﻿Aquatic beetle diversity from Volcán Tacaná, Mexico: altitudinal distribution pattern and biogeographical affinity of the fauna

﻿Abstract Results of an aquatic beetle survey at Volcán Tacaná, Mexico, are presented with five altitudinal levels in a monthly sampling regime, aiming to estimate both diversity and altitudinal distribution patterns of the aquatic beetle fauna. The first list of aquatic beetle species from this mountain is presented, comprising 40 species in 32 genera and nine families, with four species recorded for the first time from Mexico and six recorded for the first time from Chiapas. The aquatic beetle fauna is characterized by Elmidae with 20 species, Dytiscidae with eleven species, Dryopidae with three, and Epimetopidae, Hydraenidae, Hydrophilidae, Gyrinidae, Lutrochidae, and Noteridae with one species each. The species composition through the sampled altitudinal gradient (670–1,776 m) was not homogeneous, with the elmid genera Macrelmis, Heterelmis, Microcylloepus, and Austrolimnius present at all levels, while Hexanchorus, Neoelmis, and Onychelmis were present at levels 1–3 (673–1,214 m); dytiscids were mostly present at levels 4 and 5 (1,619–1,776 m), and dryopids were present only at levels 1–3. A Parsimony Analysis of Endemicity supports a general partition between altitudinal levels 1–3 and levels 4 + 5.


Introduction
Among the aquatic insects, aquatic beetles (Coleoptera) are one of the largest groups, with ca. 13,000 described species distributed in 30 families in three of the four coleopteran suborders (Short 2017). Within this insect group, the families Dytiscidae and Hydrophilidae are the largest, with ca. 4,300 and 2,900 species, respectively (Szczepański et al. 2018;Nilsson and Hákej 2020). Aquatic beetles are considered to have a great potential for biodiversity and conservation assessment of water habitats, besides their use as water quality indicators (Samir 2017). They have been recorded in all continents, except Antarctica, and inhabit almost all kinds of aquatic habitats from the smallest phytotelmata to large lakes and rivers (Bilton et al. 2019). Their distribution is determined by different ecological factors, including altitude, which plays an important role in aquatic beetle assemblage composition (Pérez-Bilbao et al. 2014).
Previous studies in the Neotropics have found that altitude may have a significant influence on the composition and structure of an aquatic insect community, as some genera may show a wide range of distribution, while others are characteristic of a particular altitudinal level (e.g., Arias 2004;Henriques-Oliveira and Nessimian 2010, in Brazil;González-Córdoba et al. 2015, 2020Mosquera-Murillo and Sánchez-Vázquez 2018, in Colombia;Huanachin-Quispe and Huamantico-Araujo 2018, in Peru).
The Tacaná volcano, in the southern Mexican state of Chiapas and bordering Guatemala, is a key element of Volcán Tacaná Biosphere Reserve, a protected area relevant for its rich biotic, cultural, and economic value. This reserve is at the northernmost range of the Central American Nucleus or Central American Volcanic Arc and lies within the Mesoamerican Biological Corridor (CONANP 2013), a dynamic biogeographical area resulting from the assembly of biotas of Nearctic and Neotropical origin. Understanding the geographical distribution and the local diversity of aquatic insects is important to assess the patterns and processes of biological diversification (Benzina et al. 2019). This study aims to record the aquatic beetle diversity from Volcán Tacaná as well as to assess their altitudinal distribution patterns and the biogeographic affinities of the fauna to aid our understanding of biological diversification in the region.

Sampling procedures
Five sampling localities were established, each at an altitude level along the volcano (levels 1-5; Figs 1, 2; Table 1), in order to estimate an altitudinal distribution pattern of species. Besides single sampling sites at each level (locality), levels 3-5 each had a second sampling site (i.e., there was a total of eight sampling sites; Fig. 1, Table 1). Water body and level selection essentially followed availability of lotic systems, as lentic systems are generally missing except for a crater lake at the top of the volcano; absence of permanent streams at higher elevations precluded sampling at uniformly separated levels, particularly between levels 4 and 5.
Level 3. Ejido El Águila, municipality of Cacahoatán. The vegetation is cloud forest. On this locality, two rivers were sampled. The first river, La Resbaladilla (R1), belongs to the Cahotán basin, and the sampling site (15°05.564'N, 92°10.849'W) is at 1,214 m asl.
The aquatic beetles were sampled monthly over a year (February 2018-February 2019). In each water body (sampling site) three points were selected, separated by 30 m from each other. Samples were obtained using a D-type benthos net (500 µm mesh), with a dimension of 30.5 cm wide × 53.3 cm long). A second trapping technique, a bucket black-light trap, was used for 3 hours at each sampling site. Captured specimens with organic matter surplus were stored in zippered plastic bags with 80% ethyl alcohol, which was replaced with clean alcohol after 24 hours; aquatic beetles were then sorted from other insect groups in the laboratory using a dissecting microscope, and subsequently identified.

Taxonomic identification
The aquatic beetle specimens were dissected and identified to species using features of the genitalic structures; individual genitalia were extracted and stored in microvials with glycerin. Specimens were mounted on entomological pins, together with their associated labels and genitalia; specimens smaller than 12 mm were placed in paper cartons (points).
Identification was performed through introductory genus-level keys (White and Roughley 2008;Archangelsky et al. 2009;Miller and Bergsten 2016;Benetti et al. 2018;Passos et al. 2018), and subsequently with specialized revisions and original species descriptions.
All the material examined was deposited in the Colección Nacional de Insectos (CNIN) of the Instituto de Biología, Universidad Nacional Autónoma de Mexico.

Parsimony Analysis of Endemicity (PAE)
To aid unravel a general distribution pattern of the aquatic beetle fauna along the altitudinal gradient in the volcano, we performed a Parsimony Analysis of Endemicity (PAE). According to Morrone (2009) "…PAE constructs cladograms based on the cladistic analysis of presence-absence data matrices of species and supraspecific taxa". A matrix was built with distributional units (i.e., sampling sites) used as "terminals" and species serving as "characters", aiming to obtain a hierarchical structure in the resulting most parsimonious cladograms. Because PAE has been applied to discern a biogeographical signal, such as delimiting areas of endemism or historical relationship between preexisting areas of endemism (Crisci et al. 2003), our assumption is that even a general pattern between altitudinal levels may be informative of a faunistic differentiation along the gradient. Two analyses were applied: one with the main five levels of sampling (localities) as terminals (i.e., levels 3-5 had sites fused in a single unit), and a second with all eight sampling sites as distribution units or terminals (Table 1). Aquatic beetle species were used as characters, codified as present (1) or absent (0) at each of the distributional units (sampling sites or terminals). A hypothetical distributional unit with all species absent (zero vector) was used to root the trees.
The matrices (Table 1) were built with WinClada (Nixon 2002), then exported as a Nexus file to perform a parsimony analysis in TNT (Tree Analysis using New Technology, version 1.5) (Goloboff and Catalano 2016). The most parsimonious cladogram was obtained through a heuristic algorithm with parameters: random seed = 0, hold = 3000, hold / = 50 in a TBR (tree bisection and reconnection technique) of 60 replicates. The most parsimonious topology was exported to Adobe Illustrator CS5 software to be edited.

Distribution maps
Mapping of the study site with the sampling sites was done with ArcGIS version 10.2. 2. Layers of states and municipalities were obtained from the National Institute of Statistics and Geography (INEGI), with information on a 1:50,000 scale. Projection of localities with geographical coordinates was carried out with Universal Transverse Mercator (UTM). The raster of the CEM model of the Chiapas area was obtained, a cut of municipalities within the study area was made, with the help of a vector layer of municipal boundaries. The elevation model was adjusted with a reclassification of the z (altitude) values so altitude differences within our area of interest could be visualized. Seven intervals from 0 m to 4080 m were used for the reclassification. In addition, a shadow map (hillshade) was made to better visualize slopes of the terrain of the study area. Finally, layers of the watersheds are located on a scale of 1:50,000, which belongs to the Costa de Chiapas hydrographic region (key RH23).
We record the following four species from Mexico for the first time (Appendix 1): the Elmidae Cylloepus atys Hinton, 1946 (Hinton, 1934), and one of Noteridae, Notomicrus sharpi J. Balfour-Browne, 1939, were recorded for the first time from the state of Chiapas.

List of species of aquatic beetles (Coleoptera) from Volcán Tacaná, Mexico
Entries are arranged alphabetically by family and genus. Entries for genera include comments on number of species, and distribution. Species entries include the valid combination, distributional and altitudinal information, as well as type of substrate where they were collected. Altitude or elevation data are given in m above sea level.

Dryops mexicanus Sharp, 1882
Note. Dryops has a worldwide distribution and comprises 79 species (Shepard and Sites 2016), three of them are recorded from Mexico. Comments. Collected on substrates consisting of gravel, macrophytes, and leaf packs; found in all sampling months (February 2018 through February 2019, dry and rainy seasons); also collected with a bucket light trap.

Genus Elmoparnus Sharp, 1882
Note.This genus includes eight species recorded in the Neotropics (Kodada and Jäch 2005), two of them are recorded in Mexico.

Comments.
Collected on substrates of gravel, macrophytes, and leaf packs (June 2018, rainy season).

Genus Clarkhydrus Fery & Ribera, 2018
Note. This genus has a Nearctic and Neotropical distribution and comprises 10 species, seven of which have been recorded in Mexico (Nilsson and Hájek 2020).

Clarkhydrus sp.
Comments. This species was collected at levels 4 (rivers 1 and 2, 1,448-1,619 m) and 5 (river 1, 1,763 m) on substrates of macrophytes and leaf packs, and was present throughout sampling months (February 2018 through February 2019, dry and rainy season). Specimens did not match known described species of the genus; however, they are close to C. decemsignatus, yet male genital morphology differs.

Genus Laccophilus Leach, 1815
Note. This cosmopolitan genus is the largest of the subfamily Laccophilinae, with 285 species (Nilsson and Hákej 2020) Genus Neoclypeodytes Young, 1967 Note. This genus is present from southwestern Canada south through western United States and Mexico, with a few species in Panama and one in Jamaica (Miller and Bergsten 2016;Nilsson and Hájek 2020). It comprises 27 species (Nilsson and Hájek 2020), 15 of which are present in Mexico (Arce-Pérez and Roughley 1999; Arce-Pérez and Novelo-Gutiérrez 2015; Nilsson and Hájek 2020).
Comments. Collected on macrophytes and leaf packs, throughout the sampling period (February 2018 through February 2019, dry and rainy season).
Comments. Collected on macrophytes and leaf packs, throughout the sampling period (February 2018 through February 2019, dry and rainy season); also collected with a bucket light trap.

Genus Thermonectus Dejean, 1833
Note. This genus is distributed across the Americas and comprises 20 species and two subspecies (Nilsson and Hajék 2020), with eight species recorded from Mexico (Arce-Pérez and Roughley 1999; Nilsson and Hájek 2020). Note. This genus occurs in the Australasian and Neotropical regions, with more than 100 described species (Manzo 2005(Manzo , 2007Jäch et al. 2016). Twenty species of this genus have been recorded in the Americas, from northern Mexico through southeastern Argentina (Hinton 1971;Manzo 2007), with four species recorded from Mexico (Santiago-Fragoso and Spangler 1995;Jäch et al. 2016).
Comments. Collected on substrates of gravel, macrophytes, and leaf packs, throughout the sampling period (February 2018 through February 2019, dry and rainy season).
Comments. Collected on substrates of gravel, macrophytes, and leaf packs, throughout the sampling period (February 2018 through February 2019, dry and rainy season).

Cylloepus atys
Comments. Collected on substrates of gravel, macrophytes, and leaf packs, through most of the sampling months (except March, July, and September 2018, dry and rainy season).

Heterelmis glabra
Comments. Collected on substrates of gravel, macrophytes, and leaf packs, through all sampling months (February 2018 through February 2019, dry and rainy season).
Comments. Collected on substrates of gravel, macrophytes, and litter, throughout all sampling months (February 2018 through February 2019, dry and rainy season).

Heterelmis simplex
Comments. Collected on substrates of gravel, macrophytes, and leaf packs, throughout sampling months (February 2018 through February 2019, dry and rainy season).

Genus Macrelmis Motschulsky, 1859
Note. This is a Nearctic and Neotropical genus, distributed from southern United States to South America, and comprises 49 species, 10 of which have been recorded from Mexico (Hinton 1940b;Passos et al. 2015;Jäch et al. 2016). Comments. Collected on substrates of gravel, macrophytes, and leaf packs, throughout sampling months (February 2018 through February 2019, dry and rainy season).
Comments. Collected on substrates of gravel, macrophytes, and leaf packs, throughout sampling months (February 2018 through February 2019, dry and rainy season).

Macrelmis sp.
Comments. This species was collected at level 5 (river 2, 1,776 m) on substrates of macrophytes and leaf packs, and was present throughout sampling months (Feb-ruary 2018 through February 2019, dry and rainy season). Specimens, including males, did not match known described species of the genus, although they are similar to M. leonilae. Male parameres of the specimens, in dorsal view, are slightly wider from the base to the apical portion, while in M. leonilae they are wider through the basal half. Comments. Collected on substrates of gravel, macrophytes, and leaf packs, throughout sampling months (February 2018 through February 2019, dry and rainy season). Hinton, 1940 Distribution. Mexico (Chiapas, Estado de Mexico). Previous altitudinal records of M. troilus are from 1,707 to 2,286 m (Hinton 1940b). In this study, M. troilus was found at all sampled levels (670-1,776 m).

Microcylloepus troilus
Comments. Collected on substrates of gravel, macrophytes, and leaf packs, throughout sampling months (February 2018 through February 2019, dry and rainy season).

Microcylloepus sp.
Comments. This species was collected at levels 1 (670 m), 2 (934 m), 3 (1,126-1,194 m), 4 (river 2, 1,619 m), and 5 (river 2, 1,776 m) on substrates of gravel, macrophytes, and leaf packs, throughout sampling months (February 2018 through February 2019, dry and rainy season). Specimens, including males, did not match exactly known described species of the genus, being close to M. angustus. Male genitalia of the specimens have the medium lobe slightly wider than M. angustus.
Comments. Collected on substrates of gravel, macrophytes, and leaf packs, in about half of the sampling period (February to May, and August 2018, and February 2019, dry and rainy season).

Genus Onychelmis Hinton, 1941
Note. This genus is distributed in Central and South America, contains eight described species (Linský et al. 2021), and this study provides the northernmost point of its range.
Comments. Collected on substrates of gravel, macrophytes, and leaf packs, during three months of the sampling period (February, April, and May 2018, dry and rainy season).

Genus Phanocerus Sharp, 1882
Note. This genus is distributed from North America through northern South America, with six described species (Jäch et al. 2016), one recorded in Mexico (Santiago-Fragoso and Spangler 1995;Jäch et al. 2016).
Comments. Collected on substrates of gravel, macrophytes, and leaf packs through four months of the sampling period (May, June, July, and August 2018, rainy season); also collected with a bucket light trap.
Comments. Collected on substrates of gravel, macrophytes, and leaf packs, throughout the sampling months (February 2018 through February 2019, dry and rainy season).

Family Epimetopidae Zaitzev, 1908
Genus Epimetopus Lacordaire, 1854 Note. This genus is distributed across the Nearctic and Neotropical region, with 56 species described (Perkins 2012 Lutrochus sp. Comments. This species was present at level 1 (670 m) and was collected on leaf packs (May 2018, rainy season). Specimens key out to an undescribed genus and species included in Maier (2016), an unpublished doctoral thesis, so a preliminary identification is maintained.
Comments. Collected on substrate of macrophytes (February 2018, dry season).

Altitudinal distribution of the aquatic beetle fauna
The aquatic beetle fauna from Volcán Tacaná is distributed throughout the sampled altitudinal gradient (670-1,776 m), however our initial hypothesis is that species distribution would not be homogeneous. We applied a Parsimony Analysis of Endemism (PAE) as a fast approach to detect a potential faunal partition, with a general finding of the three lower altitudinal levels grouping together (i.e., sharing similar species) and about 40% of the species with a widespread altitudinal distribution. A first PAE (Fig. 3A), including the five levels, each as a single unit, recovered a topology distinguishing two well-defined groups, one composed by the three lower levels (673, 998, and 1,150-1,214 m), and another composed by the two higher levels (1,619-1,741 m and 1,763-1,776 m). A second PAE (Fig. 3B), including each sampled river (i.e., rivers of levels 3-5 considered each as a unit) also recovered a group composed by the three lower levels (levels 1-3), nevertheless two rivers of levels 4 and 5 (i.e., R2 of levels 4 and 5, respectively) were recovered as closer to rivers from levels 1-3 than to other rivers of levels 4 and 5 (i.e., R1 of levels 4 and 5, respectively), yet support for the latter group (levels 1-3 + R2 of L4 and L5) is quite weak. This means that the next well supported group would be all rivers excluding river 1 of level 4. The most diverse family was Elmidae (see some representatives on Fig. 4), with most species widespread along the five altitudinal levels, with the genera Austrolimnius (A. formosus and A. sulcicollis), Xenelmis (X. bufo), and Heterelmis (H. glabra, H. obesa, H. obscura, and H. simplex), occurring in all levels (except H. simplex, absent from R2 and R1 of levels 3 and 4, respectively). Cylloepus atys shares the same distribution pattern as H. simplex, while Hexacylloepus metapa and Huleechius spinipes, both occur in all altitudinal levels but are curiously absent from river 1 of level 4. Macrelmis graniger and M. leonilae are present in all rivers, while Macrelmis sp. is present only in river 2 of level 5. Microcylloepus (M. inaequalis, M. troilus, and M. sp.) are present in all altitudinal levels, however M. troilus is absent in river 1 of level 4 and river 1 of level 5. Phanocerus clavicornis has a fragmented distribution, occurring in levels 1, 3, and 4, while Hexanchorus usitatus, Neoelmis apicalis, and Onychelmis longicollis are present in all rivers from levels 1-3.
Dytiscidae (see some representatives on Fig. 4), the second most diverse family, is characteristic of the higher levels (i.e., levels 4 and 5), with all genera represented by only one species. Copelatus distinctus, present in all rivers of levels 3-5, has the largest vertical distribution. Platambus americanus, Ilyobiosoma flohrianum, Laccophilus proximus, and Clarkhydrus sp. are present in levels 3 and 4, however only P. americanus is present in all four rivers of these levels. Bidessonotus championi, Liodessus affinis, Uvarus subornatus, and Neoclypeodytes fryii are present in level 4, nevertheless only the latter species occurs in both sampled rivers. Rhantus gutticollis and Thermonectus nigrofasciatus are only present in river 2 of the highest level.
Among the 20 species of Elmidae, 14 occur only in the Neotropical region, while the remaining six, particularly those of Heterelmis, have a wide distribution (i.e., they occur in the Nearctic and Neotropical regions). Most dytiscid species, six out of 11, have a wide distribution through the Nearctic and Neotropics, while the other five occur only in the Neotropical region. Elmidae and Dysticidae have 80 and 50% of their distribution in the Brazilian subregions and the Mexican Transition Zone, respectively, with especial affinity to the Mesoamerican and Pacific domains. Dryopidae is represented by three species, two of them with records in the Neotropical region (Brazilian subregions and the Mexican Transition Zone) and one with Nearctic and Neotropical distribution. Gyrinidae (Gyretes boucardi), Hydraenidae (Hydraena sp.), and Lutrochidae (Lutrochus sp.), also have species with Neotropical affinity, whereas Epimotopidae (Epimetopus thermarum), Hydrophilidae (Tropisternus fuscitarsis), and Noteridae (Notomicrus sharpi) have species with a wide distribution in the New World. The latter six families also have an affinity to the Brazilian subregions, particularly to the Pacific and Mesoamerican domains.

Discussion
Aquatic beetles were present at the five sampling levels (L1, 673 m; L2, 998 m; L3, 1,150-1,214 m; L4, 1,619-1,741 m; and L5, 1,763-1,776m). This agrees with the widespread distribution of aquatic beetles, as well as their high capacity to inhabit different aquatic environments from sea level to mountains of 4,000 m high or more (Jäch and Balke 2008;White and Roughley 2008). Despite their broad presence in the volcano, aquatic beetle species were not distributed homogeneously along the altitudinal gradient, which is congruent with a high endemism in almost all families of this group, particularly those of lotic systems in warm climates (Jäch and Balke 2008).
Elmidae (riffle beetles) was the dominant group (20 spp.) and was present in all sampling levels. This coincides with previous findings in the Neotropics (e.g., Arias-Díaz et al. 2007; Huanachin-Quispe and Huamantico-Araujo 2018; Mosquera-Murillo and Sánchez-Vázquez 2018; Passos et al. 2018). General characteristics of the streams on a volcanic bedrock with a variety of substrates, such as gravel, leaf litter, logs, and aquatic macrophytes, probably contributed to maintain a high diversity of elmids as reported by Elliot (2008) and Mosquera-Murillo and Sánchez-Vázquez (2018). Species of the New World genera Heterelmis, Macrelmis, and Microcylloepus, and of the Neotropical Austrolimnius were present at all levels, while the Neotropical Hexanchorus, Neoelmis, and Onychelmis were restricted to levels 1-3.
Dytiscidae (predaceous diving beetles) was the second most diverse group (11 spp.) and was present mostly at levels 4 and 5, with only one species (Copelatus distinctus) at levels 3-5. Three species, Bidessonotus championi, Ilybiosoma flohrianum, and Uvarus subornatus were only observed at L4 (R2, 1,619 m), while Rhantus gutticollis and Thermonectus nigrofasciatus appeared only at L5 (R2, 1,776 m). This distribution may relate to the size of the streams at the higher levels, which were generally smaller and with weaker currents, so pools were more common, which appeared to be a suitable habitat for dytiscids; most collecting of dytiscids was at depositional zones of the stream. This agrees with a general preference of this family for lentic systems (Miller and Bergsten 2016;Benetti et al. 2018).
Dryopidae was the third family in species richness (3 spp.) and was present at lower elevations, with Dryops mexicanus and Helichus suturalis at levels 1-3, and Elmoparnus pandus only at level 1. This is a mostly tropical family, which appears to explain their presence at low elevations, although there are records at higher elevation in other areas (Huanachin-Quispe and Huamantico-Araujo 2018). This family includes species that may be observed in both lotic and lentic environments, however, many of the species may be present near the water margin or even outside (Jäch and Balke 2008), also their larvae are terrestrial. This particular biology may indirectly restrict the presence of adult dryopids at such lower elevation sites. During collecting, specimens were only found submerged associated to substrates.
The rest of the families were represented by one species each. Hydraenidae (Hydraena sp.) was observed at the five sampling levels, which agrees with the broad distribution of the group and that species of this genus occupy different types of habitats, from small streams to large rivers (Trizzino et al. 2013). Noteridae (Notomicrus sharpi) was only present at level 4 (river 2, 1,619 m), which is above the previous known altitudinal record; as dytiscids, noterids prefer environments with slow current and some depth (Megna and Deler 2006), which includes the small pond (with macrophytes) at one side of the main stream where the only specimen was captured. Epimetopidae (Epimetopus thermarum), Gyrinidae (Gyretes boucardi), and Lutrochidae (Lutrochus sp.) were only recorded at level 1 (693 m). It is known that Epimetopus is attracted to lights (Perkins 2012), this agrees with our findings as specimens were captured with a bucket light trap. G. boucardi was collected in October, agreeing with White and Roughley's (2008) time of emergence of late summer and early fall for the species; specimens were captured in an adjacent pool forming a large aggregation, Lutrochus sp. was only found at level 1, with specimens captured on macrophytes; this group is typically from lotic systems; however, it has been little studied in Mexico. Finally, Hydrophilidae (Tropisternus fuscitarsis) was only recorded at level 2; it is interesting this representative family was only present with one species, which was collected with bucket light trap, probably indicating a not very suitable habitat for the group in a volcanic-based ecosystem.
Species observed in levels 1-3 are usually of Neotropical affinity, while in levels 4 and 5 species with both Nearctic and Neotropical distribution increase. In general, most of the species are of Neotropical distribution with an affinity for the Pacific and Mesoamerican domains, which coincides with Morrone and Márquez (2001), who observed that the Coleoptera fauna of the Chiapas Highland province is related to the Veracruzan and Pacific Lowlands provinces, which are part of the Mesoamerican domain. The relationship between the Chiapas Highland province and Veracruzan and Pacific Lowlands provinces was confirmed by Morrone (2019). This general partition in two groups of altitudinal levels, 1-3 and 4 + 5, is supported by a PAE analysis, pointing out to a preliminary general pattern of altitudinal distribution for the aquatic beetle fauna of Volcán Tacaná.

Conclusions
The aquatic beetle fauna of Volcán Tacaná presents a high diversity, with Elmidae, Dystiscidae, and Dryopidae as the most species-rich families, being responsible for 85% of the species. Some families (e.g., Hydraenidae and Elmidae) are distributed along all the altitudinal range, while Dytiscidae is present particularly at the higher altitudinal levels (1,619-1776 m); Noteridae is also present at high altitude, but only in a river located at 1,741 m. Remaining families, Dryopidae, Epimetopidae, Gyrinidae, Hydrophilidae, and Lutrochidae are present in lowlands (670-1,214 m). The aquatic beetle fauna of Volcán Tacaná presents a general partition in two well-defined groups: a lower altitude fauna (between 670, 934 and 1,150-1,214 m, levels 1-3) and a higher altitude fauna (between 1,619 and 1,776 m, levels 4 and 5). This fauna has an affinity to the Pacific and Mesoamerican biogeographic domains.

Acknowledgements
Our thanks go to the authorities of the localities Finca Alianza, Finca Monte Perla, Ejido El Águila, Ejido Benito Juárez El Plan, and Cantón San Isidro, as well as to Francisco J. Jiménez González, director of the Tacaná Volcano Biosphere Reserve, for authorizing our research. We also thank Rodolfo Cancino and Johar Almaraz for help and encouragement during field work. Thanks to Hellen Martínez-Roldán for her support in making the map of sampling points, thanks also to Alana Brunini for help with reference and text editing. We thank associate editor Ernesto Rázuri-Gonzales and one anonymous reviewer for helpful and constructive reviews. CCM acknowledges Programa de Becas Posdoctorales DGAPA-UNAM 2019-2021 for a postdoctoral fellowship. ACR thanks project "Biodiversidad de Neuroptera en México: un enfoque taxonómico integrativo" (CONACYT CB2017-2018, A1-S-32693) for general support. Alba Magali Luna-Luna thanks Consejo Nacional de Ciencia y Tecnología (CONA-CYT, Mexico) for a scholarship through Doctorado en Ciencias Biológicas y de la Salud, Universidad Autónoma Metropolitana, Mexico City; this contribution is part of the requirements of AMLL to obtain her doctoral degree at Universidad Autónoma Metropolitana. This work was supported by project PAPIIT-UNAM IN207517 (Aportaciones a la taxonomía y filogenia del orden Neuroptera (Insecta) en México, 2017-2019) granted to ACR.
Appendix 1 Table A1. Collecting data of examined material of the new records of species for Mexico; all specimens are deposited at Colección Nacional de Insectos (CNIN), UNAM. LV1-LV5 = sampled levels; R1 and R2 = sampled rivers; m = male, f = female; * = specimens collected with bucket light trap (as explained in materials and methods).  LV3R1 (12m,19f )