Sphinx moth caterpillars wield an eruptive defense
Sphinx months have an array of identifiers, one being an unusual defense mechanism
Cindy Burkhardt Maynard had been watching a sphinx moth caterpillar in her garden for several days, but then, it disappeared. Soon after, she saw another trundling across a dirt road on Rabbit Mountain. These were Hyles lineata white-lined sphinx moths which are the most common in this area. As caterpillars progress through 5 developmental stages or instars, their colors and patterns change. In addition, their background colors change from dark green in eastern states to light green in the mountains west to yellow in California. Due to this variability, it can be difficult to identify sphinx moth species from the caterpillar stage alone.
Color patterns of white-lined sphinx moth caterpillars vary with age and geography. Photos by Cindy Burkhardt Maynard.
A caterpillar transforms into a colorful moth with a heavy body about 3 inches long and a wingspan of about 4 inches. Against a brown background, the wings and body have stripes and zones of black, white and salmon or pink.
You may have seen hawkmoths but mistaken them for hummingbirds. Hummingbirds and hawkmoths are about the same size, both are active at dusk and both hover while taking nectar from flowers. Sphinx moths can fly up to 30 mph and while hovering they beat their wings 41 cycles per second (up and down), so fast that they make a buzzing sound.
If I am not thinking critically, the plight of a sphinx caterpillar evokes my sympathy. Imagine—a very large, soft caterpillar that does not bite or sting, devoid of poisonous spines and urticating (barbed) hairs, unable to fly or run away. Admittedly, the colors and patterns of some sphinx moths provide camouflage, but if they cross a road or path or patch of sand, the camouflage fails them. The sphinx caterpillars are also called hornworms due to an ominous, threatening horn projecting upwards from their posterior end, but it is all bluff--the horn is harmless.
Caterpillars of many butterfly species consume plants that have toxic defensive chemicals, which they sequester to specific tissues, such as fat tissues. Predators quickly learn that these caterpillars are toxic and thus inedible. However, the sphinx moths do not either transport or sequester plant molecular defenses. They utilize plant molecular defenses but with another technique.
Endoparasite pupae spill from a sphinx moth puparium.
If a predator approaches a sphinx caterpillar, the caterpillar rears its head in an unmistakably threatening way. If the predator persists and approaches closer, the caterpillar engages in projectile vomiting, spewing a fetid slop of semi-digested food laced with the toxic compounds synthesized by the plants it was eating. The predator, now drenched with a stinking and stinging slime and probably suffering impaired vision and sense of smell, would likely lose its concentration and break off its attack.
Even if all predators can be dissuaded, the caterpillar faces more challenges. A large number of wasps and flies are endoparasitic, meaning that the adult female injects an egg beneath the skin of a caterpillar. If you have ever been stung by a wasp, you know that the wasp can sting while swooping by, without landing. Endoparasitic species may insert one or many eggs, dooming the caterpillar. Endoparasites remind me of "Alien" a horror sci-fi movie that appeared in 1979.
A neighbor was removing a lilac bush when he found a sphinx pupa. After completing all of its instars, it had burrowed into the roots of the lilac and transformed into a pupa. My neighbor showed me the pupa, and we agreed that I would give it loving care so that we could identify the species when the moth emerged as an adult. Neither of us was aware of the load of endosymbiont eggs that were embedded in the pupa. In late summer, too much time had passed, and I knew something was wrong. I cracked open the puparium and endosymbiont pupae spilled out. They had completely consumed all the moths, leaving the sphinx puparium fully packed with endosymbiont pupae.
To identify the species of endosymbiont, I took the capsules to the laboratory of Deane Bowers, a CU entomologist whose lab could rear the endosymbionts. But the endosymbionts were dead and thus unidentified, so I am unable to end this story as I had hoped.
I mentioned that I had sympathy for sphinx moths for they had few defenses against a horde of enemies and thus each had a low probability of surviving to reproduce. An adult female sphinx moth can lay up to 1,000 eggs, but if her population is neither shrinking nor expanding, the average number of eggs that survive to reproduce is two, one for herself and one for her mate. Sphinx moths run a gauntlet to complete their life cycle, but this is the way it is for all species.
Top photo: A sphinx moth hovers while choosing its next nectar source.
Did you enjoy this article? Passionate about ecology and evolutionary biology? Show your support.