Why do crustaceans have two pairs of antennae?

Crustaceans possess two pairs of antennae—the antennules (antennae 1) and the antennae proper (antennae 2)—as a defining characteristic of their head morphology.12

These appendages serve as critical sensory organs in adult crustaceans, equipped with various setae that function as chemoreceptors to detect chemical substances in their environment. The antennules, in particular, are often highly specialized; for instance, they house statocysts, which are essential organs that allow the animal to orient itself with respect to gravity.134

While their primary role in adults is sensory, these appendages are versatile. In the nauplius larval stage, the antennae are frequently utilized for locomotion (swimming) and feeding. Additionally, processes at the base of the antennae can assist the mandibles in manipulating and pushing food into the mouth. The presence of these two distinct pairs of sensory and functional appendages is a key feature that distinguishes Crustacea from other arthropod groups.12