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Should we be afraid of the Cheiracanthium mildei spider?

Should we be afraid of the Cheiracanthium mildei spider?
Credit: Laurent Desaulniers
Should we be afraid of the Cheiracanthium mildei spider?
Should we be afraid of the Cheiracanthium mildei spider?

Cheiracanthium mildei, in English the yellow sac spider, is a small greenish or yellowish spider. In our homes it can be found in the upper corners of rooms, where walls meet the ceiling, and where it manufactures small shelters out of silk. Some people believe that this spider bites us while we sleep and that its bite can lead to severe necrotic lesions. But what’s the real story?

Does a yellow sac spider bite really produce necrosis?

The Cheiracanthium genus groups together 200 species described worldwide, and according to the scientific literature, fewer than 10 species would be capable of biting humans. In 2006 researchers studied the bites of these spiders with a view to disentangling what is true from what is false. To that end they considered the facts associated with 20 verified Cheiracanthium bites,1 including those of C. mildei, in Australia and in the United States. They also focused on 39 cases published in international medical journals. The results: reactions to bites are minor and localized. They vary from simple redness to swelling and itching. As it happened, there was a single case entailing a wound, which after a few days had disappeared – but what was involved here was a bite associated with C. punctorium, a spider nonexistent in the Québec region. In 2014 a second study, comprising 10 cases of C. mildei bites in Oregon, drew the same conclusions. Finally, other studies demonstrate that phospholipases D, regarded as the agents responsible for the development of necrosis after a spider bite, are missing from the Cheiracanthium species’ venoms.

Does the yellow sac spider try to bite us while we sleep?

C. mildei is a wary specie that like all spiders is a predator of other arthropods. Moreover, as is the case with all spiders, its venom has evolved to immobilize prey in order to obtain nutrients without that prey being able to escape. The yellow sac spider, again just like other spiders, absolutely does not go looking to bite human beings, and no case involving a bite has been recorded in Québec. Of course, it can happen that a spider bites us when it feels cornered or threatened. Nevertheless, we can sleep in peace – because, believe it or not, it’s only in movies that spiders bite human beings intentionally!

To learn more


1A verified case of spider bite requires that all of the following criteria be met:
1) at the time of the bite, clinical reactions are observed;
2) the spider is captured immediately at that moment;
3) the spider is identified by an expert.

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