Spiders’ anatomy sets them apart from insects. As members of the class Arachnida, they possess a specialized body plan that has allowed them to colonize nearly every terrestrial environment. Their unique physical structures facilitate locomotion, sensation, predation, and silk production. Their body is functionally divided into two main segments, unlike the three segments of an insect.
The Two Main Body Segments
A spider’s body is organized into two fused sections, or tagmata. The anterior section is the Prosoma, also known as the Cephalothorax, which is the head and thorax combined. This segment is covered on the top by a hard, shield-like plate called the carapace, which serves as a protective exoskeleton.
The Prosoma houses the spider’s brain, the sucking stomach, and the musculature that controls the legs and mouthparts. The posterior section is the Opisthosoma, or Abdomen, which is soft and flexible. This bulbous area contains the majority of the internal organs, including the heart, respiratory organs (such as book lungs), the digestive tract, and the reproductive organs.
The two main segments are connected by a slender, flexible waist called the pedicel. This narrow connection allows the Opisthosoma to move freely, which is important for maneuvering the spinnerets during silk production. The pedicel is a defining feature of spiders, as most other arachnids lack this connection between their body parts.
Appendages for Locomotion and Sensation
The Prosoma is the attachment point for the spider’s eight walking legs, arranged in four pairs. Each leg is a complex structure made up of seven segments: the coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus. This jointed structure provides a wide range of motion necessary for their diverse lifestyles, from web-building to active hunting.
Spiders move their limbs using muscles and hydraulic pressure. While they possess flexor muscles to pull their legs inward, they lack extensor muscles in the major leg joints. To extend a leg, the spider rapidly increases the pressure of its internal fluid, or hemolymph, which acts like a hydraulic fluid to push the limb outward.
Located near the front of the Prosoma are the Pedipalps, positioned between the chelicerae and the first pair of legs. These two small, leg-like appendages are used primarily for sensing the environment, handling food, and cleaning the body. In adult male spiders, the tips of the pedipalps are modified to transfer sperm to the female during mating.
Feeding and Defensive Structures
The Chelicerae, a pair of structures located at the front of the Prosoma, form the spider’s feeding apparatus. These mouthparts terminate in sharp, hollow fangs, which are used to inject venom into their captured prey. The venom, produced in glands located either in the chelicerae or under the carapace, quickly subdues the victim.
After immobilization, the spider begins external digestion. The spider secretes digestive enzymes through its chelicerae into the prey’s body, which dissolves the internal tissues. The spider then uses its straw-like mouth to suck up the now-liquefied contents of the meal.
A spider’s visual system is also attached to the Prosoma, where most species possess eight eyes, though the number can vary. These eyes are arranged in two rows; the lateral eyes are effective at detecting movement, while the two middle eyes in some species are capable of forming images and sensing color. This arrangement provides the spider with a wide field of view, which is useful for active hunters like jumping spiders.
Silk Production Apparatus
Silk production is a defining characteristic, managed by structures on the Opisthosoma. The silk is secreted through the Spinnerets, which are short, movable appendages located at the posterior end of the abdomen. Most spiders possess six spinnerets, though the number can vary.
Internally, the abdomen is packed with numerous Silk Glands, which can number up to seven different types in a single species. Each gland produces a unique type of silk protein, which is stored as a liquid. This liquid protein solidifies into a strong thread not by exposure to air, but by the mechanical stress and tension applied as the spider draws the fiber out through the spinnerets’ minute spigots.
The resulting silk is used for many purposes beyond the familiar prey-catching web. Spiders use different silks for creating protective egg sacs, laying down draglines, and building shelters. The different glands allow the spider to produce silk with various properties, from the strong, non-sticky foundation silk to the elastic, adhesive silk used for trapping prey.