At each different stage of development, the young will eat different foods. The larvae may eat stems, the nymph and adult may eat leaves.
According to Wikipedia the answer is, "Wound healing. Wound healing is a complex process in which the skin, and the tissues under it, repair themselves after injury. ... This process is divided into predictable phases: blood clotting (hemostasis), inflammation, tissue growth (proliferation), and tissue remodeling (maturation)."
It Could be Carbon Dioxide,Carbon Monoxide, Sulfur Dioxide,Nitrogen Oxides, or Particulate Matter.