Friday, August 14, 2009
Biodiversity Corner - Summer Insects
Two of the joys of the natural world in the dog days of summer are the primal buzzing sound of cicadas and the occasional glimpse of a giant silkworm moth. One, the inescapable background buzz of the cicadas, is accessible to all. The other is a rare privilege.
Polyphemus moth: Jean Keskulla on Concord Street, and Reed and Zahara Savory on Fern Lane were each treated to a sighting of the huge Polyphemus moth on July 26. The Polyphemus spends less than a week of its life cycle as an adult moth. The adult doesn’t feed but sustains itself entirely on the body mass built up during the caterpillar stage of its life. In the short adult stage, usually on the first evening after emerging from the cocoon, it will mate and the female will deposit her eggs. Seize the night! The tight schedule is made possible by a very potent pheromone produced by the females. A millionth of a gram is all that is needed to attract males from the other side of town. The male Polyphemus moth, sensitive guy, is equipped with very broad feathery antennae with which to sense the pheromone and locate the female.
The moth is classified as Antheraea polyphemus. It is a member of the giant silkworm moth family and was named after the mythological one-eyed giant son of Poseidon, probably because of the single large eye spot on each of the hind wings. The moth is distinguished by its large size (a typical wing span is just under six inches), its furry body, large hind wing eye spots, smaller fore wing eye spots, and reddish to yellowish brown coloring. Both pairs of wings have a pinkish line near the margin.
The caterpillars are stereotypically voracious. They have a lot of potential food plants including oak and maple but birch, rose and willow are among the preferred. After growing to almost three inches in length, the caterpillar spins a cocoon and remains in the pupal form until the following year.
Cicada: On July 22 Marjorie Johnson found one of the first cicadas of this summer in her yard on Ember Lane. There are 157 species of cicada in the U.S. so I sent a photo of it to bugguide.net where it was identified as a female Tibicen lyricen, the lyric cicada. It is a large
wedge-shaped insect about two inches long overall including the wings. It is black, brown and green with largely transparent wings and very dark eyes. Cicadas in the genus Tibicen are collectively known as dog-day cicadas. Those in the genus Magicicada are known as “periodical” cicadas and have life cycles of 13 years or 17 years. The dog-day life cycle is not known for sure but unlike the periodical cicada, the broods overlap such that adults appear every year.
Male cicadas buzz by vibrating a drumskin-like membrane. The sound is amplified in hollow chambers in the abdomen. (Grasshoppers, locusts, crickets, and katydids, all of which are unrelated to cicadas, make their sounds by rubbing body parts together, like a leg against a wing.) The cicada buzz attracts other males who contribute their buzz until the chorus is strong enough to attract females. The buzz is slightly different for each species. Experts can identify species by ear. The rest of us can see the differences in audiospectrographs.
Female cicadas cut slits in twigs in which they lay their eggs. The eggs hatch in about a month and the nymphs fall to the ground where they burrow into the soil and feed on roots. As they grow they shed their skins four times all the while remaining underground. When they are ready for the fifth and final molt,
they dig their way out of the ground and look for something to climb which they clasp while they break their way out as an adult. You are quite likely to find these skins remaining on a tree trunk or on a stalk in the garden, long after the cicada has emerged. The empty skin is pale brown and has a slit down the center back. It doesn’t seem large enough to contain a cicada but the wings are soft and tightly folded until it emerges.
Sources: Borror and DeLong’s Introduction to the Study of Insects; Caterpillars of Eastern North America, David L. Wagner; Insects, Their Natural History and Diversity, Stephen A. Marshall; www.butterfliesandmoths.org; ; A Guide to Observing Insect Lives, Donald W. Stokes.
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