What term describes something short-lived, such as an insect that lives only for a day?

The term "ephemeral" specifically refers to something that lasts for a very short period of time. It derives from the Greek word "ephemeros," which means "lasting a day." This makes it an apt choice for describing an insect that has a life cycle of just one day. The concept of ephemerality can apply to other phenomena as well, such as fleeting moments, transient flowers, or brief events, which aligns with the notion of things that are temporary and not lasting.

In contrast, the other options signify different attributes. "Transient" refers to something that is temporary but may imply a longer duration compared to "ephemeral." "Perpetual" conveys the idea of something that lasts indefinitely or is eternal. "Continual" suggests something that happens frequently or repeatedly but does not necessarily imply a brief duration. Therefore, "ephemeral" is the most precise term to describe something that is fleeting or exists for a very short time, making it the correct answer.