SKY ISLANDS, DESERT SEA-Part II, Hornworm Highway

By Arthur V. Evans

Charged by the onset of the summer monsoons in July or August, the arthropods of Arizona’s Sky Islands and desert seas are stirred into action.  The sudden and intense infusion of life-giving moisture triggers a flush of activity: eggs hatch, hungry larvae gobble up new leaves, adults are released from their earthen or wooden chambers, eager to mate and reproduce.  It is this marvelous intensity of arthropod activity that has drawn me to the mountains, desert scrub and grasslands of southeastern Arizona for nearly 40 years.

My earliest impressions of Arizona’s desert seas were formed by numerous overland trips from California in the 1970’s. Within minutes of crossing the Colorado River, the first saguaros would greet us, stationed like lone sentinels high on the rocky ridges of the Dome Rock Mountains. Although these giant columnar cacti have become symbolic of all arid regions of the southwest, they are strictly indigenous to the Sonoran Desert. These and other nearby desert ranges are capable only of supporting plants and animals adapted to fleeting amounts of rain. The summer monsoons, even at their height, seldom penetrate this far north and west.

Driving east from Gila Bend to the Maricopa Mountains, the stands of saguaros become taller and denser. Compared to the deeply pleated trunks of their western brethren, the almost bulging flesh of these plants is a clear sign of increased precipitation. The saguaros’ spongy inner tissues rapidly expand to absorb and store seasonal supplies of water as a hedge against the inevitable drought ahead.  Even the spiny ocotillo stand taller and greener here, surrounded by dense thickets of palo verde.  Here all living things enjoy the increased benefits of living under the blanket of the summer monsoons.

Another sure sign of increased rainfall is the sporadic population explosion of green and black-striped hornworms, caterpillars of the white-lined sphinx moth, Hyles lineata. Dozens to hundreds of these insects race across the hot, blistering highway in a scramble for tender desert greens. Some years there are so many of the caterpillars that the pavement becomes slick with their crushed bodies. At night marauding three-inch-long shield-backed katydids of the genus Capnobates rip chunks of sun-dried caterpillar from the road and grind them up with their powerful jaws, while scores of ants carve up the leftovers and carry them back to their underground brood.

© 2010, A.V. Evans

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