Humans have a combined respiratory and circulatory system, whereas insects do not. In humans, the air is brought in through the mouth/nose, through the windpipe, and into the lungs, the oxygen is absorbed into the bloodstream where it can be transported to muscles and organs. However, insects take in air through tiny holes in their thorax and abdomen. These are called spiracles. The oxygen is stored in many dorsal air sacs to be used. Then carbon dioxide is expelled through the spiracles.
