The genus Liodessus belongs to the predaceous diving beetle family Dytiscidae, within the tribe Bidessini, a group of minute aquatic predators found throughout the New World. These beetles are among the smallest of their family, reaching a maximum length of only 3.2 millimeters. Despite their diminutive size, they are remarkably successful in high mountain habitats. In the high Andes, at altitudes from roughly 3,000 to almost 5,000 meters, Liodessus are the most abundant aquatic beetles, thriving in cold, oxygen-poor waters where few other insects manage to persist. Yet this abundance has historically masked a problem: the beetles of high-altitude Páramo and Puna ecosystems remain poorly studied, and taxonomists have only recently begun to unravel how many species actually live there.

Since 2019, research efforts across Colombia, Ecuador, Peru and Venezuela have uncovered 11 new species and one subspecies of Liodessus, raising the number known from the entire Neotropical Region to 33 species and 10 subspecies. Many of these appear to be microendemics, species confined to single mountain massifs or even single drainage basins. The new Costa Rican discoveries fit this pattern strikingly well and represent the first mid- to high-altitude representatives of the genus ever reported from that country, extending the known range of these mountain specialists northward into Central America.

The first of the new species, Liodessus cerrodelamuerte, takes its name from Cerro de la Muerte, the imposing mountain in San José province where it was found. The type series was collected at around 3,300 meters on a gravel road called Calle de Torres, with paratypes gathered from nearby sites at similar elevations and even a historical specimen from the Páramo de Chirripó summit region dating back to 1971. The species appears restricted to Costa Rica’s highest altitudes, living above 3,000 meters in small, shallow to very shallow puddles beside the road, pools that likely receive running seepage water for much of the year, as well as in water at a small well nearby.

Morphologically, Liodessus cerrodelamuerte is dark brown to blackish on its upper and lower surfaces, with the basal segments of its antennae and the bases of its middle and hind legs standing out in a lighter shade. The beetle shows a slight but obvious discontinuity between the pronotum, the plate covering the thorax, and the elytra, the hardened wing cases that protect the abdomen. Its head bears a fine surface mesh known as microreticulation, interrupted only in the middle between the eyes, while the pronotum and elytra are shiny and studded with moderately dense setiferous punctures, small pits that carry tiny hairs. The holotype male measures 2.0 millimeters in total length, with paratypes ranging up to 2.3 millimeters.

Two features of this species are especially telling. Its hind wings are drastically reduced, reaching only about half the length of the elytra, which means the beetle is effectively flightless and must disperse over ground across its mountain home. And as with most diving beetle taxonomy, the decisive characters lie in the male genitalia: the median lobe of the aedeagus is slender, with a simple curvature in its basal third before broadening into a convexity on the dorsal margin and then narrowing sharply toward the tip. The parameres, paired accessory structures, are of the simple two-segmented type characteristic of the tribe Bidessini. Female specimens show variable degrees of microreticulation on the pronotum and elytra, with a few individuals displaying a completely dull dorsal surface.

The second new species, Liodessus quetzalicus, was named after Paraíso Quetzal, its type locality in Cartago province at 2,760 meters, an area renowned for the resplendent quetzal, the spectacular bird that shares these montane forests. Unlike its high-altitude relative, this beetle occupies a remarkably broad elevational range, from roughly 1,380 meters near San José up to around 2,800 meters, and it tolerates a variety of habitats. Specimens have been collected from small shallow puddles and the edge of a lagoon in the Páramo, from a densely vegetated roadside ditch at Coris, and from the vegetation-choked margin of the Rio Coris creek. With 224 paratypes, the type series is extensive, and older museum specimens from Alto de Ochomogo, collected as far back as 1955, turned out to belong to the same species, showing how long undiscovered diversity can hide in institutional drawers.

Liodessus quetzalicus differs from its congener in several respects. Its hind wings are fully developed, in sharp contrast to the vestigial wings of L. cerrodelamuerte, suggesting it can fly between water bodies and explaining its wider distribution. Its dorsal coloration is very dark brown to blackish with the pronotum lighter toward the lateral sides and a yellowish lateral margin, and the elytra carry lighter longitudinal patches. The most diagnostic feature is again the shape of the male median lobe, which is simply curved and relatively broad, narrowing toward an apex with a broadly rounded tip in ventral view that the authors describe as having a characteristic spoon- or duck-beaked shape. Size variation among paratypes runs from 2.2 to 2.3 millimeters in total length, and some specimens from the Ochomogo area appear dark orange, possibly an artifact of long storage or preservation method.

Perhaps the most intriguing aspect of the discovery is the biogeographic puzzle the two species present. The distance as the crow flies between the highest known point of Liodessus quetzalicus and the closest locality of Liodessus cerrodelamuerte is only 12 kilometers, both within the same mountain massif. Yet the two species do not appear to meet. One occupies a broad elevational band and multiple habitat types, while the other is confined to the coldest, highest waters in small Páramo pools. The researchers suggest that microhabitat or microclimatic factors may constrain where each species can occur, and they propose that these Costa Rican Liodessus may represent an interesting system for comparative studies on the thermal tolerance of high-altitude species. If one species thrives only in near-freezing waters while its relative spans a wide thermal range, the pair could offer a natural experiment in how mountain insects adapt to cold.

The description also underscores how much remains to be learned about tropical mountain freshwater biodiversity. The authors note that many additional Liodessus species likely await discovery, most of them probably microendemics with tiny ranges, making them vulnerable to climate change and habitat disturbance. Because high-altitude aquatic insects depend on cold water, warming temperatures could squeeze species like L. cerrodelamuerte into ever smaller refuges atop Cerro de la Muerte, with nowhere higher to go. Detailed morphological descriptions, backed by high-resolution imaging of beetle structures using focus-stacking macro photography, are the essential first step in documenting this diversity before it disappears. For now, two more of the world’s smallest diving beetles have names, a home, and a place in the growing catalog of the Neotropical mountains’ hidden fauna.

Subject of Research: Taxonomic description of two new Liodessus diving beetle species from Costa Rican highlands

Article Title: Two new species of Liodessus diving beetles from Costa Rica ( Coleoptera , Dytiscidae , Bidessini )

Article References: Balke, M., Bilton, D., Springer, M., Villastrigo, A., Neven, K., & Hendrich, L. (2026). Two new species of Liodessus diving beetles from Costa Rica (Coleoptera, Dytiscidae, Bidessini). Alpine Entomology, 10, 147-154. https://doi.org/10.3897/alpento.10.193709

Image Credits: AI Generated

DOI: 10.3897/alpento.10.193709

Keywords: Liodessus, diving beetles, Dytiscidae, Costa Rica, new species, taxonomy, Páramo, high-altitude biodiversity, entomology, microendemism, Neotropics, Cerro de la Muerte