Why do chelicerates such as spiders and scorpions lack antennae?

Chelicerates, including spiders and scorpions, lack antennae because they belong to the subphylum Chelicerata, a group defined by a specific body plan that does not include these sensory appendages.1

In the chelicerate body plan, the first pair of appendages are the chelicerae, which are positioned in front of the mouth and are typically used for feeding or defense. Because these appendages occupy the anterior position where antennae are found in other arthropod groups (such as crustaceans or insects), chelicerates do not possess them.213

Instead of antennae, spiders and scorpions rely on other specialized structures to interact with their environment. They utilize pedipalps as organs of touch and possess various sensory receptors, or sensilla, located on their body surface and legs. For instance, spiders and scorpions have slit sense organs in their exoskeleton that allow them to detect vibrations in their surroundings, such as those produced by approaching prey.24