Question

Is there any aquatic insect? Do they breathe through spiracle?

Answer 1

Very few insects are truely aquatic and they are beetles, but there are insects with aquatic larva in life cycle.
Aquatic beetles possess trachea like all other insects, and yes, they breathe through spiracles.

Explanation:

Several aquatic insect larvae are there, of which only few are truely gill breathing. These are naiads (aquatic larvae) of dragon fly, damsel fly, stone fly and may fly.

On the other hand aquatic larvae of mosquito breathe air through siphon.

Openings of spiracles could be contracted, are often surrounded by hairs. Spiracles help in exchange of respiratory gases between respiratory system of insect and the environment.