Insects secrete a variety of substances for purposes of communication and defense. Sometimes the best defense is communicating with the enemy. Insects may be inedible or unappetizing to predators. However, if a predator must bite the insect to discover its foul properties, the insect might not survive the attack. One function of insect glands is to present a potential predator with a “taste” of the defensive chemical prior to the predator taking a bite. Glands are one way that an insect can advertise its unpalatability.
The milkweed bug, Oncopeltus fasciatus, feeds on milkweeds that contain toxic cardenolides. These cardenolides can cause regurgitation in vertebrates and disrupt normal heart rhythm. The milkweed bugs sequester these toxins as defense against predators. The male and female adult milkweed bugs have scent glands on the thorax that secrete a variety of substances including cardenolides. As a defense, predators encountering the cardenolides may be more inclined to pass on attacking a milkweed bug.