An analytical synopsis of caddisfly (Insecta, Trichoptera) taxonomic history and progress in Canada and the United States

105 An analytical synopsis of caddisfly (Insecta, Trichoptera) taxonomic history and progress in Canada and the United States Phillip N. Hogan1, David C. Houghton2, Kelly Murray-Stoker3, R. Edward Dewalt4, Andrew K. Rasmussen5, John C. Morse6 1 Department of Entomology, University of Illinois, Urbana-Champaign, Urbana, IL 61801, USA 2 Department of Biology, Hilldale College, Hillsdale, MI 49242, USA 3 Department of Natural Sciences and Mathematics, Oxford College of Emory University, Oxford, GA 30054, USA 4 Illinios Natural History Survey, Champaign, IL, 61820 USA 5 Center for Water Resources, Florida A&M University, Tallahassee, FL, 32307 USA 6 Department of Plant & Environmental Sciences, Clemson University, Clemson, SC, 29634 USA Corresponding author: Phillip N. Hogan ([email protected]) Copyright: © Phillip N. Hogan et al. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). Research Article Abstract A total of 1,510 caddisfly species representing 28 families and 155 genera are reported from the 63 states and provinces of Canada and the United States of America (USA). These species have been described over a period of nearly 270 years, with the most prolific period occurring during the 1930–1940s. The families Hydroptilidae (307), Limnephilidae (255), and Hydropsychidae (159) contain the most species, whereas six families contain less than five species each. Canada and the USA host 644 and 1,487 species, respectively. The states and provinces with the greatest species richness are Tennessee (384), Virginia (383), and Alabama (378), and those with the least are Rhode Island (27), Prince Edward Island (23), and Nunavut (15). Differences in state species assemblages largely followed a geographic pattern, with a non-metric multidimensional scaling ordination suggesting six regions of caddisfly diversity corresponding to the central, far north, northeastern, northwestern, southeastern, and southwestern portions of the study area. Caddisfly species richness was highest in the southeastern region, despite being the smallest region of the six, and lowest in the far north. Species rarefaction predicted 129–181 species remain to be discovered within the two countries, while multiple linear regression modeling using common environmental variables suggested 17 states and provinces with at least 50 species remaining to be found in each. Key words: Aquatic insects, faunistic, North America, species checklist Introduction Trichoptera represents the most species-rich order of primarily aquatic insects, with the number of known, extant species approaching 17,300 in 52 families globally (Morse et al. 2019; Morse 2025). Of the orders containing aquatic taxa, only Diptera contains more freshwater species (Adler and Courtney 2019). Caddisflies are present on all continents except Antarctica. The proliferation of caddisfly diversity is, in part, the result of inhabiting a wide variety of freshwater Academic editor: Ralph W. Holzenthal Received: 26 January 2025 Accepted: 8 April 2025 Published: 10 December 2025 ZooBank: https://zoobank. org/4461CE83-4708-4A92-BC98EA8AC4BB577C Citation: Hogan PN, Houghton DC, Murray-Stoker K, Dewalt RE, Rasmussen AK, Morse JC (2025) An analytical synopsis of caddisfly (Insecta, Trichoptera) taxonomic history and progress in Canada and the United States. In: Ríos-Touma B, Frandsen PB, Holzenthal RW, Houghton DC, Rázuri-Gonzales E, Pauls SU (Eds) Proceedings of the 18th International Symposium on Trichoptera. ZooKeys 1263: 105–122. https://doi.org/10.3897/ zookeys.1263.147986 ZooKeys 1263: 105–122 (2025) DOI: 10.3897/zookeys.1263.147986 106 ZooKeys 1263: 105–122 (2025), DOI: 10.3897/zookeys.1263.147986 Phillip N. Hogan et al.: Trichoptera of Canada and the USA habitat types and is tied to the evolution of portable case-making behavior and other uses of labial silk, with subsequent exploitation of many habitat types (Mackay and Wiggins 1979; Wiggins 1996; Frandsen et al. 2024). Caddisflies perform many ecologically important services within freshwater ecosystems, made even more valuable by the large proportion of invertebrate macrofaunal biomass that caddisflies represent (Morse et al. 2019). Larvae contribute greatly to the cycle and transfer of carbon and nutrients through the processing of fine and coarse particulate organic matter, herbivory of plant tissue, and predation upon other aquatic macroinvertebrates. Adult dispersal from their natal aquatic habitats into the riparian region of terrestrial systems links habitats in food webs through the transfer of organic material, and nutrients (Hicks et al. 2005; Winder et al. 2005; Francis et al. 2006). Species that spin and maintain silken nets (e.g. Annulipalpia) can be very abundant within stream reaches (Wallace and Merritt 1980), leading to the consolidation of sediments within stream beds and supporting habitat stability for other organisms (Cardinale et al. 2004). Further, the diversity of functional feeding groups and robust response to anthropogenic disturbances across different habitat types promotes use of caddisfly assemblages in water quality biomonitoring (Resh and Unzicker 1975; Lenat 1988; Dohet 2002; Houghton 2025). Projections of global caddisfly richness estimate that only 20–25% of species have been described, with most undescribed taxa occurring in the Neotropical, Oriental, and Palearctic regions (de Moor and Ivanov 2008). The Neotropical and Oriental regions contain the highest known species richness (Morse et al. 2019) and also the highest rates of species descriptions during the past four decades (Morse 2016; Santos et al. 2020). In contrast, the rate of caddisfly species descriptions over the same period appears to have declined within the Nearctic and eastern Palearctic regions (Wiberg-Larsen 2008). The adjacent countries of Canada and the United States of America (USA) collectively compose > 90% of the ice-free land area of the Nearctic region (de Moor and Ivanov 2008), encompassing ~20,000,000 km2 within their 63 political subunits (hereafter called “states and provinces”) (Fig. 1). While the caddisflies of the two countries have been studied for almost 270 years (Fig. 2), data on species distributions remain coarse and incomplete, and these knowledge gaps limit the ability to answer questions about richness and endemism. Moreover, regional caddisfly assemblages are currently being restructured by local anthropogenic impacts and ongoing climate change, obscuring historical distributions (Murray-Stoker et al. 2019). Such changes, including species extirpations and changes in functional feeding group ecology, have been clearly identified within the northcentral USA where upstream habitat quality is now the primary driver of caddisfly community structure and species richness (Houghton and Holzenthal 2010; Houghton and DeWalt 2021; Houghton and DeWalt 2023). The caddisfly fauna of Canada and the USA is summarized in the Distributional Checklist of Nearctic Trichoptera (Rasmussen and Morse 2023) and has been the basis for regional (e.g. Houghton et al. 2022) and state and provincial checklists. To date, no study has attempted to analyze or quantify large-scale caddisfly distribution patterns within the two countries. Thus, the objective of this study was to extract data from the Nearctic Checklist and elsewhere to 107 ZooKeys 1263: 105–122 (2025), DOI: 10.3897/zookeys.1263.147986 Phillip N. Hogan et al.: Trichoptera of Canada and the USA provide the first summary of caddisfly taxonomic work from the past three centuries, identify the most prolific taxonomists, determine regions of high endemicity and coarse patterns of species distributions and richness, and to provide direction for future research within Canada and the USA. Methods A species presence or absence matrix, including authors and description dates, was generated for all 63 states and provinces within Canada and the USA from the Nearctic Checklist, as well as from recent studies not yet incorporated into the checklist and our own unpublished data. State and province species records that were noted as dubious or erroneous in the Nearctic Checklist were marked as absent. Species considered nomina dubia were not included. Maps were generated in ArcGIS Pro (ESRI 2024). Chao1, bootstrap, and first-order jackknife extrapolations of species richness within the region were produced from the package “vegan” (Oksanen et al. 2022) Figure 1. The 63 political units (states and provinces) composing continental Canada and the USA, showing the number of caddisfly species reported from each. Caddisfly regions determined by NMDS ordination (Fig. 7). CN: central, FN: far north, NE: northeastern, NW: northwestern, SE: southeastern, SW: southwestern. Data from Rasmussen and Morse (2023) as well as smaller studies and our own unpublished data. State abbreviations: https://www.fs.usda.gov/database/feis/format.html. 108 ZooKeys 1263: 105–122 (2025), DOI: 10.3897/zookeys.1263.147986 Phillip N. Hogan et al.: Trichoptera of Canada and the USA in R v. 2024.09.0+375. Species richness estimates were generated using states and provinces as sample locations and the presence–absence matrix as species incidences within states and provinces. To estimate how the level of taxonomic research in a particular state or province influenced its known species richness, the mean number of citations per species per state or province, based on the Nearctic Checklist, was correlated with the total species richness known per state and province. This relationship was tested for significance using a Spearman rank correlation. Differences in caddisfly assemblages relative to geography were examined with a non-metric multidimensional scaling (NMDS) ordination using the program PCORD v. 7 for Windows (Peck 2016). The data matrix consisted of presence (‘1’) or absence (‘0’) values for each species for each of the 62 analyzed states and provinces. Nunavut was excluded from the analysis since only 15 species are known from that province, and extreme outliers tend to distort the overall ordination (Peck 2016). All species were weighted equally. The NMDS ordination was conducted using the default program settings, 250 randomized runs, and a Jaccard distance measure. A Monte Carlo test was conducted on each determined axis to assess its difference from a random ordination structure (Dexter et al. 2018). To examine trends in species richness relative to size of state or province and environmental data, seven variables were tested for their collective ability to predict known species richness in the 62 states and provinces using multiple linear regression modeling. The analysis was conducted using Excel for Windows with the Real Statistics add-in (http://ww.real-statistics.com). Nunavut was again excluded from the analysis. Latitude and longitude were determined from the approximate middle of each state or province using Google Earth. Total land area, total freshwater area, and percentage of state or province area composed of freshwater were determined from http://www.census.gov for the United States and http://www.statcan.gc.ca for Canada. Mean temperature and precipitation were determined for each state and province from http:// www.currentresults.com. Once a model was produced, its predicted species richness values were compared to reported species richness values for each individual state and province, including Nunavut. This analysis identified states and provinces with reported richness below predicted richness as those for which further research will more likely discover additional species. In addition, the density of species for each state and province was calculated as a number of species divided by the size of the state or province in Mm2. Data resources The data underpinning the analysis reported in this paper are deposited in the Zenodo data repository at https://doi.org/10.5281/zenodo.15176532 (Hogan et al. 2025). Results The history of caddisfly taxonomy in Canada and the USA currently spans nearly 270 years (Fig. 2). The first three North American species were described by Linnaeus in 1758 from Sweden and are Holarctic in distribution. The first caddisfly described from a strictly Nearctic distribution, Glyphopsyche irrorata (Fa- 109 ZooKeys 1263: 105–122 (2025), DOI: 10.3897/zookeys.1263.147986 Phillip N. Hogan et al.: Trichoptera of Canada and the USA bricius) (Limnephilidae), was described 23 years later in 1781. Only 26 species were described during the next 75 years, whereas 127 were described during the 1850s–1870s by Walker and other workers (Fig. 3). Several hundred species were described in the early 1900s by Banks and others. The most prolific period occurred during the 1930s–1940s, when nearly a third of the total fauna (495 species) was described by Ross, Milne, Denning, and others. Ross himself described a quarter (375 species) of the entire fauna from 1938–1971. Species descriptions have averaged around 90 per decade from the 1950s to the 2010s, with about half of the hydroptilid fauna (151 species) described since 1970 by Blickle, Harris, and others (Hogan et al. 2025). Currently, 1,510 caddisfly species are known from Canada and the USA, representing 28 families and 155 genera (Hogan et al. 2025). A total of 644 species representing 111 genera and 24 families are known from Canada, whereas 1,487 species representing 154 genera and 28 families are known from the USA. The states and provinces with the greatest species richness are Tennessee (384 species), Virginia (383), Alabama (378), and California (366), whereas those with the least are the District of Columbia (55), Rhode Island (27), Prince Edward Island (23), and Nunavut (15) (Fig. 1). The overall mean density of species per area among all states and provinces is 2.77 per Mm2, with the highest species densities found in the small northeastern states of District of Columbia (550), Delaware (46), and Connecticut (23) and the lowest in the large far north states and provinces of Alaska (0.16), Northwest Territories (0.10) and Nunavut (0.01) (Hogan et al. 2025). Among families, Hydroptilidae (307) has the greatest species richness, followed by Limnephilidae (255), and Hydropsychidae (159) (Fig. 4). The families Beraeidae (3), Hydrobiosidae (3), Ptilocolepidae (2), Rossianidae (2), Xiphocentronidae (2), and Ecnomidae (1) all are represented by fewer than five species in Canada and the USA. Oecetis inconspicua (Walker) (Leptoceridae) is the most widespread species, occurring in 60 of the 63 (95%) states and provinces analyzed. Triaenodes tardus Milne (Leptoceridae) and Helicopsyche borealis Figure 2. The number of caddisfly species described per decade from Canada and the USA, and the cumulative total from the 1750s to the 2020s. 110 ZooKeys 1263: 105–122 (2025), DOI: 10.3897/zookeys.1263.147986 Phillip N. Hogan et al.: Trichoptera of Canada and the USA (Hagen) (Helicopsychidae) both occur in 55 (87%) states and provinces (Fig. 5). In contrast, 357 species are endemic to a single state or province (Hogan et al. 2025). The state of California (97) has the most endemic species, followed by Oregon (30), Florida (30), Alabama (29), Arizona (29), and Texas (22). All other Figure 3. The total number of caddisfly species described by the top 10 most prolific taxonomists from Canada and the USA. Figure 4. The total number of species for each of the 28 known caddisfly families from Canada and the USA 111 ZooKeys 1263: 105–122 (2025), DOI: 10.3897/zookeys.1263.147986 Phillip N. Hogan et al.: Trichoptera of Canada and the USA states and provinces have < 10 endemic species, and 51 of the 63 states and provinces have ≤ 5 (Hogan et al. 2025). Total known species richness within each state and province was positively (Spearman’s ρ = 0.70, P < 0.001) correlated with the mean number of citations per species of each state and province in the Nearctic Checklist (Fig. 6). The NMDS ordination of species presence or absence per state and province produced a two-dimensional solution (Fig. 7). The two determined axes reflected almost 80% of variation within the dataset. Distribution of the 62 states and provinces in ordination space had a high congruence with states and provinces in geographic space, with the three most notable outliers—Rhode Island, Prince Edward Island, and the District of Columbia—also being the states and province of lowest species richness (Fig. 1). The ordination suggested six faunal regions of caddisfly assemblages, corresponding to the central, far north, northeastern, northwestern, southeastern, and southwestern portions of Canada and the USA. The far north, northwestern, and southwestern regions were more distinct from each other than the central, northeastern, and southeastern regions, although there was no geographic overlap between any of the states or provinces in ordination space. Caddisfly species richness was highest in the southeastern region (684), followed by the central (663), northwestern (608), southwestern (527), northeastern (510), and far north (228) (Table 1). The southeastern region had the highest species richness relative to area and per state. The species richness per area of the southeastern region was especially noteworthy, as it was > 1.5× greater than any other region and > 10× higher than the far north region. A multiple linear regression analysis combining all seven variables produced a significant model (P = 0.04) with low predictive power (R2 = 0.12) (Table 2). Species richness increased with increasing temperature and precipitation and decreased with increasing latitude. Comparing reported species richness to species richness predicted by the model suggested 17 states and provinces with ≥ 50 species remaining to be found in each Figure 5. The most widespread caddisfly species, color-coded by family, from Canada and the USA based on the number of states that are known to contain the species. 112 ZooKeys 1263: 105–122 (2025), DOI: 10.3897/zookeys.1263.147986 Phillip N. Hogan et al.: Trichoptera of Canada and the USA (Table 3). Estimates of total species richness for the entire study area ranged from 1,674 (±72) to 1,861 (±117) from the bootstrap and first-order jackknife methods respectively (Fig. 8). Figure 6. Correlation of species richness and mean citations per species by state and province (n = 62, Spearman’s ρ = 0.70, P < 0.001). Citations that represent the documentation of a particular species in a particular state or province were counted in the Nearctic Checklist. Figure 7. The 62 analyzed states and provinces of Canada and the USA delineated by the results of an NMDS ordination of caddisfly presence or absence per state. The six caddisfly regions determined by ordination results and geographic proximity. Nunavut was excluded from analysis due to only 15 species being reported from it. State abbreviations: https://www.fs.usda.gov/database/feis/format.html. 113 ZooKeys 1263: 105–122 (2025), DOI: 10.3897/zookeys.1263.147986 Phillip N. Hogan et al.: Trichoptera of Canada and the USA Table 1. Summary statistics for the six determined caddisfly regions (Fig. 7) of Canada and the USA. Region Area (Mm2) Total species Species per area Mean species per state Southeastern 1327 684 0.52 276.4 Central 5528 663 0.12 200.4 Northwestern 3275 608 0.19 245.7 Southwestern 1554 527 0.34 202.0 Northeastern 2638 510 0.19 182.6 Far North 4750 228 0.05 107.3 Table 2. Results of a multiple linear regression model of the combined ability of seven variables to predict the known number of caddisfly species within each of the 62 tested states and provinces. Nunavut excluded due to having only 15 species reported to date. Overall model R2 = 0.12, P < 0.043. VIF: variance inflation factor. Variable Coefficient SE T-statistic PVIF Intercept 857.93 268.09 3.20 0.002 Mean latitude −19.23 6.22 −3.09 0.003 19.46 Mean precipitation 3.58 1.43 2.50 0.015 2.67 Mean temperature 20.17 8.20 2.46 0.017 16.98 Mean longitude −2.12 1.03 −2.05 0.045 3.25 Total water area 0.00 0.00 0.65 0.516 7.90 Total land area 0.00 0.00 0.40 0.691 8.46 Percent water area −37.93 193.35 −0.20 0.845 2.06 Table 3. The 17 states and provinces, and their land and freshwater areas, that are predicted to contain at least 50 more caddisfly species based on difference in reported species richness relative to the model generated in Table 2. 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