Ask The Entomologist

Thoughts on Invertebrate Houseguests

Just last night, my wife asked me to identify a spider that was perched on the wall of our shower. A small thing, its body was only about 3 millimeters long, or about 1/10 an inch. It was an immature spider, and I didn’t manage to get a good look at the layout of its eyes, so I was unable to identify it as well as I’d like. (Many identification resources tend to be more helpful for identifying adult bugs than immature bugs).

While I myself couldn’t identify it to genus level, I shared the observation to iNaturalist, in hopes that somebody with more expertise in spiders would be able to get a closer identification. Regardless of what it is, the chances of its bite and venom being of medical importance are just about nill. The only spider of real medical importance we have here in Montana is the Western Black Widow.

From just in front of the eyes to the tip of the abdomen, about 3 mm.
Sadly, I could only make out the two central eyes, and I’d need to see the others to identify her.

Insects and their kin have been living alongside humans for a very long time – it’s thought that some of our current house-dwelling insects started out as cave dwellers, also living alongside our human forbears. Some early cave art even features cave crickets, as in the Cave of the Three Brothers in southwestern France.

An ongoing project on iNaturalist, “Never Home Alone”, attempts to learn more about our long-term arthropod houseguests. Thus far, this project has led to the discovery of a number of new species – organisms whose behavior “in the wild” is entirely unknown. They’ve only ever been observed and collected from human dwellings.

The scientific paper that this project arose from found that upwards of 100 species of insects could be found in just about every home they surveyed. Defying the stereotype, folks with larger homes in better parts of town had more species of insects, not fewer! Regardless of homeowner’s income, the vast majority of the insect houseguests discovered were non-pest species, and some hadn’t been observed in the region before.

If an entomologist finds that your home is full of bugs, you shouldn’t feel too bad – chances are good that your neighbors’ homes have similar numbers of insects. They’re mostly harmless, and some are even beneficial…. Case in point, at closing time, the spider from the shower had been identified as a harmless variety of cellar spider, Pholcophora americana, who had likely been feeding on moth flies from our drains.

At present, the cellar spider has been relocated to the holly to join our Christmas Spiders.

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