Final answer:
Insects, crustaceans, and arachnids all have jointed appendages, which are specialized limbs used for various functions.
Step-by-step explanation:
Insects, crustaceans, and arachnids are similar in that they all have jointed appendages. Jointed appendages are specialized limbs that arthropods use for various functions such as sensing their environment, capturing and manipulating food, and for walking, jumping, digging, and swimming. These appendages are a key characteristic of arthropods and contribute to their success and diversity in the animal kingdom.