The term “bug” is often used casually to describe any small creature that crawls or flies, leading to frequent confusion about spiders. While spiders fit the general, non-scientific description of a “creepy-crawly,” they are definitively not bugs in a biological sense. The differences between spiders and insects—the class that contains true bugs—are fundamental, separating them into distinct evolutionary groups. Understanding these distinctions requires examining the specific anatomical traits that define each class of animal.
Spiders Belong to Arachnida
Spiders are members of the Class Arachnida, a classification that also includes scorpions, ticks, mites, and harvestmen. All arachnids belong to the Phylum Arthropoda, meaning they share characteristics like an exoskeleton and jointed appendages with insects and crustaceans.
The spider body is divided into two main segments, known as tagmata. The front section is the cephalothorax (prosoma), which is a fusion of the head and thorax. The rear segment is the abdomen (opisthosoma), which contains the digestive and reproductive organs, as well as the spinnerets used for silk production.
Spiders lack antennae, which are a defining sensory feature of insects. Instead, spiders possess specialized mouthparts called chelicerae, often equipped with fangs for injecting venom, and a second pair of appendages called pedipalps. These pedipalps are used for manipulating prey, sensing the environment, and in some cases, for reproduction.
Key Differences Between Spiders and Insects
The most immediate way to distinguish a spider from an insect is by counting the number of legs. Adult spiders possess eight walking legs, arranged in four pairs, all attached to the cephalothorax. In contrast, all adult insects have six legs, organized into three pairs attached to the thorax segment.
Another major difference is the number of body segments. Spiders have only two body parts—the cephalothorax and the abdomen—while insects are characterized by having three distinct segments: the head, the thorax, and the abdomen. This three-part body plan is unique to the Class Insecta.
Insects typically have one pair of antennae for sensing chemicals, vibrations, and movement, an appendage that is completely absent in spiders. Most adult insects also possess one or two pairs of wings, allowing for flight and dispersal, but spiders are wingless.
The Meaning of “Bug”
The confusion over whether a spider is a “bug” comes from the word’s dual meaning in common language and scientific taxonomy. Colloquially, “bug” is a catch-all term used by the general public for nearly any small, terrestrial arthropod, including spiders, beetles, and worms. Scientifically, however, the term “bug” refers to a very specific group of insects known as “True Bugs,” which belong to the Order Hemiptera.
The Order Hemiptera is a large group containing over 80,000 species worldwide, including cicadas, aphids, stink bugs, and bed bugs. These true bugs are defined by a unique anatomical feature: specialized mouthparts modified for piercing and sucking. This beak-like structure, called a rostrum, allows them to penetrate plant tissue or the skin of other animals to feed on fluids.
True bugs, like all insects, still possess six legs and three body segments, which immediately separates them from spiders. While a person might use the term “bug” broadly to describe a spider, a biologist reserves that label for an insect that belongs to the Order Hemiptera.