Spiders belong to the class Arachnida, a group of arthropods that also includes scorpions and mites. Like all arachnids, spiders possess eight jointed legs and lack antennae, distinguishing them from insects. The posterior section of the body is often informally referred to as the “butt.” This unique, often bulbous rear section is a highly functional and complex segment. Understanding the specific anatomy of a spider reveals a creature perfectly adapted for its predatory lifestyle.
The Two Primary Body Segments
A spider’s body is organized into two primary segments, or tagmata, unlike the three segments found in insects. These two parts are joined by a narrow, flexible stalk known as the pedicel. The anterior, or front, segment is called the Prosoma, or Cephalothorax, representing a fusion of the head and thorax. This segment serves as the spider’s locomotor and sensory center, bearing all the walking legs, eyes, and mouthparts. The posterior, or rear, segment is called the Opisthosoma, or Abdomen. This segment houses most of the spider’s major internal organ systems, including the respiratory, digestive, and reproductive organs.
Opisthosoma: Housing the Spinnerets
The Opisthosoma contains the structures responsible for the spider’s most iconic ability: silk production. This posterior segment is typically unsegmented in most modern spiders, appearing as a smooth, rounded, or oval structure.
Located at the posterior end of the Opisthosoma are the spinnerets, which are short, movable, finger-like appendages. Most spiders possess six spinnerets, though the number can vary from two to eight. These external structures are the mechanical apparatus used to extrude and manipulate silk.
Each spinneret is tipped with numerous microscopic openings called spigots, which connect to an internal silk gland. Spiders can possess up to six different types of silk glands, each producing silk with a unique protein composition. The silk protein is stored as a liquid inside the Opisthosoma and hardens into a fiber as it is mechanically drawn out by the spider, which changes the internal structure of the protein molecules. This extruded silk is used for:
- Building webs
- Creating egg sacs
- Lining burrows
- Wrapping captured prey
- Dispersal, a process called ballooning
The Opisthosoma also contains openings for the respiratory organs, such as book lungs, and the anus, which is located just above the spinnerets.
Prosoma: Appendages and Sensory Organs
The Prosoma, or Cephalothorax, acts as the command center, coordinating movement and sensory input. It is covered by a hard, protective shield called the carapace, which provides a rigid surface for muscle attachment. Attached to this segment are six pairs of appendages, including the four pairs of walking legs.
The eight legs are primarily used for locomotion, but they also serve a sensory role, covered in fine hairs that detect vibrations, airflow, and chemical cues. Directly in front of the legs are the pedipalps, a smaller pair of appendages. Pedipalps are used for handling food, sensing the surroundings, and in male spiders, they are modified to transfer sperm during reproduction.
The spider’s “face” is dominated by the chelicerae, the mouthparts that contain the fangs. These structures are used for seizing and holding prey. Most spiders use them to inject venom produced in glands within the Prosoma. The venom, which often contains neurotoxins or digestive enzymes, quickly immobilizes or breaks down the prey’s tissues. The Prosoma also bears the eyes, typically eight simple eyes, though many species rely more on vibrations than eyesight.