Spiders lack a mouth structure similar to humans, possessing no teeth, jaws, or lips for traditional ingestion. Their feeding system is specialized for a predatory lifestyle requiring the consumption of liquid nutrients. The spider’s anatomy is a complex apparatus designed for capturing, subduing, and liquefying prey before the resulting fluid is consumed. This mechanism involves external appendages for preparation and internal structures for suction, which allow the arachnid to process its meal.
The Anatomy of Ingestion
The spider’s opening for food intake is a small aperture located on the underside of its cephalothorax, positioned just behind the external mouthparts. This opening leads into the pre-oral cavity, which serves as the immediate staging area for the liquid meal.
The closest equivalent to a true mouth is the pharynx, a narrow passage that follows the pre-oral cavity. The pharynx is highly muscular and sclerotized, meaning it has hardened walls, allowing it to function as a powerful pump. This structure generates the suction force necessary to draw the liquefied prey through the small opening.
The pharynx connects directly to the sucking stomach, a larger, muscular chamber that is part of the foregut. This system operates like a specialized vacuum cleaner, incapable of handling solid food particles. Muscular contractions of the sucking stomach facilitate the rapid and efficient intake of the nutrient-rich fluid.
Chelicerae and Fangs
The chelicerae are paired, jaw-like appendages located at the front of the cephalothorax. Each chelicera consists of a large basal segment that articulates with the body and a movable, hinged fang at its tip. The primary role of the chelicerae is to hold, pierce, and, in many species, crush the prey while injecting venom and digestive fluids.
The orientation of the chelicerae dictates how a spider handles its meal, and this is a defining characteristic across spider groups. Primitive spiders, such as tarantulas and other Mygalomorphae, exhibit an orthognathous orientation. Here, the fangs move parallel to the body’s axis, essentially stabbing downward and upward. This vertical motion is suited for large, burrowing hunters that rely on a powerful, direct strike.
In contrast, the vast majority of modern spiders, or Araneomorphae, possess a labidognathous orientation. In this configuration, the chelicerae are rotated so the fangs move in a pincer-like fashion, swinging side-to-side at right angles to the body axis. This scissor-like motion provides greater dexterity in manipulating and wrapping prey, making it highly effective for web-building and active hunting species.
The fang itself is a sharp, hollow structure, often tipped with a minute opening connected to the venom gland. When the spider bites, the fang penetrates the prey’s exoskeleton, delivering a cocktail of venom that rapidly immobilizes the victim. In many species, the basal segment of the chelicerae also features a cheliceral furrow lined with teeth, which are used to grip and mash the prey, further preparing it for the liquefaction process.
The Mechanism of External Digestion
Spiders perform the majority of their digestion outside their body, a process termed extra-oral digestion. After subduing their prey with venom, the spider releases a potent cocktail of digestive enzymes onto or into the captured animal. These digestive fluids, which contain powerful hydrolases like proteases, lipases, and carbohydrases, are regurgitated from the midgut and salivary glands.
The enzymes immediately begin to break down the prey’s internal tissues, turning the solid organs and muscles into a nutrient-rich liquid broth. In some cases, the spider actively pumps the enzyme-rich fluid back and forth into the prey, a process called refluxing, which ensures maximum saturation and digestion efficiency. This action effectively transforms the prey’s body into a temporary external stomach.
Once the tissues are liquefied, the spider uses the muscular pharynx and sucking stomach to draw the fluid into its digestive tract. The ingested material is not pure liquid, so the spider employs a critical filtering mechanism to ensure no solid debris enters the delicate midgut. This filter system includes thick brushes of fine setae within the pre-oral cavity and a structure known as the filtering plate within the pharynx.
These filters are highly efficient, capable of straining out particles as small as one micrometer, ensuring that only pure liquid is passed into the midgut for final absorption. Any remaining indigestible solids, such as the prey’s exoskeleton, are simply discarded after the nutritional fluids have been completely extracted.