Trachea of a tiger moth. Scanning electron micrograph (SEM) of a trachea (centre) of a garden tiger moth caterpillar (Arctia caja). Air passes into spiracles (breathing pores) on the surface of the insect before reaching the tracheae, respirat- ory tubes, in the caterpillar's body. The diameter of the tubes becomes ever smaller, until they form tracheoles. In insects, oxygen is not carried in the blood but diffuses across these tracheal tubes directly into each cell. Tracheae are ingrowths of the body wall made of chitin. Spiracles occur in pairs on either side of most body segments. Magnification: x6300 at 6.5x9.5 inch size.