Spiders can be identified from the DNA they leave on webs, say US scientists.
Analysis of genetic material stuck to spiders webs also reveals what they have eaten weeks after catching their prey.
The research may have future uses in monitoring endangered species or tracking down spider pests, experts report in the journal, Plos One.
The study looked at black widow spiders kept in a zoo.
If the technique works on other types of spider, it could have widespread practical uses, say experts from the University of Notre Dame in Indiana.
Lead researcher, Charles Xu, extracted mitochondrial DNA from the webs of black widow spiders kept at Potawatomi Zoo in Indiana.
He found that both the spider species and its prey - in this case crickets - can be identified from DNA spider web samples.
Spider webs can potentially be used to collect DNA without having to capture the spiders themselves, he says.
"In the past, identification of spiders has relied on morphology, especially looking at the genitalia of spiders because they're very different between different species of spider," he told BBC News.
"But there are a lot of errors associated with these kinds of methods and now with the advent of new genetic technologies we can more accurately identify these species.
"The really cool part about our study is that we used non-invasive samples - so these web samples - where we don't even have to directly observe or capture these spiders to get their DNA."
The experts say DNA analysis of spiders webs may be useful for monitoring and conservation purposes.
For example, DNA "fingerprinting" of spiders webs could be used to find out where a poisonous spider is living or to map the locations of endangered species.
Spider webs have been used in the past by citizen scientists to assess spider biodiversity by examining the structure of webs.
Web DNA samples collected by citizen scientists around the world might also have potential in this area, say the researchers.
"Spider web DNA as a proof-of-concept may open doors to other practical applications in conservation research, pest management, biogeography studies, and biodiversity assessments," they report in Plos One.
Black widow spiders, found in temperate regions around the world, are feared for their poisonous bite.
The female black widow spider can be twice as big as the male and will, on occasion, kill and eat the male after mating.
The spiders spin large webs in which females suspend a cocoon with hundreds of eggs.
They also use their webs to trap prey such as flies, mosquitoes, grasshoppers, beetles and caterpillars.