Late last month, Anne Danielson-Francois, an associate biology professor at the University of Michigan, received an unusual package. She had instructed the sender to make sure the contents — spiders — were cushioned because she did not want their legs to break off.
When Dr. Danielson-Francois opened the little brown box, there they were — a male spider stuck on a glue trap, two female spiders suspended in an alcohol solution, and a few juvenile spiders.
Dr. Danielson-Francois, who works at the university’s Dearborn campus, had been enlisted by university officials as a sort of spider-buster. Last month, pest management teams found that unfamiliar spiders had moved into the basement of the Shapiro Undergraduate Library on the Ann Arbor campus, a space fewer people were visiting because of the coronavirus pandemic, the university said.
Her job: Identify the arachnids and report back.
Dr. Danielson-Francois usually studies another type of spider, a giant nephila, or orb-weaving spider. But she probed the desiccated body of the male, examined its genitalia, consulted taxonomic literature, noted its six eyes and arrived at a conclusion: Mediterranean recluse spiders were occupying the library basement.
Their body length can measure up to about half an inch long, and their leg span to over an inch, about the size of a half-dollar. And in the rare case that they bite someone, there could be tissue necrosis, or skin death.
In response to the infestation, the library, which was open by appointment only, closed on Sunday and Monday to deal with the spiders.
The university issued a statement on Tuesday announcing the discovery of the arachnids in which it explained that Mediterranean recluse spiders are even more solitary than their U.S. cousins, brown recluse spiders. Given the low risk posed by the spiders, the university said that the decision to close the library was “a misunderstanding of the situation.”
“Based on what we all know now, library managers agree that it was a mistake to close the building and they apologize for the inconvenience to the university community,” Kim Broekhuizen, a university spokeswoman, said in the statement published on the university’s website.
The pest management team was treating the tunnels of the buildings with pesticides, cleaning out remote spaces and asking people to report any further sightings.
Some spiders were also found in the Stearns Building on the university’s North Campus, The Michigan Daily, a student newspaper, reported this week. Ms. Broekhuizen said that recluse spiders were found in about a dozen academic buildings on the Ann Arbor campus, which has roughly 300 buildings. None were found in residence halls.
“Due to the pandemic, these are almost all low-occupancy buildings as the university prioritized social distancing, reduced density and increased cleaning in all of our facilities,” she said in an email on Thursday.
Recluse spiders are aptly named. They like to be alone, and generally stay away from places where there is foot traffic, instead choosing to crawl around in basements or wherever they can find a crevice.
Dr. Danielson-Francois said in an interview on Thursday that people were unlikely to encounter them “unless you are the unfortunate plumber who has to go into a crawl space.” If they do bite, she said, it “might be a dry bite.”
“They do not see you as a prey item,” she added. “They don’t want to consume you. It’s more of a defense.”
The episode generated minor tremors on campus. On the Facebook page Overheard at UMich, students commented on The Michigan Daily’s coverage. “That’s insane lol,” one student wrote. “Makes me glad I don’t go on campus.”
The University of Michigan Library sought to address concerns, saying in a statement on Tuesday that another spider had just been caught that day at the Shapiro Undergraduate Library, but that “their presence in a mechanical room doesn’t pose risks to library patrons.”
The discovery was similar to other instances during the pandemic in which creatures moved in as people moved out of buildings because of stay-at-home orders.
“People do get concerned where recluses are involved — there is a lot of fear there,” Dr. Danielson-Francois said. “But browsing the library stacks, you would be so unlikely to encounter one of these spiders.”
Recluse spiders can be found throughout the United States but are not widespread, said Richard S. Vetter, a retired research associate from the University of California, Riverside, who is one of the country’s foremost experts on recluse spiders.
“They may overpopulate that building, but go to the next building and there are no recluses,” he said. “A few buildings in every state does not mean they are widespread causing damage.”
Skin lesions are often misdiagnosed as being caused by spider bites, he said, noting that a brown recluse spider bite can possibly lead to a “very, very, very rare” fatal case. “We are talking a couple of bites a year,” he said.
He added that a fear of spiders was often irrational. “They have many legs, move fast, are hairy, they have many eyes and they show up unexpectedly,” he said. “They just look different and act different.”
Because of an editing error, an earlier version of this article misstated the average size of Mediterranean recluse spiders. Their leg span is a little over an inch, or the size of a half dollar, not half an inch.