Tubercles are small rounded outgrowths present on some species of caterpillars. Tubercles are sometimes a defining characteristic of a caterpillar. For example, the gypsy moth caterpillar, Lymantria dispar, has rows of tubercles, five blue rows anterior and 6 red rows posterior. The long irritating hairs of the gypsy moth caterpillar arise from the centers of the tubercles.
The tubercles provide support for the hairs and allow them to stand erect and away from the body, which may be useful in predator deterrence. What is the reason for the color? Why are they red and blue? Why not a single color? Are the colors a signal to predators to avoid eating the caterpillars? Presumably, the tubercle color is due to pigments and not structural color. What are the pigments and why does the caterpillar produce red in the posterior and blue in the anterior? Perhaps someday we will learn the answers to these questions. However, the answer may have to wait while questions of a higher priority are solved first.