What does '$a$' represent in the given figure?

Blood does not transport oxygen in

Which of the following statement$(s)$ is/are correct regarding the respiratory system of a cockroach?
$(i)$ It consists of a network of trachea that open through $12$ pairs of small holes called spiracles present on the lateral side of the body.
$(ii)$ Thin branching tubes carry oxygen from the air to all the parts.
$(iii)$ The opening of the spiracles is regulated by sphincters.
$(iv)$ Exchange of gases takes place at the tracheoles by diffusion.

How many spiracles are present in a cockroach (in $pairs$)?

The conglobate gland occurs in:

Phallic organs in cockroach are related to