Which one of the following has an open circulatory system?

The color of the hemolymph found in a cockroach is .....

Malpighian tubules remove excretory products from:

What is the similarity between the trachea of a cockroach and a rabbit?

Which of the following statement$(s)$ is/are correct regarding the digestive system of cockroach?
$(i)$ Mouth opens into a short tubular pharynx,leading to a narrow tubular passage called oesophagus.
$(ii)$ Gizzard helps in grinding the food particles.
$(iii)$ $A$ ring of $6-8$ blind tubules called hepatic or gastric caecae is present at the junction of foregut and midgut,which secrete digestive juice.
$(iv)$ At the junction of midgut and hindgut is present another ring of $100-150$ yellow coloured thin filamentous Malpighian tubules which help in removal of excretory products from haemolymph.

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