Cut open a potato and leave it exposed to the air. The potato will turn from white to brown. This is due to an enzyme, polyphenol oxidase that leads to production of a dark pigment, melanin. Wound a caterpillar and the hemolymph cells (insect blood) will plug the hole and produce melanin. The caterpillar will have a black spot at the site of the wound. Phenol oxidase reactions are found in most plants and animals. Although the name may be unfamiliar, we are familiar with the phenomena. Phenol oxidase reactions produce the brown color of tea and coffee. Phenol oxidase reactions produce the defensive “ink” of an octopus.
Phenol oxidase has an important role in insect defense against pathogens. At the site of a wound, phenol oxidase will generate reactive chemicals. These chemicals react with the cells of bacteria and other pathogens present at the wound site. Some of the insect cells at the wound site are affected as well. Once released, the reactive chemicals that do not react with cell components, will polymerize to form melanin. Melanin at the site of an insect wound is evidence of the pathogen fighting process.