> The vast spider population is attributed to an abundant food supply: more than 2.4 million midges in the cave, ready to be entangled in the intricate web.
...although I guess the question then is what sustains the millions of midges!
So the whole food-chain here is: sulfur -> bacteria -> midges -> spiders.
Akin to hydrothermal vents[1] in the ocean, and the lifeforms that eat that effluent.
[0] https://subtbiol.pensoft.net/article/162344/ ("An extraordinary colonial spider community in Sulfur Cave (Albania/Greece) sustained by chemoautotrophy")
> "Stable isotope analyses (δ¹³C and δ¹⁵N) revealed that the trophic web sustaining this assemblage is fueled by in situ primary production from sulfur-oxidizing microbial biofilms then transferred through chironomid larvae and adults to higher trophic levels."
This is a hugely erroneous claim that is much too frequently encountered in the popular publications.
Both in this cave and in hydrothermal vents, most autotrophic bacteria use free oxygen to oxidize the hydrogen sulfide, producing thus the energy needed for autotrophy.
The free oxygen comes from the phototrophic algae and plants (located elsewhere), i.e. from solar energy.
On Earth, there are only 2 kinds of autotrophic bacteria and archaea that may be independent from solar energy, the acetogenic bacteria and archaea and the methanogenic archaea. Both kinds obtain energy from free hydrogen and carbon dioxide, the former producing acetic acid and the latter producing methane.
These 2 kinds of bacteria and archaea need free hydrogen and most of them are killed by free oxygen. Sometimes the free hydrogen is produced by fermentation of organic substances, like in our intestines, so also coming from solar energy, but free hydrogen is also produced by the oxidation of volcanic rocks by water, when its origin is independent of solar energy and dependent only on the internal heat of the Earth, which produces volcanic rocks that are in chemical equilibrium at high temperatures deep inside the Earth's mantle, but they are no longer in chemical equilibrium after reaching the cold surface of the Earth.
Thus deep underground or in certain places on the bottom of the oceans, where free dihydrogen is abundant and there exists no free dioxygen, there are communities of acetogenic and methanogenic bacteria and archaea that are independent of solar energy, but this is not the case for this cave and for many of the hydrothermal vents, where both hydrogen sulfide and free dioxygen are abundant, so aerobic bacteria are dominant.
Anywhere where there is either air or water with dissolved dioxygen, the living beings use the most efficient energy source, i.e. the oxidation of either organic or anorganic substances with the free dioxygen, so they depend on solar energy, even when there is no light in that place.