Jumping spiders — members of the Salticidae family — have solved the problem of vision in a way that puts most larger animals to shame, and they do it with a brain the size of a poppy seed. They have eight eyes arranged around their cephalothorax, but the two large forward-facing principal eyes are the ones that matter most, and they are extraordinary.
The principal eyes have a narrow field of view, approximately 2 to 3 degrees, but within that field they resolve detail at a level that exceeds the visual acuity of a cat and approaches that of a human, despite the spider’s entire body being smaller than a fingernail. The eyes achieve this through a retina with four layers of photoreceptors, including receptors sensitive to ultraviolet light that humans cannot see, and a retinal tube that moves independently of the eye to scan the visual field without the spider moving its head.
The secondary eyes, arranged around the sides and back of the cephalothorax, provide wide-angle motion detection. When the secondary eyes detect movement, they trigger the spider to turn and bring its principal eyes to bear on the source. The system combines a wide-angle motion alarm with a narrow-angle high-resolution camera, and the two work together seamlessly.
Damian Elias at the University of California Berkeley has studied jumping spider sensory biology and behavior extensively, and what his work reveals is an animal that is paying close attention to the world in ways that its size would not suggest. Jumping spiders track moving objects with their principal eyes, turning to follow prey or potential mates across a visual field. They plan routes to prey that take them out of sight of the prey temporarily, suggesting they maintain a mental representation of the prey’s location even when they cannot see it.
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The Jumping Spider Courtship Display
Male jumping spiders perform elaborate visual courtship displays that are among the most complex in the arthropod world. The peacock spiders of Australia, Maratus species, have abdominal flaps covered in iridescent scales that they raise and vibrate during display, producing a visual performance that has been compared to a tiny, eight-legged peacock. The displays are species-specific, with different species producing different color patterns and movement sequences that females use to identify conspecific males.
The displays are performed at close range, within a few centimeters of the female, and the female’s response determines whether the male survives the encounter. A female that is not impressed, or that mistakes the male for prey, will eat him. The male’s entire display is a negotiation between attraction and survival, performed in front of an audience with eight eyes and a female who might eat him.
Cognition in a Poppy Seed
The cognitive capacities documented in jumping spiders have forced a reconsideration of the relationship between brain size and intelligence. These spiders plan detours, recognize individual humans, show curiosity about novel objects, and solve problems in ways that suggest flexible reasoning rather than fixed behavioral programs.
Fiona Cross at the University of Canterbury has studied jumping spider cognition in detail, finding that some species can solve multi-step problems that require holding information in working memory across several sequential decisions. The neural architecture that produces this capacity in a brain of a few hundred thousand neurons is not understood, but the behavior is documented well enough that dismissing it is no longer scientifically credible.
The spider that sees better than a cat, that plans routes to prey it can no longer see, that performs a species-specific dance in front of a female who might eat it, is doing all of this with a brain that would fit comfortably on the head of a pin. Size, it turns out, is not the point.