Spiders do not possess crushing mouthparts like the mandibles found in insects. The structures spiders use, called chelicerae, are highly specialized for piercing, injecting venom, and initiating the breakdown of prey from the outside. The absence of true crushing jaws necessitates a different method for consuming a meal, relying on potent digestive fluids and a powerful sucking mechanism.
The Anatomy of Chelicerae and Fangs
A spider’s mouthparts, the chelicerae, consist of two segments: a large basal segment that connects to the cephalothorax and a sharp, terminal fang. This structure functions more like a hypodermic needle than a traditional jaw designed for chewing or crushing. The basal segment often contains the venom glands, which contract to force venom through a small opening near the tip of the fang.
The chelicerae are fundamentally different from the mandibles of insects, which are hardened, toothed structures that move side-to-side to shear and grind solid food particles. Spider chelicerae are primarily designed for penetration and envenomation. The fangs penetrate the prey’s exoskeleton or skin, allowing the spider to inject venom to quickly immobilize its capture.
The movement of the chelicerae varies across different groups of spiders. Most spiders, known as Araneomorphs, have labidognathous chelicerae, meaning the fangs cross over each other like pincers and move at right angles to the body axis. This scissors-like action aids in manipulation and securing the prey.
More ancient spiders, such as tarantulas and trapdoor spiders (Mygalomorphs), possess orthognathous chelicerae, where the fangs are parallel and strike downward in a stabbing motion. In both forms, the apparatus’s goal is to pierce and inject, not to apply sustained crushing force.
The Process of External Digestion
Since the narrow digestive tract of a spider cannot accommodate solid food, the captured prey must first be liquefied in a process known as extra-oral digestion. After subduing the prey with venom, the spider injects a cocktail of digestive enzymes secreted from its midgut into the prey’s body. These enzymes, which include peptidases, lipases, and nucleases, begin to break down the internal tissues of the victim.
The spider repeatedly pumps digestive fluids back and forth, a reflux action that ensures the enzymes are thoroughly mixed and distributed throughout the prey’s body. This process turns the internal organs and muscles of the prey into a nutrient-rich “soup.”
Once the tissues are fully broken down, the spider uses a powerful muscular pharynx and a sucking stomach located in the cephalothorax to draw the liquefied meal into its gut. The sucking stomach acts as a pump, creating the necessary suction force to ingest the liquid food. This specialized mechanism allows the spider to consume the entire contents of the prey, leaving behind only the indigestible exoskeleton.
Specialized Feeding Adaptations
While the majority of spiders rely on venom and external digestion, some species have evolved adaptations that allow for a form of mechanical processing that comes closest to crushing. Certain spiders possess serrated edges or teeth on the basal segment of their chelicerae. These structures are used to physically mash or shred the prey while simultaneously flooding it with digestive enzymes.
In these cases, the chelicerae and the bases of the pedipalps work together to create a preoral cavity where the prey is pulpified before the liquefied contents are ingested. This allows some species to handle prey too large or too tough to fully dissolve with enzymes alone, sometimes consuming a semi-solid pulp.
The pedipalps, which are small, leg-like appendages located between the chelicerae and the first pair of walking legs, play a supplementary role in feeding. They are often used to manipulate, hold, and rotate the prey during the injection of enzymes and the subsequent ingestion process. Adaptations, such as the modified chelicerae of woodlouse-eating spiders, demonstrate how the apparatus is adapted to overcome the hard protective shells of their specific prey.