CHULA VISTA, Calif. -- The discovery of black widow spiders at several Chula Vista elementary schools has prompted the district to hire exterminators to spray all 18 campuses.

A parent first spotted the venomous spiders -- the arachnids are found across the Southwest -- at Kellogg Elementary a week ago, then found them at several other schools, The San Diego Union-Tribune reported.

Chula Vista Elementary School District Superintendent Lowell Billings told the newspaper that five campuses got a "spot spraying" on Saturday, and 13 other campuses will be sprayed this weekend.

So far, no children have been reported bitten. Letters were sent home with students Tuesday, saying that the large population is probably the result of the unusually rainy winter.

The neurotoxin in the spider's venom can cause pain that radiates to the abdomen, back and limbs, and sometimes causes vomiting and breathing problems, according to Marshal Hedin, an associate professor of biology at San Diego State University.

Hedin said he was unaware of any deaths due a black widow bite in the United States in more than 20 years.

The female black widow, jet black and about 1.5 inches long, is distinguished by a reddish, hourglass-shaped marking on its belly. Males are harmless and about half the size.