Tarantulas are often recognized for their hairy bodies and distinctive movements. Despite having multiple eyes, their navigation relies on a combination of senses, raising questions about their visual effectiveness.
Number and Arrangement of Eyes
Tarantulas typically possess eight eyes, arranged in four pairs and clustered on the front-top part of their cephalothorax (the fused head and thorax region). Two larger eyes are centrally located, with six smaller eyes surrounding them, usually positioned with two pairs below and one pair on each side. The exact position of these smaller eyes can vary slightly between different tarantula species. This specific arrangement provides them with a wide field of view, covering a large area around them without needing to move their heads.
What Tarantulas Can See
Despite having multiple eyes, a tarantula’s vision is generally considered poor, especially when compared to human eyesight. Their eyes are simple in structure, lacking the complex focusing lenses found in more visually acute animals. This means they cannot form sharp, detailed images. Tarantulas primarily detect changes in light and shadow, and are sensitive to movement, distinguishing between light and darkness to navigate. Some research also suggests their eyes can detect polarized light, which assists with orientation and navigation. Tarantulas are thought to have limited color vision, seeing the world mostly in shades of blue.
How Tarantulas Navigate Their World
Tarantulas compensate for their limited eyesight through other highly developed sensory mechanisms. They rely extensively on their sense of touch and their ability to detect vibrations. Their bodies and legs are covered in specialized, sensitive hairs (setae) that pick up subtle ground vibrations from prey or predators, and air currents. This allows them to sense distant movement. Tarantulas also possess slit sensilla, tiny slits in their exoskeleton that function as internal pressure sensors, enabling them to feel changes in surface tension and ground movement. Chemoreception, similar to smell and taste, is another important sense, allowing them to detect chemicals in their environment, such as pheromones or prey trails, using specialized hairs on their legs and pedipalps.
A Glimpse into Spider Vision
The visual systems of spiders, including tarantulas, show considerable diversity across different species. While most spiders have eight simple eyes, the number can range from none in some cave-dwelling species to six or even occasionally twelve in others. Spider eyes are broadly categorized into principal eyes and secondary eyes.
Principal eyes, typically the anterior median eyes, are thought to be responsible for forming more detailed images, though still with lower resolution than human vision. Secondary eyes, which include the anterior lateral, posterior median, and posterior lateral eyes, are generally more sensitive to light and motion. The specific arrangement and relative size of these eyes vary greatly among different spider families and are often used as a characteristic for classification. This variation in eye structure and arrangement reflects the diverse ecological niches and hunting strategies of different spider species.