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Diet of Florida’s elusive red widow spider revealed by MU biologist

March 20, 2014

image is a close up of the red widow spider

The red widow spider (Latrodectus bishopi) gets its name for the reddish-orange coloring of its head, carapace, and legs. Although venomous, no bites from this spider have been recorded. (Photo courtesy of J. Carrel)

Beetles. They’re what’s for breakfast—or at least for the red widow spider (Latrodectus bishopi), according to a new study by University of Missouri biologist James Carrel. The study, which appears in the March issue of the Florida Entomologist, provides a first-time glimpse at the diet of this enigmatic spider found only in Florida’s “scrub” habitat.

“The pine scrub habitat is ancient, one of the oldest in North America, and is found on sandy ridges in Central and Southeastern Florida,” said Carrel, Curators Professor Emeritus of biological sciences. “There is a high level of endemism on these ridges, meaning that plants and animals that are found on these ridges, including the red widow spider, are pretty much restricted to these high dry areas in Florida.”

While confinement to this unique habitat contributes to this spider’s rarity, its web has cloaked its natural history in mystery for years. The spider builds its web on palmetto shrubs with its funnel-shaped retreat hidden in an unopened palmetto leaf. A loose tangle of threads spun between the vertical tips of the palmetto frond is the only clue of its presence, and these threads are visible only on foggy mornings four months of the year, making this species extremely difficult to locate and study.

Since 1987, Carrel has been monitoring populations of this spider at the Archbold Biological Station, which protects a 5,193-acre Florida scrub preserve near Lake Placid. Only twice in those 23 years – in March 1989 and in May 2003 – have enough webs been located to study the dietary habits of these elusive spiders.

During both months, Carrel collected all the prey sacs from 30 red widow webs for 5 consecutive days and nights, replacing each sac with a beetle or cricket of approximate nutritional value. The sacs were dissected to identify the species of prey and to calculate its dry mass weight. Carrel worked with Mark Deyrup, senior research biologist for the Archbold Biological Station, on species identification.

The scientists identified 43 species of insects among the 98 specimens collected. The study revealed that the primary prey of the spider, especially in early spring, are five species of scarab beetles endemic to the Florida scrub habitat.

“These beetles, most of which are larger and stronger than the spiders themselves, fly just above the tops of scrub vegetation, probably an instinctive way of avoiding the bats and nighthawks that patrol the night sky,” said Deyrup, who co-authored the study. “Now and then one of these beetles hits the strands spun between tips of palmetto fronds and tumbles into the denser tangle of threads below.”

The finding helps explain why the spider is restricted to the Florida scrub habitat. “The data suggest that these spiders have evolved to specialize on scarab beetles that are specific to this scrubby habitat because it’s a reliable food source,” Carrel said.

He adds that the spider’s diet and its high degree of endemism make an important case for conservation of this dwindling habitat. “While the preservation of species diversity is an end in itself, species diversity also represents a vast library of undiscovered bioactive compounds.”

Funding for the study, titled “Red widow spiders (Araneae: Theridiidae) prey extensively on scarab beetles endemic in Florida scrub,” came in part from a grant from the University of Missouri.

the images shows a scrub palmetto scarab on a flower. The beetle is black with yellow markings.

The scrub palmetto scarab (Trigonopeltastes floridana) is one species of scarab beetles endemic to the Florida scrub that the red widow spider preys on. (Photo courtesy Tim Lethbridge)

two part image: one image shows a dewy web visible on a palmetto front; the second image shows a palmetto frond where the web is not visible

The red widow spider builds its web between the vertical tips of palmetto fronds. The threads, which are visible only on foggy mornings (left), are invisible during the day (right).

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Written by: Melody Kroll

Related research strengths:
Behavior, Ecology, Evolution