Monday, May 11, 2009

Webspinners


One of the most fascinating and unique insects to be found here in Tucson is the “webspinner.” They even have their own order, the Embiidina (formerly Embioptera). Adult males like the one pictured above are drawn to lights at night, and at under 10 mm, they are easily mistaken for alate (winged) termites.

I first became aware of webspinners because one of my childhood idols, insect photographer extraordinaire Edward S. Ross was also the world authority on these obscure insects. He still is the expert on embiids, in fact. Discovering new species of webspinners took Dr. Ross all over the globe, as the diversity in this order is mostly tropical. During his expeditions, Ross also took amazing images of other insect species, many of which graced the pages of National Geographic magazine.

Nearly everything about webspinners is bizarre. They go through a simple metamorphosis, with immature individuals resembling adults, but the females never develop wings. Most species are live in galleries of silken tubes under loose bark and stones, or leaf litter, often in a matriarchal congregation of a parent female and her offspring. The insects can move quickly through their silken tunnels, usually moving backwards. The wings of adult males flip forward so they don’t catch on the silken walls during backwards maneuvers.

Perhaps the oddest feature of their anatomy are the front legs. The swollen “feet” (tarsal segments) contain the glands from which the silk is spun. This appearance of having ankle weights or Popeye forearms easily separates webspinners from winged termites, small stoneflies, and other look-alikes. Even the youngest web-spinner can do just that: spin silk.

Webspinners feed mostly, if not exclusively, on decaying vegetable matter, though adult males do not feed at all. The mandibles (jaws) of males are modified for chewing holes in the galleries of females, and for grasping the woman of his dreams during courtship.

Three families of webspinners are known to occur in the southern United States, with one species reaching as far north as southern Oregon. Our common Tucson species is Oligotoma nigra, but it is not native, having come traveled in date palm cuttings from Egypt and the Persian Gulf sometime in the 1800s. It is believed that India is the true place of origin for this insect. Now it is widespread in the tropics, and ranges from San Antonio to southern California, and north to Utah in the U.S. There is also a documented record from Arkansas.

For more information on these amazing invertebrates, I recommend the World Wide Webspinners website, recently constructed.

4 comments:

  1. Faskinating. I'll be looking for them from now on.

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  2. Very cool -- great blog, Eric!

    Keep an eye out for Sclerogibbid wasps which parasitize these guys (Probethylus schwarzi is in AZ).

    -George

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