The Odonata are considered among the most endangered freshwater faunal taxa. Their DNA‐based monitoring relies on validated reference data sets that are often lacking or do not cover important biogeographical centres of diversification. This study presents the results of a DNA barcoding campaign on Odonata, based on the standard 658‐bp 5′ end region of the mitochondrial COI gene, involving the collection of 812 specimens (409 of which barcoded) from peninsular Italy and its main islands (328 localities), belonging to all the 88 species (31 Zygoptera and 57 Anisoptera) known from the country. Additional BOLD and GenBank data from Holarctic samples expanded the data set to 1,294 DNA barcodes. A multi‐approach species delimitation analysis involving two distance (OT and ABGD) and four tree‐based (PTP, MPTP, GMYC and bGMYC) methods was used to explore these data. Of the 88 investigated morphospecies, 75 (85%) unequivocally corresponded to distinct molecular operational units, whereas the remaining ones were classified as ‘warnings’ (i.e. showing a mismatch between morphospecies assignment and DNA‐based species delimitation). These results are in contrast with other DNA barcoding studies on Odonata showing up to 95% of identification success. The species causing warnings were grouped into three categories depending on if they showed low, high or mixed genetic divergence patterns. The analysis of haplotype networks revealed unexpected intraspecific complexity at the Italian, Palearctic and Holarctic scale, possibly indicating the occurrence of cryptic species. Overall, this study provides new insights into the taxonomy of odonates and a valuable basis for future DNA and eDNA‐based monitoring studies.

37Freshwater macroinvertebrates, and specifically Odonata, are considered among the most 38 endangered faunal groups. Their biomonitoring has been progressively supported by DNA-39 based tools whose performance and accuracy rely on validated reference datasets that, in some 40 cases, are lacking or do not cover important biogeographical centres of diversification. 41This study reports the results of a DNA barcoding campaign on Odonata, involving the 42 collection of 812 specimens (448 of which barcoded) from Italy and its main islands (328 43 localities), belonging to the 88 species (31 Zygoptera and 57 Anisoptera) inhabiting the country. 44 Additional BOLD and GenBank data from Holarctic samples of the same taxa expanded the 45 dataset to 1294 DNA barcodes. An integrative species delimitation analysis involving two 46 distance (OT and ABGD) and four tree-based (PTP, MPTP, GMYC, bGMYC) approaches 47 identified warnings of possible taxonomic relevance. Of the 88 investigated species, 85% could 48 be successfully identified by their DNA barcodes, with damselflies showing a percentage of 49 warnings (29%) higher than dragonflies (7%), contrasting with the other available DNA 50 barcoding studies on Odonata (showing up to 95% of identification success). The species 51causing warnings were grouped in three categories depending on if they showed low, high or 52 mixed genetic divergence patterns. Moreover, for the second class of warnings, the analysis of 53 haplotypes revealed unexpected structure at the Italian, Palearctic and Holarctic scale. Overall, 54 the DNA barcoding inventory assembled in this study will provide valuable insights into the 55 systematics and conservation of many odonate species with implications for future DNA and 56 eDNA monitoring-based studies. 57 58

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The nests of secondary cavity-nesters located in tree cavities may form specific microhabitats of conservation importance due to their limited accessibility and availability. Species-specific nesting materials in nests of different secondary cavity-nesters may furthermore provide very different microhabitats for arthropods. The potential differences in arthropod communities inhabiting nests of different bird species in excavated cavities or nest boxes have, however, rarely been studied despite their relevance for conservation. Here we investigated the diversity and composition of arthropod communities in these different cavity types and bird species’ nests in managed boreal forests. We identified morphologically and by DNA-metabarcoding arthropods in nest materials that were collected in and compared between (i) woodpecker-size cavities from seven different combinations of cavity type (nest box or excavated cavity), tree species (aspen or pine) and accumulation history of nest materials (single-season cleaned or uncleaned nest boxes that accumulated nests of passerines or an owl species); and (ii) nests of two different passerine species in small nest boxes. We identified 64 arthropod taxa in ten orders, from which Diptera, Coleoptera, Siphonaptera, and Lepidoptera were the most abundant. Shannon diversity index was similar among the cavity-nest-type combinations, but taxa richness was the highest in the owl nests. The arthropod communities (especially Histeridae beetles) deviated most from the other types of nests in owl and aspen cavity nests with more advanced decomposition of nest material (soil or wet environment related taxa). The differences in arthropod communities between the different nest types point out the importance of the ecological chain “tree cavities—bird nests—arthropod communities”.