Caves, biogeography and tiny arachnids

Palpigrades are as precious as pebbles from the Moon … hidden in the deepest fractures of rocks of caves and other kinds of subterranean habitats.

Above: A cave-dwelling palpigrade found in an Alpine caves. Photo: Alberto Chiarle

The Austrian professor Dr. Erhard Christian, one of the few experts worldwide on the taxonomy of the enigmatic arachnid order of Palpigradi (microwhip scorpions), used to say that “palpigrades are as precious as pebbles from the Moon”. Now that Americans and Chinese are chauffeuring different rovers around Mars, we should probably revise this quote and match the new incredible advances of space technology because palpigrades remain very precious organisms, especially in zoological collections around the world.

Cover article: (Free to read online for a year.)
Mammola, S., Souza, M.F.V.R., Isaia, M. and Ferreira, R.L. (2021), Global distribution of microwhip scorpions (Arachnida: Palpigradi). J Biogeogr. 48:1518–1529. 

The rarity of palpigrades is a fact: apart from a few relatively common species traveling around the world with the soil used in greenhouses (e.g., Eukoenenia mirabilis, the first palpigrades ever described in 1885), most palpigrades live hidden in the deepest fractures of rocks of caves and other kinds of subterranean habitats. Very few researchers have seen them alive. Some lucky subterranean biologists accidentally find them trapped in secluded water ponds or in the footprints that speleologists left in the mud covering the ground of the deepest parts of caves. And there is where the magic happens: some tiny flashing on the water ponds and a closer look reveal the presence of a clumsy and fragile long-tailed organism of less than 1 millimeter, struggling to remain atop the water surface—but perhaps also taking advantage of the possible prey sharing their secluded retreat (

Eukoenenia strinatii dwelling on the surface of a pond in a remote section of the Bossea cave, in Piedmont (NW-Italy) Photo: Alberto Chiarle

When you tell such a story to a wildlife photographer keen on arthropods, the endeavor becomes even more challenging: first you have to find the palpigrades and, second, you necessarily have to squeeze the photographer and their one-thousand dollar equipment into the hairline-crack-like entrance of the cave where the palpigrade lives. The wonderful cover photograph of the Journal of Biogeography issue hosting our research is the successful outcome of such a caving trip, embellished by the use of a U.V. lamp to illuminate a specimen of Eukeonenia strinatii, a specialized species inhabiting a very few caves in the South-Western Italian Alps. For this picture we took advantage of the propriety of the cuticle of the palpigrades to reflect UV light ( The specimen was photographed by Emanuele Biggi in the Bossea cave in Piedmont (NW-Italy) which is also the so-called “type locality” of this species, namely the place where it was first collected and described.

A microwhip scorpion under U.V. light. Photo: Emanuele Biggi.

In this paper, we examined the global distribution of these tiny creatures, asking ourselves about their typical range size, the ecological factors driving their distributions, and to what extent sampling bias may influence the observed patterns. Despite their rarity, we manage to assemble a dataset of over 1,000 localities of more than 120 species. This led us to test for differences in range sizes of soil- and cave-adapted species and to explore how different factors such as climate, nutrient availability, and geology drive the observed distribution patterns. In asking whether the pattern we detected should reflect the distribution of experts rather than palpigrades themselves, we verified the so-called people-species correlation, i.e. whether the number of occurrence records and the number of palpigrades’ researchers was related (a ‘palpigradologist effect’). We also found the typical range of any species of palpigrade is very small, only 0.01 km2. Europe and Brazil are the most relevant centers of diversification of cave-dwelling palpigrades, while soil species are mostly distributed over a broader geographical range, mainly in the Southern Hemisphere. The distribution of palpigrades seems to primarily match specific climatic conditions and historical biogeographic factors.

However, all these observations are inevitably generic, not only because of the global scale of our study, but also because we are just scraping the surface about the distribution of these enigmatic organisms. We see this study just as a start, a jumping-off point for future studies on the ecology and conservation of these poorly known organisms. There are certainly so many surprises in store into the fractures of rocks, in the voids among particles of soils, and in the darkest recesses of caves.

Written by:
Prof. Marco Isaia (University of Turin, Italy)
Dr. Stefano Mammola (Finnish Museum of Natural History, Finland ; National Research Council, Italy)

Additional information:
Twitter: @Italian_Spiders ; @stefanomammola1

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