The March of the Hornworms

Hard to see the ears

White lined sphinx moth
Photo courtesy Wikipedia

If you’ve been out in the desert in the past few days you will have noticed one of the last big shows of the monsoon season, the march of the hornworm caterpillars. Hard to miss, the hornworms are three inches long with lime green and black stripes and horns on their cabooses.

This is the larval stage of the white lined sphinx moth. Near the end of the summer rains the season’s last generation of hornworms creep off in every direction in a mass dispersal. It is believed this migration is sparked by shortening days or changes in humidity.

The caterpillars I’ve seen were munching on spurge like weeds, but their main interest is to put distance between themselves and the local ‘hood and to find some soft soil where they can burrow in and form a pupa. Inside their cozy casing they develop wings and a new form. In less than two weeks the good sized moth emerges, flexing its impressive three and a half inches of wingspan.

Sphinx moths feed day and night, hovering by flowers and unrolling their long proboscis to reach the nectaries of the deepest blooms. They are often mistaken for hummingbirds.

Sphinx moths are important pollinators in the Southwest. They are also a food source for bats. The moths have ears on both sides of their abdomens that tune into the frequency emitted by hunting bats. This allows them to detect the hunters and attempt evasive action.

Moths link up for mating by sending out air-borne chemical signals called pheromones. After mating the females lay tiny eggs. These eggs hatch into caterpillars that shed their skin as they grow. You will see the smallest caterpillars are a very light green and blend in well with their surroundings. Each new size of caterpillar is called an instar. Creeping, but not creepy!


4 responses to “The March of the Hornworms

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  2. How cool…..I learned about instars….have never run into that term before….yippee!

  3. Awesome educational info! Now I know what all those caterpillars I see along the trails are! Thanks!

  4. I have got some spurge I would willingly donate to attract a caterpillar or two!

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