Spiders, as arachnids, possess a body divided into two main sections: the cephalothorax and the abdomen. The cephalothorax (prosoma) is the fused head and thorax, anchoring the legs and housing the primary control centers. The abdomen (opisthosoma) connects to the cephalothorax via a thin, flexible stalk called the pedicel. This posterior section contains the bulk of the internal organs, including the respiratory system, heart, digestive tract, and the silk-producing apparatus. The soft abdomen can expand significantly to accommodate large meals or developing eggs.
How Spiders Process Food
Spiders cannot chew their prey, so digestion begins externally, a method called extra-oral digestion. After injecting venom, the spider pumps digestive enzymes into the victim’s body. These enzymes break down the prey’s internal tissues into a nutrient-rich liquid, liquefying the meal inside its exoskeleton.
The spider uses a powerful, muscular pump called the sucking stomach, located within the cephalothorax, to draw the liquefied tissues into its gut. This organ creates the necessary suction to ingest the fluid, which is filtered to remove any solid particles. The ingested liquid passes into the midgut, where blind extensions called ceca increase the surface area for nutrient absorption.
These numerous ceca branch throughout the abdomen and even into the legs, allowing the spider to extract maximum sustenance from the liquid meal. This specialized strategy allows spiders to consume prey much larger than their mouths, leaving behind only the dry, empty husk of the victim.
Respiration and Blood Flow
Spiders breathe using specialized organs on the underside of the abdomen, typically combining book lungs and, in some species, a supplementary tracheal system. Book lungs consist of 10 to 80 thin, leaf-like plates called lamellae, stacked like the pages of a book within a hardened pocket. Air enters through external slits, and oxygen diffuses across the lamellae into the hemolymph, the spider’s circulating fluid.
The tracheal system, when present, consists of simple tubes that deliver oxygen directly to the tissues. This system is more efficient and likely evolved to resist water loss in smaller species. This dual system provides an effective means of oxygen uptake, especially for larger or more active spiders.
The circulatory system is open, meaning the hemolymph bathes the organs directly within the body cavity, known as the hemocoel. The heart is a simple, tube-shaped organ running along the dorsal side of the abdomen, equipped with small, valved openings called ostia. When the heart contracts, hemolymph is pumped forward through an artery into the cephalothorax and backward into the hemocoel.
This fluid, which is not true blood, contains the respiratory pigment hemocyanin, a copper-based protein that turns bluish when oxygenated. The hemolymph circulates freely, delivering oxygen, nutrients, and hormones to all internal organs. It is then collected and drawn back into the heart through the ostia.
Neural Control and Waste Management
The spider’s nervous system is highly centralized, with all ganglia concentrated entirely within the cephalothorax. This fused structure, called the synganglion, surrounds the esophagus. It is composed of the supraesophageal ganglion (the brain proper) and the larger subesophageal ganglion.
This centralized arrangement allows for rapid processing and motor control. The subesophageal ganglion contains nerve clusters that control the eight legs. Sensory information, such as vibrations detected by hairs on the legs, is quickly relayed to this compact control center, enabling near-instantaneous reactions and complex behaviors.
Waste management is handled by a specialized excretory system designed for water conservation. The primary organs for filtering metabolic waste are the Malpighian tubules, small tubes in the abdomen that function similarly to kidneys. These tubules extract nitrogenous waste products from the hemolymph and convert them into guanine, a white, solid compound. This conversion allows the spider to excrete waste as a dry material, conserving precious body water.
The Internal Silk Factories
The production of silk is managed by specialized silk glands located within the abdomen. These glands can occupy substantial space, especially in web-building species. A single spider can possess up to seven different types of silk glands, each producing a unique liquid protein for specific purposes, such as sticky capture silk or strong dragline silk.
The liquid silk protein, or spidroin, is stored in these glands as a viscous solution. When the spider begins to spin, the liquid is channeled through a long, progressively narrowing duct that acts like an internal spinning reel. As the protein solution travels through this duct, cells lining the passage extract water, and the decreasing pH causes a structural rearrangement of the protein molecules.
This combination of water extraction and acidification forces the liquid protein to transition into a semi-solid, ordered fiber. The resulting fiber is then extruded through tiny spigots located on the external, movable appendages called spinnerets. The final solidification into a strong silk thread occurs as the protein fiber is physically drawn out and exposed to the air.