Phylogenetic systematics of the colorful, cyanide-producing millipedes of Appalachia (Polydesmida, Xystodesmidae, Apheloriini) using a total evidence Bayesian approach

A second species of the pill millipede genus Nearctomeris Wesener, 2012 (Diplopoda, Glomerida) from the Great Smoky Mountains, USA

We describe a second species of Nearctomeris Wesener, 2012, a genus of pill millipede endemic to the southern Appalachians, based on morphological and molecular evidence. The fauna of Glomerida in America is characterized by its low diversity, and Nearctomerissmokysp. nov. is only the fifth species of the order known from the eastern United States. Our phylogenetic analyses based on COI sequences recover a tentatively monophyletic lineage including both eastern American genera Onomeris Cook, 1896 and Nearctomeris, with a common ancestor in the Late Cretaceous to Mid Eocene and extant diversity within genera dating back to the Miocene. Our results suggest that the observed low diversity of the group in the eastern US is likely caused by extinction events, but it is also possible that new species are yet to be found. We provide new records for Nearctomerisinexpectata Wesener, 2012, Onomerisunderwoodi Cook, 1896 and O.australora Hoffman, 1950; the latter is here reported for the first time from South Carolina. We also present DNA barcoding data for all species of Glomerida present in the US that are not yet publicly available.

New species in old mountains: integrative taxonomy reveals ten new species and extensive short-range endemism in Nesticus spiders (Araneae, Nesticidae) from the southern Appalachian Mountains

This revision is based on sampling efforts over the past three decades in the southern Appalachian Mountains which have provided Nesticus (Araneae, Nesticidae) collections of approximately 2100 adult specimens from more than 475 unique collecting events. Using a “morphology first” framework we examined recently collected specimens plus museum material to formulate morphology-based species hypotheses for putative new taxa (discovery phase). Using sequence capture of nuclear ultraconserved elements (UCEs) we analyzed 801 nuclear loci to validate new (and prior) morphology-based species hypotheses (validation phase) and reconstructed a robust backbone phylogeny including all described and new species. Sanger sequencing and UCE-bycatch were also used to gather mitochondrial data for more than 240 specimens. Based on our integrative taxonomic framework ten new Nesticus species are herein described, including N. binfordaesp. nov., N. bondisp. nov., N. caneisp. nov., N. cherokeensissp. nov., N. dellingerisp. nov., N. dykemanaesp. nov., N. jemisinaesp. nov., N. lowderisp. nov., N. roanensissp. nov., and N. templetonisp. nov. Previously unknown males are also described for N. bishopi Gertsch, 1984, N. crosbyi Gertsch, 1984, and N. silvanus Gertsch, 1984, as well as the previously unknown female for N. mimus Gertsch, 1984. Based on combined evidence N. cooperi Gertsch, 1984 is placed in synonymy with N. reclusus Gertsch, 1984. Overall, the montane radiation of Appalachian Nesticus reveals a general lack of species sympatry and compelling biogeographic patterns. Several regional Nesticus taxa are rare, microendemic habitat specialists that deserve conservation attention and detailed future monitoring as conservation sentinels.

The Complete Mitochondrial Genome of Spirobolus bungii (Diplopoda, Spirobolidae): The First Sequence for the Genus Spirobolus

Millipedes (Diplopoda) comprise one of the most important groups of large soil arthropods in terrestrial ecosystems; however, their phylogenetic relationships are poorly understood. Herein, the mitochondrial genome (mitogenome) of Spirobolus bungii was sequenced and annotated, which was 14,879 bp in size and included 37 typical mitochondrial genes (13 protein-coding genes (PCGs), two ribosomal RNA genes (rRNAs), and 22 transfer RNA genes (tRNAs)). Most of the 13 PCGs had ATN (AT/A/T/G) as the start codon except for COX1, which used CGA, and most PCGs ended with the T end codon. By comparing the gene arrangements of the mitogenomes among Diplopoda species, rearrangement occurred between and within orders. In contrast to Narceus annularus, the mitogenome genes of S. bungii had consistent orders but were transcribed in completely opposite directions, which was a novel finding in Spirobolidae. Moreover, the phylogenetic relationships within Diplopoda, which were based on the sequences of 13 PCGs, showed that S. bungii was clustered with N. annularus, followed by Abacion magmun. This indicated that there might be a close relationship between Callipodida and Spirobolida. These results could contribute to further studies on the genetics and evolutionary processes of S. bungii and other Diplopoda species.

A revision of the wilsoni species group in the millipede genus Nannaria Chamberlin, 1918 (Diplopoda, Polydesmida, Xystodesmidae)

Although many new species of the millipede genus Nannaria Chamberlin, 1918 have been known from museum collections for over half a century, a systematic revision has not been undertaken until recently. There are two species groups in the genus: the minor species group and the wilsoni species group. In this study, the wilsoni species group was investigated. Specimens were collected from throughout its distribution in the Appalachian Mountains of the eastern United States and used for a multi-gene molecular phylogeny. The phylogenetic tree recovered Nannaria and the two species groups as monophyletic, with Oenomaeapulchella as its sister group. Seventeen new species were described, bringing the composition of the wilsoni species group to 24 species, more than tripling its known diversity, and increasing the total number of described Nannaria species to 78. The genus now has the greatest number of species in the family Xystodesmidae. Museum holdings of Nannaria were catalogued, and a total of 1,835 records used to produce a distribution map of the species group. Live photographs, illustrations of diagnostic characters, ecological notes, and conservation statuses are given. The wilsoni species group is restricted to the Appalachian region, unlike the widely-distributed minor species group (known throughout eastern North America), and has a distinct gap in its distribution in northeastern Tennessee and adjacent northwestern North Carolina. The wilsoni species group seems to be adapted to mesic microhabitats in middle to high elevation forests in eastern North America. New species are expected to be discovered in the southern Appalachian Mountains.

A revision of the minor species group in the millipede genus Nannaria Chamberlin, 1918 (Diplopoda, Polydesmida, Xystodesmidae)

Millipedes in the family Xystodesmidae (Polydesmida) are often referred to as "colorful, flat-backed millipedes" for their bright aposematic coloration and tendency to form Müllerian mimicry rings in the Appalachian region. However, there are many species of Xystodesmidae that do not display colorful warning patterns, and instead have more cryptic appearances. Perhaps for this reason, groups such as the genus Nannaria have remained understudied, despite containing a large number of undescribed species. Before his death in 2012, R. L. Hoffman worked on a revision of the genus Nannaria, and synthesized material and drawings since 1949. Here the work is continued, inferring a molecular phylogeny of the Nannariini (Nannaria + Oenomaea pulchella), and revealing two clades within the genus. One clade is named the minor species group, and the second is the wilsoni species group. This revision, using a molecular phylogenetic framework, is the basis for descriptions of 35 new species in the minor species group. A multi-gene molecular phylogeny is used to make taxonomic changes in the taxon. Eleven putative species of Nannaria are also illustrated and discussed. Additionally, detailed collection, natural history and habitat notes, distribution maps, and a key to species of the Nannaria minor species group are provided. These items are synthesized as a basis for a revision of the genus, which hopefully will aid conservation and evolutionary investigations of this cryptic and understudied group.

Is geography an accurate predictor of evolutionary history in the millipede family Xystodesmidae?

For the past several centuries, millipede taxonomists have used the morphology of male copulatory structures (modified legs called gonopods), which are strongly variable and suggestive of species-level differences, as a source to understand taxon relationships. Millipedes in the family Xystodesmidae are blind, dispersal-limited and have narrow habitat requirements. Therefore, geographical proximity may instead be a better predictor of evolutionary relationship than morphology, especially since gonopodal anatomy is extremely divergent and similarities may be masked by evolutionary convergence. Here we provide a phylogenetics-based test of the power of morphological versus geographical character sets for resolving phylogenetic relationships in xystodesmid millipedes. Molecular data from 90 species-group taxa in the family were included in a six-gene phylogenetic analysis to provide the basis for comparing trees generated from these alternative character sets. The molecular phylogeny was compared to topologies representing three hypotheses: (1) a prior classification formulated using morphological and geographical data, (2) hierarchical groupings derived from Euclidean geographical distance, and (3) one based solely on morphological data. Euclidean geographical distance was not found to be a better predictor of evolutionary relationship than the prior classification, the latter of which was the most similar to the molecular topology. However, all three of the alternative topologies were highly divergent (Bayes factor >10) from the molecular topology, with the tree inferred exclusively from morphology being the most divergent. The results of this analysis show that a high degree of morphological convergence from substantial gonopod shape divergence generated spurious phylogenetic relationships. These results indicate the impact that a high degree of morphological homoplasy may have had on prior treatments of the family. Using the results of our phylogenetic analysis, we make several changes to the classification of the family, including transferring the rare state-threatened species Sigmoria whiteheadi Shelley, 1986 to the genus Apheloria Chamberlin, 1921-a relationship not readily apparent based on morphology alone. We show that while gonopod differences are a premier source of taxonomic characters to diagnose species pairwise, the traits should be viewed critically as taxonomic features uniting higher levels.

The first entomophthoralean killing millipedes, Arthrophaga myriapodina n. gen. n. sp., causes climbing before host death

A new species and genus of entomophthoralean fungus, Arthrophaga myriapodina kills polydesmid millipedes. This species was first seen over a century ago but never described. It is the first millipede pathogen known from the order Entomophthorales, species of which are best known as pathogens of a wide diversity of insects. The fungus induces pre-death climbing behavior in its hosts, enabling the fungus to broadcast its forcibly-discharged conidia from a high vantage, which presumably increases the fitness of the fungus. Study of herbarium specimens and photographic discoveries on the internet suggest the fungus occurs widely in eastern North America.

A general methodology for collecting and preserving xystodesmid and other large millipedes for biodiversity research

Background

With an estimated 80% of species remaining undescribed (but see Brewer et al. 2012), millipede taxonomy offers the opportunity to discover new species and explore biodiversity. The lack of basic alpha taxonomic information regarding millipedes belies their significant ecological role and potential as premier models in ecological and evolutionary studies. The group possesses many fascinating biological properties (e.g., bioluminescence, mimicry, and complex chemical secretions) that have been the focus of several recent studies and are emerging avenues of future investigation.

New information

Here we summarize a methodology for large-bodied millipede collection, curation, and preservation for genetic analyses with the hope that sharing these techniques will stimulate interest in these charismatic detritivores.

Discovery of a glowing millipede in California and the gradual evolution of bioluminescence in Diplopoda

The rediscovery of the Californian millipede Xystocheir bistipita surprisingly reveals that the species is bioluminescent. Using molecular phylogenetics, we show that X. bistipita is the evolutionary sister group of Motyxia, the only genus of New World bioluminescent millipedes. We demonstrate that bioluminescence originated in the group's most recent common ancestor and evolved by gradual, directional change through diversification. Because bioluminescence in Motyxia has been experimentally demonstrated to be aposematic, forewarning of the animal's cyanide-based toxins, these results are contrary to aposematic theory and empirical evidence that a warning pattern cannot evolve gradually in unpalatable prey. However, gradual evolution of a warning pattern is plausible if faint light emission served another function and was co-opted as an aposematic signal later in the diversification of the genus. Luminescence in Motyxia stem-group taxa may have initially evolved to cope with reactive oxygen stress triggered by a hot, dry environment and was repurposed for aposematism by high-elevation crown-group taxa colonizing new habitats with varying levels of predation. The discovery of bioluminescence in X. bistipita and its pivotal phylogenetic location provides insight into the independent and repeated evolution of bioluminescence across the tree of life.

Evolution of body shape in differently coloured sympatric congeners and allopatric populations of Lake Malawi's rock-dwelling cichlids

The cichlid fishes of Lake Malawi represent one of the most diverse adaptive radiations of vertebrates known. Among the rock-dwelling cichlids (mbuna), closely related sympatric congeners possess similar trophic morphologies (i.e. cranial and jaw structures), defend overlapping or adjacent territories, but can be easily distinguished based on male nuptial coloration. The apparent morphological similarity of congeners, however, leads to an ecological conundrum: theory predicts that ecological competition should lead to competitive exclusion. Hence, we hypothesized that slight, yet significant, ecological differences accompanied the divergence in sexual signals and that the divergence of ecological and sexual traits is correlated. To evaluate this hypothesis, we quantified body shape, a trait of known ecological importance, in populations of Maylandia zebra, a barred, widespread mbuna, and several sympatric nonbarred congeners. We found that the barred populations differ in body shape from their nonbarred sympatric congeners and that the direction of shape differences was consistent across all barred vs. nonbarred comparisons. Barred populations are generally deeper bodied which may be an adaptation to the structurally complex habitat they prefer, whereas the nonbarred species have a more fusiform body shape, which may be adaptive in their more open microhabitat. Furthermore, M. zebra populations sympatric with nonbarred congeners differ from populations where the nonbarred phenotype is absent and occupy less morphospace, indicating potential ecological character displacement. Mitochondrial DNA as well as published AFLP data indicated that the nonbarred populations are not monophyletic and therefore may have evolved multiple times independently. Overall our data suggest that the evolution of coloration and body shape may be coupled as a result of correlational selection. We hypothesize that correlated evolution of sexually selected and ecological traits may have contributed to rapid speciation as well as the maintenance of diversity in one of the most diverse adaptive radiations known.

Molecular phylogeny of the Thyropygus allevatus group of giant millipedes and some closely related groups

Giant cylindrical millipedes of the family Harpagophoridae, especially species of the genus Thyropygus, are broadly distributed in Thailand and nearby countries. They show a great deal of variation in body size, color patterns and gonopodal characters. Phylogenetic analyses of 26 nominate species from six genera in the subfamilies Harpagophorinae and Rhynchoproctinae, as well as nine new morphotypes (regarded as new species), were performed with the DNA sequences from two mitochondrial gene fragments (16S rRNA and COI). The genus Thyropygus (Harpagophorinae) was recovered as monophyletic under all analyses, whilst the representatives of Rhynchoproctinae also formed a monophyletic group. However, the analyses suggested that the tribe Gonoplectini should be elevated to a separate subfamily, Gonoplectinae. The molecular analyses were largely (but not totally) congruent with, and so supported the usefulness of, gonopodal characters for the classification and identification of harpagophorid millipedes, and additionally supported previous studies on the delimitation of species and subgroups. This is the first molecular study inside the family Harpagophoridae and provides the basis for further studies of the evolutionary processes and biogeographic patterns of millipedes in Southeast Asia.

Ordinal-level phylogenomics of the arthropod class Diplopoda (millipedes) based on an analysis of 221 nuclear protein-coding loci generated using next-generation sequence analyses

BACKGROUND

The ancient and diverse, yet understudied arthropod class Diplopoda, the millipedes, has a muddled taxonomic history. Despite having a cosmopolitan distribution and a number of unique and interesting characteristics, the group has received relatively little attention; interest in millipede systematics is low compared to taxa of comparable diversity. The existing classification of the group comprises 16 orders. Past attempts to reconstruct millipede phylogenies have suffered from a paucity of characters and included too few taxa to confidently resolve relationships and make formal nomenclatural changes. Herein, we reconstruct an ordinal-level phylogeny for the class Diplopoda using the largest character set ever assembled for the group.

METHODS

Transcriptomic sequences were obtained from exemplar taxa representing much of the diversity of millipede orders using second-generation (i.e., next-generation or high-throughput) sequencing. These data were subject to rigorous orthology selection and phylogenetic dataset optimization and then used to reconstruct phylogenies employing Bayesian inference and maximum likelihood optimality criteria. Ancestral reconstructions of sperm transfer appendage development (gonopods), presence of lateral defense secretion pores (ozopores), and presence of spinnerets were considered. The timings of major millipede lineage divergence points were estimated.

RESULTS

The resulting phylogeny differed from the existing classifications in a number of fundamental ways. Our phylogeny includes a grouping that has never been described (Juliformia+Merocheta+Stemmiulida), and the ancestral reconstructions suggest caution with respect to using spinnerets as a unifying characteristic for the Nematophora. Our results are shown to have significantly stronger support than previous hypotheses given our data. Our efforts represent the first step toward obtaining a well-supported and robust phylogeny of the Diplopoda that can be used to answer many questions concerning the evolution of this ancient and diverse animal group.

Arthropod phylogenetics in light of three novel millipede (myriapoda: diplopoda) mitochondrial genomes with comments on the appropriateness of mitochondrial genome sequence data for inferring deep level relationships

BACKGROUND

Arthropods are the most diverse group of eukaryotic organisms, but their phylogenetic relationships are poorly understood. Herein, we describe three mitochondrial genomes representing orders of millipedes for which complete genomes had not been characterized. Newly sequenced genomes are combined with existing data to characterize the protein coding regions of myriapods and to attempt to reconstruct the evolutionary relationships within the Myriapoda and Arthropoda.

RESULTS

The newly sequenced genomes are similar to previously characterized millipede sequences in terms of synteny and length. Unique translocations occurred within the newly sequenced taxa, including one half of the Appalachioria falcifera genome, which is inverted with respect to other millipede genomes. Across myriapods, amino acid conservation levels are highly dependent on the gene region. Additionally, individual loci varied in the level of amino acid conservation. Overall, most gene regions showed low levels of conservation at many sites. Attempts to reconstruct the evolutionary relationships suffered from questionable relationships and low support values. Analyses of phylogenetic informativeness show the lack of signal deep in the trees (i.e., genes evolve too quickly). As a result, the myriapod tree resembles previously published results but lacks convincing support, and, within the arthropod tree, well established groups were recovered as polyphyletic.

CONCLUSIONS

The novel genome sequences described herein provide useful genomic information concerning millipede groups that had not been investigated. Taken together with existing sequences, the variety of compositions and evolution of myriapod mitochondrial genomes are shown to be more complex than previously thought. Unfortunately, the use of mitochondrial protein-coding regions in deep arthropod phylogenetics appears problematic, a result consistent with previously published studies. Lack of phylogenetic signal renders the resulting tree topologies as suspect. As such, these data are likely inappropriate for investigating such ancient relationships.

Millipede taxonomy after 250 years: classification and taxonomic practices in a mega-diverse yet understudied arthropod group

BACKGROUND

The arthropod class Diplopoda is a mega-diverse group comprising >12,000 described millipede species. The history of taxonomic research within the group is tumultuous and, consequently, has yielded a questionable higher-level classification. Few higher-taxa are defined using synapomorphies, and the practice of single taxon descriptions lacking a revisionary framework has produced many monotypic taxa. Additionally, taxonomic and geographic biases render global species diversity estimations unreliable. We test whether the ordinal taxa of the Diplopoda are consistent with regards to underlying taxonomic diversity, attempt to provide estimates for global species diversity, and examine millipede taxonomic effort at a global geographic scale.

METHODOLOGY/PRINCIPAL FINDINGS

A taxonomic distinctness metric was employed to assess uniformity of millipede ordinal taxa. We found that ordinal-level taxa are not uniform and are likely overinflated with higher-taxa when compared to related groups. Several methods of estimating global species richness were employed (Bayesian, variation in taxonomic productivity, extrapolation from nearly fully described taxa). Two of the three methods provided estimates ranging from 13,413-16,760 species. Variations in geographic diversity show biases to North America and Europe and a paucity of works on tropical taxa.

CONCLUSIONS/SIGNIFICANCE

Before taxa can be used in an extensible way, they must be definable with respect to the diversity they contain and the diagnostic characters used to delineate them. The higher classification for millipedes is shown to be problematic from a number of perspectives. Namely, the ordinal taxa are not uniform in their underlying diversity, and millipedes appear to have a disproportionate number of higher-taxa. Species diversity estimates are unreliable due to inconsistent taxonomic effort at temporal, geographic, and phylogenetic scales. Lack of knowledge concerning many millipede groups compounds these issues. Diplopods are likely not unique in this regard as these issues may persist in many other diverse yet poorly studied groups.

Phylogenetics of the millipede genus Brachycybe Wood, 1864 (Diplopoda: Platydesmida: Andrognathidae): patterns of deep evolutionary history and recent speciation

The genus Brachycybe Wood is a little known group of millipedes comprising eight nominal species distributed throughout North America, Japan, South Korea, Taiwan, and China. The group's species are relatively morphologically homogenous and have been described primarily on the basis of differences in somatic morphology largely ignoring the often-used characters in millipede taxonomy and systematics - male genitalia (the gonopods). The objectives of this study were to survey male gonopods with the aim of evaluating inter-specific variation, assess existing species boundaries and phylogeny using molecular characters, examine the historical biogeography of the genus, and estimate the timing of lineage divergence using a molecular clock. We surveyed two mitochondrial genes (cytochrome c oxidase I and cytochrome b) and one nuclear protein-coding gene (glutamyl and prolyl-tRNA synthetase) each comprising 952, 746, and 555 aligned base pairs respectively. Phylogenetic inference coupled with an analysis of species delimitation using a generalized mixed Yule coalescent model recovered eight species, two of which were considered to be cryptic. Molecular dating analyses coupled with ancestral range reconstructions indicate that the group is quite ancient (age>50 million years) and its origins are likely traced back geographically to the mountains of California's Sierra Nevada and Coastal Ranges. The genus Brachycybe appears to have expanded its range at least twice out of present day California into eastern North America and at least once into Asia. This study highlights the need for integrative approaches to describe biodiversity and furthers the evidence for cryptic diversity even in groups where genitalia are generally thought to be rapidly evolving, diagnostic features.

Phylogeny of the millipede order Spirobolida (Arthropoda: Diplopoda: Helminthomorpha)

This study examines relationships within the millipede order Spirobolida using an exemplar approach, sampling within families to maximize geographical and morphological diversity; due to lack of available material, Allopocockiidae and Hoffmanobolidae were not included in analyses. The focus of this study was to test monophyly of the order, the suborders, and the families of Spirobolida and to propose interfamilial relationships using morphological and molecular data in a total-evidence approach. Both maximum-parsimony analyses and Bayesian inference were employed to analyse two datasets consisting of combined morphological and molecular data, one aligned using progressive alignment methods and the second aligned by secondary structure models. Rhinocricidae was recovered sister to all remaining spirobolidan millipedes and is elevated to suborder status as suborder Rhinocricidea. Trigoniulidea was recovered as monophyletic as was Spirobolidea excluding Rhinocricidae; Spirobolidea is redefined to reflect this change. All previously recognized families were recovered, with the exception of Spirobolidae; in all instances, this family was paraphyletic or part of a polytomy that lacked sufficient resolution to assess its monophyly. The results reaffirm much of the existing taxonomic foundation within Spirobolida. This study provides the first phylogenetic test of higher-level relationships within Spirobolida and will serve as a foundation for future work in this group at finer levels. © The Willi Hennig Society 2010.

The symbiotic mites of some Appalachian Xystodesmidae (Diplopoda:Polydesmida) and the complete mitochondrial genome sequence of the mite Stylochyrus rarior (Berlese) (Acari:Mesostigmata:Ologamasidae)

Millipedes of the family Xystodesmidae (Polydesmida) are often host to several symbiotic mite species, but very little work has been done to identify these acarines or to understand their relationship to the millipedes. In an attempt to better understand these associations, mites found on xystodesmid millipedes, a group for which a species phylogeny has been proposed, were collected in the Appalachian Mountains of Kentucky, Virginia, Tennessee and North Carolina. Mites in the genera Stylochyrus Canestrini & Canestrini, 1882 (Mesostigmata: Ologamasidae) and Schwiebea Oudemans, 1916 (Sarcoptiformes: Acaridae) were prevalent among millipedes in the genera Apheloria Chamberlin, 1921, Appalachioria Marek & Bond, 2006, Boraria Chamberlin, 1943, Brachoria Chamberlin, 1939, Dixioria Chamberlin, 1947, Nannaria Chamberlin, 1918, Pleuroloma Rafinesque, 1820, Prionogonus Shelley, 1982, Rudiloria Causey, 1955 and Sigmoria Chamberlin, 1939. Of the mite taxa collected, the species Stylochyrus rarior (Berlese, 1916) was found on the greatest number of sampled millipede taxa. To enhance future coevolutionary studies of xystodesmid millipedes and their mite symbionts, the complete mitochondrial genome of S. rarior associated with the millipede genus Apheloria (Polydesmida: Xystodesmidae) was sequenced. The genome is 14 899 nucleotides in length, has all the typical genes of an arthropod mitochondrion, differs in gene arrangement from that of the ancestral arthropod, and has a gene order that is unique among mites and ticks. The major difference in S. rarior is the placement of the protein-coding gene nad1, which is positioned between the rRNA gene 12S and the protein-coding gene nad2 (tRNA genes and non-coding regions excluded). There are also two non-coding control regions within this mitochondrial genome.

Current status of the Myriapod class diplopoda (millipedes): taxonomic diversity and phylogeny

The arthropod class Diplopoda, the millipedes, ranks among the most diverse groups of terrestrial organisms, with over 12,000 species described. Although they play an important ecological role in most terrestrial ecosystems, little is known about the group's diversity, morphology, and phylogeny compared with other arthropod groups. We review diplopod natural history and discuss the historical and current literature pertaining to millipede morphology, ecology, chemical defenses, and the paleontological record of the group's ancient history. Diplopod systematics, past and present, are reviewed with a focus on taxonomy, collections, and biogeography. The phylogenetics of the class is reviewed, with particular attention on diplopod placement within the Myriapoda and emphasis on recent advances using molecular approaches to phylogenetic reconstruction. We present (a) the first combined morphological and molecular analysis of the millipede orders, and (b) a list of critically evaluated characteristics of nominal clades identifying putative apomorphies.