Metamorphosis

If you had lived among the Hohokam people in this area hundreds of years ago, you would have looked forward to a bountiful harvest of hornworms at the end of monsoon season. The edible hornworms are the caterpillars, or larvae, of white-lined sphinx moths (Hyles lineata), and are named for a short spine protruding from their rear ends. This year, thousands of these fat, green caterpillars were seen along desert trails and city streets wherever there was an abundance of their favorite food, a sprawling annual plant called spiderling (Boerhavia spp.).  On the autumn equinox, there were so many trying to cross the road at Happy Valley and 33rd Drive that some sections of asphalt turned a lovely shade of chartreuse. If you were lucky enough to see them up close, you might have admired the colorful pattern of black and green stripes, accented by red spots along their sides, one of nature's most colorful works of art deco.

Still today, some indigenous people of southwest deserts gather and dry bushels of hornworms for food. In southern California, the Cahuilla people call them "piyatum." O'odham people of southern Arizona named the host plant "makkum jeej," which means "mother of the caterpillar." If you are going to try this local delicacy, it's important to pinch the head off and shake the guts out before you skewer them on a stick to roast over hot coals, according to one 19th century ethnographer who learned piyatum foraging skills from the Cahuilla. 

Lizards and birds also feast on hornworms, dead or alive, head, guts and all.

These finger-sized caterpillars are just one stage in the life cycle of an even more fascinating and beautiful moth. After they have eaten their fill, hornworms burrow into the soil and change into a brown, pod-like pupa. This resting stage varies from two weeks to six months, depending on the time of year and latitude where they live. During this time, the caterpillar transforms into a moth, a mysterious process still not fully understood by scientists. 

Longer days of spring induce moths to emerge from the pupae after spending the winter months underground. Pupae are able to perceive light and therefore, the number of hours of sunlight in a day. Here in central Arizona, there may be several cycles of metamorphosis between March and September, with moths emerging within two weeks of pupating. Additional environmental cues, including soil temperature and moisture, ensure that pupae remain underground so that the moth's emergence will coincide with flowers to feed on and the right kind of plants to lay eggs on so that caterpillars will have plenty to eat. This is a well-orchestrated miracle!

Since moths are mostly crepuscular (a fancy term for being active during shadowy hours of dawn and dusk) or nocturnal, they favor white flowers, which are more visible in low light. They are also strongly attracted by certain floral scents. Their super long tongues allow them to slurp up nectar from deep-throated flowers like evening primrose, jimson weed, columbines and night-blooming cacti. In the process, their furry bodies are dusted with pollen, which gets transferred to other flowers as they go about foraging, making them important pollinators for some species. 

As adaptable opportunists, white-lined sphinx moths often fly in broad daylight and will feed on any color of flower that has sufficient nectar. Their adroit hovering skills and rapid wingbeats make it easy to confuse a sphinx moth for a hummingbird at first glance, but the brown and white stripes on their wings and flash of orange on the underwings are unlike any hummingbird. During a moth's two to three weeklong life, their primary mission is to mate. Females may lay hundreds of eggs that hatch within a week or so to begin the cycle again.

This is all really interesting, but after consulting with several entomologists and texts, I'm still left wondering: "Why do caterpillars try to cross the road?" Is it just to get to the other side? And is it true, as some naturalists claim, that they tend to crawl north? It turns out, no one really knows. The caterpillars are probably looking for more food, or soft soil to burrow into. But wouldn't the vast corridors of hot asphalt deter them from what is almost always a suicidal journey? Fortunately, although we may notice them mostly on roads, there is still plenty of desert land for hornworms to inhabit without having to navigate through heavy traffic.