because the front-side eyes are specifically tuned to biological movement. ‘The disappearance is startling, so the spider may look that way, to check what happened’, says De Agrò.

So, wall jumping spiders depend on their front-side eyes to identify the tell-tale motions that distinguish distinctive animal movements from a falling leaf, even though their brains are significantly smaller than the brains of most vertebrates. They combine information from the front-side and rear pairs of eyes to ensure that they react effectively to unexpected turns of event.

De Agrò
,
M.
,
Rößler
,
D. C.
and
Shamble
,
P. S.
(
2024
).
Eye-specific detection and a multi-eye integration model of biological motion perception
.
J. Exp. Biol
.
227
,
jeb247061
.