What structural component is a defining trait shared by all members of the Phylum Arthropoda?

A hard exoskeleton made of chitin that must be periodically shed to allow growth

The Phylum Arthropoda encompasses a vast diversity of creatures, including insects, spiders, and crustaceans, all unified by fundamental structural characteristics. The most defining feature shared across this phylum is the presence of a tough outer cuticle, composed primarily of chitin, which functions as a rigid exoskeleton. Because this external shell cannot expand as the organism grows, members of Arthropoda must undergo a process called ecdysis, or molting, where the old exoskeleton is periodically shed to permit an increase in size. This jointed, external support system and the necessity of periodic shedding for growth are hallmark features of all arthropods.