The long hairs are urticating, or irritating. The hairs repel ants, and have caused numerous miscarriages in horses. Photo by Jeff Mitton
Soft and lustrous silk is used for royal robes and wedding gowns. But tent caterpillars use silk to build defensive fortresses, and to pave paths to lead them to food and back to safety. At this time of year, silk tents adorn aspen, cottonwoods, mountain mahogany, and currants.
Four species of tent caterpillars occur in Colorado, but the two most common are the western tent caterpillar, Malacosoma californicum, and the forest tent caterpillar, M. disstria. The western tent caterpillar defoliates an occasional tree, but forest tent caterpillars can irrupt to cause epidemics that defoliate and kill large expanses of aspen.
In mid summer, a female attaches 100-500 eggs in a band around a slender twig of a deciduous tree, such as an aspen or cottonwood. The eggs begin to develop immediately, and caterpillars are fully formed in three weeks. However, the caterpillars wait until spring to hatch out.
The caterpillar, encased in an egg near the end of a branch, must survive the cold of full winter. At high elevations or high latitudes, the coldest temperatures might plunge below –40 degrees. How do the caterpillars avoid freezing? During summer a caterpillar is about one percent glycerol, a molecule similar to glycol, which we use as antifreeze in our cars. As winter approaches, the entombed caterpillar produces more glycerol, and when winter arrives, the caterpillar is more than 1/3 natural antifreeze.
A caterpillar has a small head followed by 13 segments. Each of the first thee segments has a pair of legs. The next 10 segments contain the stomach, intestines, and silk glands.
On each side of the head, beside the mouth, are two semicircles of 6 eyes, each with two lenses. The caterpillar can only see about ¼ inch—all more distant objects are a blur.
After hatching, a caterpillar waves its head back and forth, extruding a fine thread of silk. Silk is produced by two long cylindrical glands that run the full length of the body, and are plumbed to a spinneret below the mouth. Threads are strung from branch to trunk, forming a mat. More caterpillars assist in the task, creating a multi-chambered structure with small passageways (see my website photos). This tent will be used by the caterpillars during the day, when they are not feeding, and when they are molting. Predators are unable or unwilling to penetrate the walls of strong, adhesive silk.
Caterpillars leave the safety of the tent to eat expanding buds and emerging leaves. Their world is limited to their host tree, and tree branches define their routes to food and back to safety. But caterpillars are so near-sighted that they cannot see their destination. As each caterpillar exits the tent, it pays out a silk thread, a life line; numerous foraging caterpillars pave a path of silk. The caterpillars use smell to distinguish their silk from the silk in spider webs.
The blue spots on tent caterpillars are not produced by pigments, but are Tyndall blue, a structural color produce by mats of micro-sculptured, transparent hairs. The hairs let most wavelengths of light pass through to the black pigment underneath, where they are absorbed. The hairs scatter blue wavelengths, so we see blue spots.
The bright blue, orange, black and white of the caterpillars advertise danger, even for animals as large as horses. The long hairs are urticating, or irritating. When pregnant mares come in contact with eastern tent caterpillars, they eat them. The caterpillar hairs perforate the mare’s intestines, and are subsequently distributed throughout the circulatory system, threatening the health of the mare and the life of the embryo. In the spring of 2001, in the Ohio Valley, 2,000 mares suffered miscarriages after eating tent caterpillars.