Insects, and some other invertebrates, exchange oxygen and carbon dioxide between their tissues and the air<span> by a system of </span>air<span>-filled </span>tubes<span> called tracheae. Tracheae open to the outside through small holes called spiracles. In the</span>grasshopper<span>, the first and third segments of the thorax have a spiracle on each side.</span>
Answer:
While it is difficult to determine exactly how many Natives lived in North America before Columbus, estimates range from a low of 2.1 million to 7 million people to a high of 18 million.
