Both spiders and insects belong to the phylum Arthropoda, a diverse group characterized by exoskeletons, segmented bodies, and jointed appendages. Despite their common arthropod ancestry, spiders and insects exhibit fundamental differences in their body plans, sensory organs, feeding mechanisms, and other specialized features. Understanding these distinctions clarifies why spiders are classified as arachnids rather than insects.
Body Plan Differences
A primary distinction between spiders and insects lies in their fundamental body organization. Spiders typically have two main body segments: a fused head and thorax called the cephalothorax (or prosoma) and an abdomen (or opisthosoma). In contrast, insects possess three distinct body parts: a head, a thorax, and an abdomen.
Another clear difference is the number of legs. Spiders have eight legs, arranged in four pairs, all attached to the cephalothorax. Insects, on the other hand, have six legs, or three pairs, always attached to their thorax. Insects commonly feature antennae on their heads for sensing their environment, a structure entirely absent in spiders. While many insects have wings, enabling flight, spiders lack them.
Sensory and Feeding Apparatus
Spiders and insects differ in how they perceive their surroundings and consume food. Spiders have multiple simple eyes, known as ocelli, often numbering eight and arranged in various patterns. Insects have large, complex compound eyes, which can be supplemented by simple ocelli.
The mouthparts of these two groups differ significantly. Spiders have chelicerae, jaw-like appendages tipped with fangs. These fangs are typically hollow and inject venom into their prey, liquefying the internal tissues for consumption. Insects, conversely, have a wide array of mouthpart types adapted to their specific diets, such as mandibles for chewing in beetles or grasshoppers, piercing-sucking mouthparts in mosquitoes, or a coiled proboscis for siphoning nectar in butterflies.
Specialized Features
Spiders possess unique adaptations that distinguish them from insects. Spiders are well-known for their ability to produce silk from specialized organs called spinnerets on the underside of their abdomen. This silk serves various purposes, including building webs for capturing prey, constructing egg sacs, and aiding in movement. While some insect larvae, like silkworms, produce silk for cocoons, it is not a defining or universal characteristic of adult insects in the same way it is for spiders.
Their respiratory systems also differ significantly. Spiders primarily use book lungs, internal structures with thin, leaf-like plates resembling pages of a book, for gas exchange. Many spiders also have a tracheal system, with tubes carrying oxygen directly to tissues, often alongside their book lungs. Insects rely exclusively on a complex network of tracheae, internal tubes that open to the outside through small pores called spiracles, allowing for direct gas exchange throughout their bodies.