Is it true that insects can hear even though they don't have ears?
It is true, that many insects can "hear" but don't necessarily have "ears" like mammals.
Many insects have good hearing, but as in almost all cases, not all.
Hearing sensory organs are also not necessarily located on their heads (e.g. crickets have them on their legs!).
Hearing in organisms is another example of an evolutionary adaptation - if it helps an organism survive, it probably emerges as a sense organ. If its not necessary, then these sense don't emerge (for example, plants don't need to hear to survive, so hearing never evolved).