Final answer:
The tick is not an insect; it is an arachnid. Dragonflies, fireflies, and deathwatch beetles are all insects as they exhibit typical insect characteristics like having six legs and undergoing metamorphosis.
Step-by-step explanation:
The question pertains to arthropod diversity and specifically focuses on identifying which of the listed options is not an insect. Insects are classified within the class Hexapoda and are characterized by having a three-part body consisting of a head, thorax, and abdomen with six legs, as well as undergoing various forms of metamorphosis during their life cycle. From the provided options, we know that dragonflies, fireflies, and deathwatch beetles fit within the insect classification due to characteristics such as their winged adult stages and metamorphosis patterns. On the other hand, a tick is not an insect but rather belongs to the class Arachnida, which includes spiders and scorpions, characterized by having eight legs.