Which of the following is not a part of the respiratory system of Periplaneta Americana?

The chitinous exoskeleton in cockroach is made of hardened plates called sclerites.The respiratory system consists of a  network of trachea, that open through 10 pairs of small holes called spiracles present on the lateral side of the body. Thin branching tubes (tracheal tubes subdivided into tracheoles) carry oxygen from the air to all the parts. The opening of the spiracles is regulated by the sphincters. Exchange of gasestake place at the tracheoles by diffusion.