The opening to the tracheal system (respiratory system) in insects is the


spiracle.

trachea.

alveolus.

bronchus.

stomata.

The opening to the tracheal system in insects is called a spiracle.

To arrive at this answer, one can use the process of elimination by considering the options provided and eliminating those that are not associated with the opening to the tracheal system.

- Trachea refers to the airways in vertebrates, not insects.
- Alveolus is a term used to describe the small air sacs in the lungs of mammals, again not relevant to insects.
- Bronchus is a part of the respiratory system of vertebrates, not insects.
- Stomata are small openings on the surface of plant leaves, not related to insects' respiratory system.

By eliminating these options, we are left with the correct answer, which is spiracle.