War Chests, The Caterpillar’s Way

Defenses From Predation Used By Lepidoptera Larvae

Posted on: February 17th, 2021

Sheltering-in-Place Blog Entry #42

Once, during our beach stay, I rescued a kitten that kept me company. The metal railing where I encountered the caterpillar of the story can be seen in the background.

When I was growing up, my family would spend a couple of weeks of our summer vacation in a rental beach house on the Caribbean coast of Venezuela, north of Caracas, my home town. For several summers, my little sisters and I enjoyed the warm and dangerous waves of the ocean. We ran the gauntlet of scalding-hot sands to reach the cooler areas, baked our small bodies red in the tropical sun and had our burned skins lathered pink with Calamine lotion in the evenings at the house. Mosquitoes were kept at bay by the smoking coils of Plagatox placed under our hammocks. While the adults played “bolas criollas” (the Venezuelan version of bocce ball), I would spend my non-beach hours exploring the neighborhood’s dry forest/chaparral-like grounds. On one of those explorations, I accidentally fell prey to one of the most passive-aggressive species of caterpillars known to the American tropics.

I was skipping down the steep grade on a set of cement steps, keeping my balance by running my hand on the metal pipe used as a handrail. My hand slid into an unnoticed small hairy mass on the tube, and the effect was instantaneous. A sharp prick was closely followed by an explosive and excruciating burning sensation on my index finger. My screams attracted the immediate attention of my father, mother, and half a dozen other adults (I guess I was very loud at setting off the alarm). My dad picked me up in his arms and examined me for the grievous injury I was screaming about, but nothing seemed amiss, except that I continued to scream in pain, holding my hand close to my chest. Finally, my hand started to swell and redden, but the culprit responsible for my injury remained undiscovered and invisible to all.

One of  several species of puss caterpillars (family Megalopygidae) that grace our continent.

Many years later, I was able to identify the likely suspect, a species of pus caterpillar (family Megalopygidae), considered to be perhaps the most poisonous group of caterpillars in the Americas. Also known in the US as flannel moths, the caterpillars are deceivingly cuddly, their bodies shrouded in a mantle of long and silky hairs that hide underneath a formidable set of glass-like, poison-filled spines that break on contact, injecting the hapless victim with a potent toxin. Birds and insectivorous mammals have learned through thousands of years of evolution to avoid feeding on or handling these and other species of caterpillars, given the effective and painful results of their power. About a dozen Lepidoptera families (butterflies and moths) have caterpillars that produce or accumulate a bewildering variety of toxins, poisons, and dangerous chemicals, all used in the constant battle to avoid predation.

Through my many travels in the tropics, I have encountered and photographed various toxic caterpillars— and even experimented a little with trying to overcome the painful memories of those meetings. The combination of lethal or semi-lethal poisons, brilliant colors, and bewildering forms is endlessly fascinating, and I’d like to share some of the most spectacular ones in this fun mini-survey of caterpillar weapons.

The larvae or caterpillar of the monarch butterfly, here feeding on milkweeds, is brightly colored and easily recognized. The adult visits the milkweeds to lay her eggs and feed on nectar.

Toxicity as a defense mechanism is well studied and reported. One of the best known is the beautiful Monarch caterpillar (Danaus plexippus). As they feed on the white sap-filled leaves of milkweeds (a diverse group of plants in the genus Asclepias), these caterpillars accumulate compounds called glycosides in their bodies. Milkweeds produce these glycosides to defend themselves from herbivores (plant eaters), but monarch caterpillars evolved to tolerate the poisons and accumulate them in their bodies, making them inedible to predators such as birds. Even the adult monarchs, with their attractive colors that make them conspicuous to birds, retain the poison. If a bird tries to eat an adult monarch or a caterpillar, it may become sick but not die. From then on, the bird will avoid monarchs (and other non-toxic species that mimic monarchs, but that’s another story). It’s a lesson well learned.

Here are a few of the thousands of species of caterpillars that use spines and toxins to defend themselves from predators. Some are mildly poisonous or non-toxic but the spines are sharp enough to deter attacks. Others have spines that break off and inject powerful venoms.

Perhaps the most noticeable feature of toxic caterpillars is their spines and hairs. There is a bewildering diversity of spines, hairs, and prickly appendages that make many species of caterpillars toxic, difficult to swallow, or both. When combined with warning colors (various bright shades of red, yellow, black, white, and others), the combination is memorable and effective. “Don’t touch! Don’t eat! Stay away from me!” One would be wise to heed the warning. Many of these hairy and spiny caterpillars pack a punch of toxins in their spines and hairs. Some can cause allergies (hairs and scales can be shed and get in the eyes or stick to the skin). Others literally inject the toxins from spines that break on contact. They can be painful and the effects can last from minutes to days.

A sampler of caterpillar colors and textures that warn potential predators. We’ll discuss the exquisite variety of forms of caterpillars in another essay.

Finally, warning colors (technically called “aposematic”) are a commonly found strategy in many caterpillars and snakes, frogs, and other species. Wild predators learn to avoid venomous or toxic prey by recognizing their looks and colors. Many non-toxic species have evolved to copy or mimic these color patterns and even hairs and spines, benefiting by visual association with the look-alike but toxic species.

Another species of puss caterpillar like the one I encountered. The hair looks silky enough to touch. But…under that silky hair hide clusters of fragile, glass-like spines filled with venom that produces intense pain on contact.
One of my students at the La Selva Biological Station was very tempted to touch the hairs, even with a stern warning. He did it as an experiment, touching the caterpillar oh-so delicately. These were the results. A burning, very sore finger, all from three minute but very effective broken spines.

Back to the initial encounter. I was in excruciating, throbbing pain for three sleepless days and nights, tended to by my mother and older cousin with ice and the ever-present Calamine (I’ll never forget the color and smell). The pain and swelling finally became bearable, and I resumed my explorations with an increased dose of mindfulness. My father eventually found the caterpillar’s crumpled hairy mass around the area where I had crushed it with my hand. He recognized it as a “gusano pollo” (literally “chicken worm;” go figure how common names get started) and showed it to me so I could be more careful and aware of their power for the rest of my life.

Here are some North American toxic caterpillars. Google any of these names to get lots of photos and information on recognizing, avoiding, and treating these species’ stings.

  • Dorsal view (from above) of one of several species of saddleback caterpillars (family Limacodidae) found in the Americas. Those tufts of hairs along the sides — and the clusters of spines on the front and back — are incredibly toxic and will give you a painful burn.

    Buck Moth (Hemileuca maia)
    Io Moth (Automeris io)

  • Puss Caterpillar (Megalopyge opercularis)
  • White Flannel Moth Caterpillar (Norape ovina)
  • Saddleback Caterpillar (Sibine stimulea)
  • Hag Moth (Phobetron pithecium)
  • Stinging Rose Caterpillar (Parasa indetermina)

Carlos L. de la Rosa
Executive Director



Cute toxic caterpillars

Is that a toupee?

Guide to common US toxic caterpillars

More stinging caterpillars to learn