The origins of Halloween began in the time of ancient Celts when summer ended and the time of harvest gave way to approaching winter. The start of the Celtic New Year on November 1 was a scary time as spirits of the dead returned to earth on the eve of October 31 to visit family and engage in frightening tricks. Orange and black became the colors of Halloween, orange representing the color of harvest and later the Jack O’ lantern, and black symbolizing the color of death. How fitting it is that we visit one more insect in orange and black this Halloween week.
In the last two episodes we met the beautiful but potentially deadly milkweed bug and its Batesian mimic the milkweed leaf beetle, insects that consume milkweed and dress in orange and black to send this warning to would-be predators, “eat me at the peril of death.” Milkweed gets its name from the sticky white sap exuded from stems and leaves when their surface is broken by hungry insects or curious humans. Milky sap and cells within the leaves contain nasty chemicals called cardiac glycosides.
To disable the latex defense of the milkweed, the longhorned beetle snips the leaf veins before dining on the nutritious leaf tissue.
As the name implies, these compounds have something to do with the heart. At high concentrations, cardiac glycosides can be heart poisons bringing death to humans and other animals that eat them. In this way, milkweed wards off attack by a horde of hungry herbivores. However, insects that eat milkweeds, including monarch and milkweed tussock moth caterpillars, are clever and have evolved ways to detoxify or avoid the toxic effects of these poisons and utilize these noxious chemicals for their own defense.
Many of the insects that live on milkweed and consume its leaves display vivid patterns of orange and black as both juveniles and adults. This convergence on a similar, easily recognizable color pattern by two or more nasty tasting insects is called Müllerian mimicry. In addition to cardiac glycoside defenses, milkweed plants have one more trick up their leaves. The gummy latex that is released by the plant becomes very sticky when exposed to air, so much so that it may actually gum-up the mouthparts of insects that chew milkweed leaves making it impossible for them to open their tiny mouths.
How do milkweed eaters deal with this sticky defense? Enter the gorgeous milkweed longhorned beetle who, like the monarch and milkweed bug, stores cardiac glycosides from the milkweed and is a full-fledged member of the milkweed Müllerian mimicry gang. The milkweed longhorned beetle is so named for the amazing long antennae that characterize this family of beetles. This masterful leaf muncher has a good trick to disable the latex defense of the milkweed. Prior to eating the leaf, the beetle uses its powerful jaws to carefully snip leaf veins that are responsible for transporting latex throughout the leaf. This prevents the latex sap from flowing to the blade of the leaf where the beetle takes its meals. With the sticky defense disabled, the milkweed longhorned beetle is free to devour the leaf without fear of gummed-up jaws.
After snipping leaf veins below, milkweed longhorned beetles consume leaf tissue and blossoms without fear of gummy mouthparts.
Remember, if you dress in orange and black this Halloween, you will become a Batesian mimic of monarchs, milkweed bugs, and milkweed longhorned beetles and you surely will avoid being eaten by predators. But please don’t eat milkweeds to become a Müllerian mimic! However you dress, have a happy and safe Halloween.
Dr. Shrewsbury, who regularly dons orange and black, served as the inspiration for this Halloween story. Information for this episode came from the “The Love of Insects” by Thomas Eisner and “Deactivation of plant defense: correspondence between insect behavior and secretory canal architecture” by Dave Doussard and Bob Denno.