Spiders are known for their intricate webs and unique predatory behaviors. These arachnids have a distinctive body structure that sets them apart. Understanding their anatomy provides insight into how they thrive in diverse environments.
Spider Body Plan
Spiders possess a body plan characterized by two segments, distinguishing them from many other arthropods. These two sections are the prosoma (cephalothorax) and the opisthosoma (abdomen). A slender, flexible stalk called the pedicel connects these segments, allowing for significant movement of the abdomen relative to the cephalothorax. This two-part division houses all essential organs and appendages.
The Cephalothorax
The cephalothorax, or prosoma, forms the anterior (front) part of the spider’s body, a fusion of head and thorax. This segment is typically covered by a hard protective plate called the carapace dorsally and a sternum ventrally. Most of the spider’s external appendages originate from the cephalothorax, including its eight jointed legs, arranged in four pairs. Spiders typically have eight simple eyes, though some species may have six, two, or even none, on the carapace.
Located at the front of the cephalothorax are the chelicerae, the spider’s mouthparts, often equipped with fangs to inject venom into prey. Behind the chelicerae are the pedipalps, a second pair of appendages that resemble small legs. These pedipalps serve sensory functions, assist in food handling, and in male spiders, are modified for sperm transfer during mating. The cephalothorax also contains the brain, venom glands, and the muscular sucking stomach, to draw liquid food into the digestive system.
The Abdomen
The abdomen, or opisthosoma, is the posterior (rear) body segment of the spider, connected to the cephalothorax by the pedicel. In contrast to the rigid cephalothorax, the abdomen is generally soft and pliable, allowing it to expand significantly after feeding or during egg development. This segment contains many internal organs, including most of the digestive tract, the heart, and the reproductive organs.
A defining feature of the abdomen is the presence of spinnerets, specialized organs at the posterior end. These spinnerets produce silk, a protein substance used for various purposes such as web construction, wrapping prey, and creating egg sacs. Spiders breathe using book lungs, respiratory organs on the ventral side of the abdomen, appearing like pages in a book. Some spiders also possess tracheae, a system of tubes for gas exchange.
Spiders Versus Insects
Spiders are often mistaken for insects, but a fundamental difference lies in their body segmentation. Insects are characterized by a three-part body plan: a distinct head, thorax, and abdomen. Each of these insect segments serves specific functions, and their legs are exclusively attached to the thorax.
In contrast, spiders are arachnids, and their body is divided into two segments: the cephalothorax and the abdomen. This fused head and thorax in spiders bears all eight legs, distinguishing them from the six-legged insects.
Insects also possess antennae and often wings, features absent in spiders. This difference in body segmentation is a primary characteristic used by biologists to classify these two groups of arthropods.