Spiders, unlike many animals, do not possess a distinct neck. This anatomical feature, common in vertebrates and insects, is absent due to the spider’s unique body structure. Their form is adapted for specific functions, which means their “head” region cannot move independently in the way a neck would allow.
Spider Anatomy: The Cephalothorax
A spider’s body is divided into two primary segments, unlike the three found in insects. The front section is known as the cephalothorax, which represents a fusion of the head and thorax. This single, integrated unit houses many organs and appendages. The cephalothorax is covered dorsally by a hard outer plate called the carapace, providing structural support and protection.
Numerous external features are attached to the cephalothorax, including the spider’s eyes, mouthparts, and all eight legs. The chelicerae, which are the spider’s jaws equipped with fangs, are located at the front of this segment, alongside the pedipalps, which serve sensory roles and aid in manipulating food.
This fusion dictates how spiders interact with their environment. While nearly all spiders lack a neck, a unique exception exists in assassin spiders (family Archaeidae), which possess an elongated “neck” connecting their head to the rest of the cephalothorax.
How Spiders Move Their Heads
Since spiders lack a neck, they cannot turn their heads independently. Instead, they reorient their entire cephalothorax, or even their whole body, to adjust their field of vision and direction. This movement is primarily achieved through the coordinated actions of their legs.
Spiders have a unique method of leg extension, as they lack extensor muscles in many of their leg joints. Leg extension in spiders largely relies on hydraulic pressure generated by their internal body fluid, called hemolymph. The cephalothorax contains strong muscles that contract, increasing the pressure of the hemolymph and forcing it into the legs, which causes them to extend. Flexor muscles are present to retract the legs, creating a push-pull system that enables walking, running, and jumping.
For instance, jumping spiders can generate hydraulic pressure for rapid movements like leaping. This hydraulic mechanism allows spiders to achieve speeds and agility, with some capable of moving 34 times their body length per second.
Beyond whole-body reorientation, some spiders, particularly jumping spiders, have a specialized mechanism for adjusting their gaze. While their eyes are fixed in place on the cephalothorax, they can internally move their retinas. This internal movement, controlled by small muscles, allows them to shift their field of view without moving their entire body. This adaptation is useful for hunting spiders that rely on precise vision to track prey.