Sunday, March 5, 2017

Announcing the Iberobaeniidae

Holotype of Urtea graeca, mustachioed with silly maxillary palporgans (Paulus, 2004)
Undoubtedly, Europe is one of the regions most thoroughly picked-over by descriptive taxonomists, particularly in the case of an insect group as comparatively popular with entomologists as the beetles (Coleoptera). Nonetheless, unexpected oddities in the beetle department do crop up in the western Palearctic from time to time. The incomparable Crowsoniella relicta (placed in a monotypic family) is known from soil samples collected from the base of a single Italian chestnut tree (Pace, 1975), while Urtea graeca (a genus and species known from a single locality in northern Greece) is the only European representative of the oddball Atractocerinae (Lymexylidae), a wood-boring taxon that aside from a single Japanese species (Arractocetus nipponicus) is restricted to the tropics and southern hemisphere (Paulus, 2004).

Male Iberobaenia minuta; scale bar=1 mm.
Last December, an entirely new beetle family was described from the mountains of southern Spain, including the single genus Iberobaenia and two species distinguished upon the basis of the male genitalia (Bocak et al., 2016). What appears to be a third, still-undescribed species is known only from a third-instar larva. The circumstances of their discovery warrant some explanation: the larvae were collected for soil biodiversity surveys in southern Spain and noticed when metagenomic analysis of these samples revealed mitochondrial sequences within the Elateroidea (click beetles and kin) that did not nest within any family; these sequences allowed linkage of these larvae with adult male specimens, which had been collected by sweeping foliage. Interestingly, the relative positioning of protein-coding and ribosomal genes in the beetle's mitogenome had been altered from the putatively ancestral condition, this being the first such rearrangement documented in the Coleoptera (Andujar et al., 2016) and an abnormal occurrence for any insect, which tend to be conservative with respect to mitochondrial gene order (Cameron, 2014).

In appearance, Iberobaenia is clearly elateroid in both known ontogenetic stages. The tiny, soft-bodied male imagos resemble, in their reduced mouthparts and lack of elytral costae or tubercles, other elateroid lineages that have undergone miniaturization relative to the superfamily's norm. Their habitus is most comparable to such genera as Paradrilus (Omalisidae) and Antennolycus (Lycidae), but iberobaeniids differ from these in their hypognathous* mouthparts (in contrast to omalisids), lack of a pronotal margin (differing from lycids and non-paradriline omalisids in this respect), and uncompressed legs (here being distinct from otherwise comparable lycids). Larvae are not dorsoventrally flattened and lack urogomphi (unlike their lycid counterparts), with robust convergent mandibles (Bocak et al., 2016).

Female Duliticola paradoxa (Lycidae): image by Bernard Dupont
A taxonomically comprehensive molecular phylogenetic analysis of the Elateroidea, using two loci each from the nuclear and mitochondrial genomes, retrieved Iberobaenia as the sister group of the Lycidae (net-winged beetles). Male miniaturization in the genus is noteworthy in terms of speculation on iberobaeniid biology. Elateroids with males of this dwarfish, soft-bodied description tend to be sexually dimorphic, and if comparison of Iberobaenia to these better-known cousins is valid, its unknown females are probably larviform crustacean-like things far larger than their paramours. This syndrome has evolved on multiple occasions throughout the Elateroidea, with examples of flightless lycid females looking more like denizens of the Sea of Corruption than inhabitants of the East Indies (see above).

While Iberobaenia is certainly distinctive in that it cannot be confidently placed in any elateroid family, I personally doubt that its assignment to an entirely new one will hold under future acquisition of more genetic data. From a morphological standpoint, the proposed Iberobaeniidae lacks any nomothetic characters, meaning that we must rely upon the phylogenetic analysis of Bocak et al. (2016) for justification of its ranking as family. This phylogeny (with Elaterophanes and Aphodiites, which are respectively the oldest known elateriforms and scarabaeiforms, as calibration priors) suggests that Iberobaenia branched from its most recent common ancestor in the mid-Jurassic: I wonder how much water mitogenomes (from which half of the data used in this analysis were drawn) hold at that timescale, given their famously rapid rate of mutation (Brown et al., 1979). 

As with so many things in our world, more data would be welcome.

*With mouthparts directed downwards.
Superficially cercus-like processes on the terminal segments of some beetle larvae.
One particular feature that is unique to the taxon in question.

Andujar, C.; Arribas, P.; Linard, B.; Kundrata, R.; Bocak, L. and Vogler, A. P. (2016). The mitochondrial genome of Iberobaenia (Coleoptera: Iberobaeniidae): first rearrangement of protein-coding genes in the beetles. Mitochondrial DNA Part A: DNA Mapping, Sequencing, & Analysis; 28(2), 156-158. Retrieved 3/3/17 from

Bocak, L.; Kundrata, R.; Fernandez, C. A.; and Vogler, A. P. (2016). The discovery of Iberobaeniidae (Coleoptera: Elateroidea): a new family of beetles from Spain, with immatures detected by environmental DNA sequencing. Proceedings of the Royal Society of Britain, 283, 1-7. Retrieved 3/5/17 from

Brown, W. M.; George, M. Jr.; and Wilson, A. C. (1979). Rapid evolution of animal mitochondrial DNA. Proc. Nat. Acad. Sci. USA, 79, 1967-1971. 

Cameron, S. L. (2014). Insect mitochondrial genetics: implications for evolution and phylogeny. Annual Review of Entomology, 59, 95-117. 

Pace, R. (1975). An exceptional endogeous beetle: Crowsoniella relicta n. gen. n. sp. of Archostemata Tetraphaleridae from central Italy. Bolletino del Museo Civico di Storia Naturale, Verona; 2, 445-458.

Paulus, H. F. (2004). Urtea graeca nov. gen. et nov. spec., der erste Vertreter tropischen Atractocerinae in Europa sowie eine Beschreibung von Hymaloxylon aspoecki nov. spec. aus Yunnan (China) (Coleoptera, Cucujiformia, Lymexylidae, Atractocerinae nov. status). Denisia, 277-290. Retrieved 2/11/17 from 

Saturday, January 28, 2017

A Flagrant Exercise in Laziness Concerning Chalcidoids

After several years of deferrals, I finally got around this May to penning a post for the Ohio State University's Triplehorn Insect Collection. I'll take this opportunity to shamelessly self-plagiarize.

I have always been fascinated by insects, but it was not until, at age 15, I took a week-long field insect taxonomy course at the Ohio State University’s Stone Laboratory. There, I learned the conventions of arthropod collection and preservation and something of proper curatorial practices. Ever since, I have steadily accumulated a collection with pretensions made modest by the limited resources of a teenager; and collection and identification remain an exceedingly enjoyable activity for me.

Therefore, it was only natural that I gravitated to the Triplehorn Insect Collection upon commencing my undergraduate career. Sorting unidentified specimens was easily my favorite task there. Although the collection contains the full range of insect diversity, those specimens that I was tasked with identifying almost always belonged to the order Hymenoptera—often casually referred to as “ants, bees, and wasps”, but technically including far more taxa than simply those that happen to have colloquial names. Being one of the four most diverse insect orders, the variety of Hymenoptera is considerable: and I encountered much of their phylogenetic span through this process, while becoming intimately familiar with Goulet and Huber’s tome Hymenoptera of the World: an Identification Guide to Families (1993) (PDF), the monochromatic line drawings within which—printed on thick, coarse paper—have caused hymenopterists to nickname it “the coloring book”.

Paratype of Dicopomorpha echmepterygis, male (Huber & Noyes, 2013)
As an exemplar of the many taxa with which I thereby became familiar, I have chosen to briefly discuss the superfamily Chalcidoidea herein. These parasitoid wasps are one of those aforementioned many prominent insect taxa that have no name in the vernacular—understandable, given that the vast majority of these particular parasitoids are a few millimeters in length or less. (Indeed, the smallest insect known to science—the 0.13-mm.-long male of Dicopomorpha echmepterygis [Mockford, 1997]—is a chalcidoid.)
Unidentified pteromalid from the Amazon Basin, photographed by yours truly (©OSU)
This diminution has also resulted in a lack of taxonomic attention from entomologists, and chalcidoid systematics is by consequence a frustratingly opaque matter—something one is immediately impressed with while attempting to identify the miniscule things: keys are peppered with qualifiers like “usually” and “most”, not to mention annotated with lengthy footnotes elucidating the exceptions to each couplet. The fundamental problem at hand, as Goulet and Huber point out, is that chalcidoid families are often defined by combinations of characters, as opposed to singular traits that are unique to that taxa and none other (autapomorphies, in cladistic terms). This has resulted in a superfamily littered with taxa whose boundaries are under constant debate (e.g., the Agaonidae) or that do not hold up to scrutiny whatsoever (the grossest wastebasket taxon of flagrant wastebasket taxa, the Pteromalidae).

Female Sycoryctes cyathistipula (Agaonidae: Sycoryctinae) (©Iziko Museums of South Africa)
Chalcidoids are hardly deserving of this neglect, considering their ecological and numerical diversity (they possibly constitute 10% of all insect species; Noyes, 2003). I would have impartially respected this significance regardless of my work at the Collection, but parsing through unit tray after unit tray of nigh-microscopic specimens representing untold numbers of species—each one a chalcidoid—gave me a concrete grasp of that abstraction.

I still have strong visual impressions of many of them: the subtly turquoise, spatula-shaped abdomen I swiftly came to associate with the Tetracampidae; the minute serrations on the inner rims of a stocky chalcidid’s femora, making its thighs appear like chitinous razors; the oar-like forewings of many an insubstantial mymarid, fringed with haloes of setae; the metallic, spindle-shaped abdomen that accounted for two-thirds the length of a sycoryctine. I am not the only one to have thought them often quite showy under sufficient magnification: Alexandre A. Girault, a notoriously verbose chalcidologist, spoke of the tiny wasps as “gem-like inhabitants of the woodlands, by most never seen or dreamt of” (Thomer & Twidale, 2014).

Suffice it to say, without my work at the Collection, I would not have seen nor dreamt of so many chalcidoids.
Huber, J. T. and Noyes, J. S. (2013). A new genus and species of fairyfly, Tinkerbella nana (Hymenoptera, Mymaridae), with comments on its sister genus Kikiki , and discussion on small size limits in arthropods. Journal of Hymenoptera Research, 32, 17-44. Retrieved 1/6/17 from

Mockford, E. L. (1997). A new species of Dicopomorpha (Hymenoptera: Mymaridae) with diminutive, apterous males. Annals of the Entomological Society of America, 90, 115-120.

Noyes, J. S. (2003). Universal Chalcidoidea Database. Retrieved 5/18/16 from
Thomer, A. K. and Twidale, M. B. (2014). How Databases Learn. In: iConference 2014 Proceedings (pp. 827-833). Retrieved 4/8/15 from

Wednesday, July 27, 2016

Yet More Haidomyrmecine Strangeness

Recently, I ran across the artwork shown below, depicting an ant (Formicidae) by the name of Ceratomyrmex ellenbergi; according to the author, the creature is known in fossil form from mid-Cretaceous Burmese amber. Since I found the supposed reconstruction on a forum nominally devoted to speculative evolution, I naturally assumed the insect to be fictitious. Not biologically implausible, by any means: but that seta-covered projection on the head, nearly touched by vertical scythe-mandibles? Give me a break. Clearly, this was merely an admirable thought experiment, too outlandish to be genuine.

Impression of Ceratomyrmex ellenbergi by Joschua Knüppe
Of course, much to my pleasure and chagrin, Ceratomyrmex is quite real (Perrichot et al., 2016): and no artistic license was taken. It is a member of the Haidomyrmecini, a basal tribe of the Formicidae known only from the mid-Cretaceous, and already characterized by unique cephalic equipment consisting of a setose median lobe (which could be interpreted as an extension the clypeus; Barden & Grimaldi, 2013) directly betwixt the antennae, scythe-shaped mandibles thatuniquely among antsswing on a vertical axis, and a singularly elongated face to accommodate these mandibles. 

It appears that Ceratomyrmex represents an exaggeration of these haidomyrmecine traits. The elevated setose portion of the clypeus* is extended forwards and to a tremendous length (hence the nickname "unicorn ant") and has been warped into a spatulate shape, with pegs restricted to its spoon-like tip and setae running the process' length. Just below the base of the antennal sockets emanate trigger hairs, which are correspondingly longer than in any other haidomyrmecine: indeed, they are more like whiskers than anything else (Perrichot et al., 2016).
Anochetus sp. (Ponerinae) with mandibles in striking position; photographed by Alex Wild (who else?)
As the term "trigger" would suggest, it has been imputed that haidomyrmecines were ambush predators with a "trap-jaw" strategy, much like living Anochetus or Myrmoteras; dissimilar to the likes of these, however, the mandibles would have snapped upwards upon triggering, pressing prey items against the spatulate clypeal process, the setae upon which would have provided gripping capability. While the majority of modern ants which conform to a "trap-jaw" habitus are generalist predators, the fact that the armature of Ceratomyrmex would have been useless for capturing prey items below a certain size suggests a considerable degree of dietary specialization (Perrichot et al., 2016). Of course, it is anyone's guess as toward what prey items this remarkable specialization was directed.

Without a doubt, we can add the "unicorn ant" to the long list of insects that we wish still lived  on this planet.

*An insect mouthpart articulating with the frons (i.e., face), from which emanates the labrum.

Barden, P. and Grimaldi, D. (2013). A new genus of highly specialized ants in Cretaceous Burmese amber (Hymenoptera: Formicidae). Zootaxa, 3681(4), 405-412. Retrieved 12/24/15 from 

Perrichot, V.; Wang, B.; and Engel, M. S. (2016). Extreme Morphogenesis and Ecological Specialization among Cretaceous Basal Ants. Current Biology, 26, 1468-1472. Retrieved 7/27/16 from

Monday, May 30, 2016

News of a Trigonalid

Potter Wasp Pot - Eumenes
A pot of Eumenes sp., collected by Troy Bartlett in Duluth, GA
And now, to demonstrate that I am not (yet) deceased, a post.

Back on April 30th, I found an adjoining pair of potter wasp nests (likely belonging to Eumenes sp.) while on a collecting trip to Zaleski State Forest with the Ohio State University's undergraduate entomology club. At the recommendation of one of my fellow students, I saved the stout clay pots in hopes that something more interesting than the offspring of their creator would emerge.

My hope paid off just one day shy of a month later, when two distinctly non-eumenine wasps emerged, one from each pot: both larvae had been parasitized (females of the Eumenini lay a single egg in each jar; Hermes et al., 2015). Much to my pleasure, the parasitoids I had unwittingly collected were Lycogaster pullata, a member of the small, seldom-encountered family Trigonalidae.

Four genera of trigonalids are present in North America (Murphy et al., 2009). They tend to be moderately sized, chunky wasps with unreduced wing venation and >16 flagellomeres, distinguished from other apocritans of this description by finger-like ventral projections at their tarsomeres' apices and reduced ovipositors in females (Goulet & Huber, 1993). Distinctive enough to warrant their own superfamily, trigonalids are currently regarded as most akin to the Megalyroidea (Heraty et al., 2011).

The reduced ovipositor is a consequence of the wasps' distinctive biology, the convolutions of which are at least partially responsible for their rarity: rather than lay eggs directly upon or within their desired hosts, trigonalid females oviposit large quantities of minute eggs in foliage, where they are ingested by herbivorous sawfly and lepidopteran larvae. This strategy is not unique, but trigonalids add a bizarre wrinkle in that they are for the most part obligate hyperparasitoids, attacking ichneumonid or tachinid larvae that are themselves parasitoids of the insect that originally ingested the eggs (Murphy et al., 2009).

Wasp - Lycogaster pullata
A Lycogaster pullata that emerged from the above pot
L. pullata has been reported both as a hyperparasitoid, and as facultatively employing a different stratagem—the one employed by the particular specimen I collected (Smith, 1996). Namely, it (and other trigonalids) may parasitize the larvae of vespid wasps (including eumenines) that provision their nests with caterpillars, infesting these hosts through inadvertent larval ingestion of minute trigonalid larvae that had hatched within said caterpillars. Evidently, one should thoroughly chew one's food.


Goulet, H and Huber, J. T. (1993). Hymenoptera of the World: an Identification Guide to Families. Ottawa: Agriculture Canada.

Heraty, J.; Ronquist, F.; Carpenter, J. M.; Hawks, D.; Schulmeister, S.; Dowling, A. P.; Murray, D.; Munro, J.; Wheeler, W. C.; Schiff, N.; and Sharkey, M. (2011). Evolution of the hymenopteran megaradiation. Molecular Phylogenetics & Evolution, 60, 73-88. 

Hermes, M. G.; Araujo, G. and Antonini, Y. (2015). On the nesting biology of eumenine wasps yet again: Minixi brasilianum (de Saussure) is a builder and a renter… at the same time! (Hymenoptera, Vespidae, Eumeninae). Revista Brasileira de Entomologia, 59(2), 121-142. Retrieved 5/30/16 from

Murphy, S. M.; Lill, J. T.; and Smith, D. R. (2009). A scattershot approach to host location: uncovering the unique life history of the trigonalid hyperparasitoid Orthogonalys pulchella (Cresson). American Entomologist, 55, 82-87. Retrieved 5/29/16 from

Smith, D. R. (1996). Trigonalyidae (Hymenoptera) in the eastern United States: seasonal flight activity, distributions, hosts. Proceedings of the Entomological Society of Washington, 98(1), 109-118.

Wednesday, December 23, 2015

Armadillo Ants "Gots Cousins", Drepanicine Mantidflies Get a Range Extension, & Umenocoleidae Troll Paleontologists Yet Again: a Few Minor Addenda

A worker of Ankylomyrma coronacantha (Agroecomyrmecinae), imaged by April Nobile
Science is by definition in a state of constant revision: and organismal systematics—the subset of which I am fondest, and therefore upon which this blog concentrates—is no exception. Therefore, despite the fact that Life, et al. will see what is merely its third birthday this month, some of what I have written here has already been superseded by new discoveries: and since I am too lazy to revise my original posts where necessary, I have resolved to provide the needful updates here.

Doubtlessly the majority of my posts have been rendered obsolete in some manner since their publication, and thus it would be a task both Sisyphean and unnecessary to keep a finger on them all: instead, I intend here to simply make note of the recent news of which I have become aware.
A female Allomantispa mirimaculata (Liu et al., 2014)
  1. A new genus (and two species) of mantidfly (Mantispidae) has been described: Allomantispa tibetana and mirimaculata (Liu et al., 2014), restricted to the southeastern fringe of the Tibetan Plateau. Notably, Allomantispa is assignable to the Drepanicinae, a subfamily heretofore known to have a relictual distribution in South America and Australia (there are fossil drepanicines known from Eurasia; Makarkin, 1990; Liu et al., 2014). This constitutes a significant addition to the limited information I gave on the taxon in "Mantidflies: Chimeras of the Insect World".
  2. The reclusive armadillo ants (of which I provided an overview in May 2014)—formerly the sole living members of the Agroecomyrmecinaenow have an extant confamiliar, in the form of the enigmatic Ankylomyrma coronacantha. Formerly shuffled about in the Myrmicinae, the monotypic Ankylomyrmini was found to be sister to Tatuidris tatusia in an extensive phylogenetic re-analysis of that immensely speciose subfamily (Ward et al., 2015). A. coronacantha is nonetheless dissimilar to its cousins in both biogeography (Afrotropical, as opposed to the Neotropical and Holarctic distribution of Agroecomyrmecini living and extinct) and morphoecology (ankylomyrmines are large and arboreal, unlike the diminutive soil-dweller T. tatusia) (Bolton, 1973). 
  3. Despite what I stated as regards the fossil taxon Umenocoleoidea in "The Origin of Earwigs", the type genus Umenocoleus has been reclassified as a beetle (Kirejtshuk et al., 2013). Its supposed relatives remain, however, an odd lineage of "roachoids" unrelated to Umenocoleus (Nel et al., 2014) or to the Protelytoptera, an order in which this exceedingly confusing Cretaceous genus has also been placed (Carpenter, 1992). 
May you suffer through the holidays with somewhat less than the accustomed amount of pain.

Bolton, B. (1973). A remarkable new arboreal ant genus (Hym., Formicidae) from West Africa. Entomologist's Monthly Magazine, 108, 234-237.

Carpenter, F. M. (1992). Treatise on Invertebrate Paleontology (vol. 3, pt. R). Boulder: Geological Society of America. 

Kirejtshuk, A. G.; Poschmann, M.; Prokop, J.; Garrouiste, R.; and Nel, A. (2013). Evolution of the elytral venation and structural adaptations in the oldest Paleozoic beetles (Insecta: Coleoptera: Tshekardocoleidae). Journal of Systematic Paleontology, 12, 575-600.

Liu, X.; Winterton, S. L.; Wu, C.; Piper, R.; and Ohl, M. (2014). A new genus of mantidflies discovered in the Oriental region, with a higher-level phylogeny of Mantispidae (Neuroptera) using DNA sequences and morphology. Systematic Entomology, 40, 183-206. Retrieved 12/19/15 from

Makarkin, V. N. (1990). Novye setchatokrylye (Neuroptera) iz verkhnego mela Azii. In: I. A. Akimov (ed.), Novosti Faunistiki I Sistematiki (pp. 63-68). Kiev: Naukova Dumka.

Nel, A.; Prokop, J.; Grandcolas, P.; Garrouste, R.; Lapeyrie, J.; Legendre, F.; Anisyutkin, L. N.; and Kirejtshuk, A. G. (2014). The beetle-like Palaeozoic and Mesozoic roachoids of the so-called "umenocoleoid" lineage (Dictyoptera: Ponopterixidae fam. nov.). Comptes Rendus Palevol, 13(7), 545-554. Retrieved 12/22/15 from

Ward, P. S.; Brady, S. G.; Fisher, B. L.; Schultz, T. R. 2015. The evolution of myrmicine ants: phylogeny and biogeography of a hyperdiverse ant clade (Hymenoptera: Formicidae). Systematic Entomology, 40, 61-81. Retrieved 12/20/15 from

Friday, August 28, 2015

A Frivolous Look at the Ohmu

Skeletal reconstruction of Godzilla as an oversized neoceratosaurian* (Carpenter, 1998)
For nerds with a zoobiological cast, there are few things more fun than speculative biology: the creation of fictitious life-forms upon extrapolation from what is known of reality. If nothing else, such speculation is a stimulating exercise when engaged in for its own sake (see "Hallelujah! Snaiad is Back" for several fine examples of this); but it takes on a decidedly silly cast when one attempts to formulate rationalizations for fictional organisms that were never intended to conform to the bounds of scientific plausibility. Godzilla, in particular, has been the focus of this sort of attention with respect to its physiology and phylogeny (Naish, 2010); although not so widely known outside of Japan, other kaiju have also received something resembling this treatment. Rather more humorous are the speculations I've seen on Elmo's morphoecology.
Original theatrical poster for Nausicaä of the Valley of the Wind, by Yoshiyuki Takani
In this post, I will speculate on the anatomy and ecology of the Ohmu: outsized arthropods which appear in Hayao Miyazaki's film Nausicaä of the Valley of the Wind (1984) and its associated manga (serially released over the course of 1982-1994; Miyazaki, 2009). I regard the literary incarnation of this anime as that director's magnum opus, which, considering this animator's reputation, is saying a lot. Within this post, I will principally refer to the manga as my primary source (in the form of the English edition from Viz Media), since a) it contains considerably more material from which to gain an accurate picture of Ohmu morphology; b) I own a copy, whereas I have not been blessed with one of the film; and c) I am a geek.

Impression of the Sea of Corruption by Roberto Nieto
Irrespective of which iteration of Nausicaä of the Valley of the Wind one uses, the Ohmu are prominently featured. The mis-a-scené is a post-apocalyptic future (which I reckon at c. 3800 A.D.) in which much of the terrestrial biosphere has been replaced by a sylvan biome stocked with gargantuan arthropods (of which the Ohmu are the most impressive) and characterized by pulmonary-toxin-emitting flora intertwined with a panoply of fungal symbionts (although there is some confusion as to which are botanical and which are fungal in classification; vol. 2, p. iii); termed a name translatable as "Toxic Jungle" or "Sea of Corruption", not only does its toxicity in and of itself displace all other ecosystems, but its spread is facilitated by the phoretic dispersal of spores via Ohmu (Miyazaki, 2012).

At the manga's conclusion, it is revealed that the Sea of Corruption and all organisms therein were genetically engineered prior to the violent denouement of industrial civilization for the purpose of cleansing the biosphere (vol. 2, p. 441): autotrophs draw pollutants from the soil and metabolically render them insoluble solids; the Sea's gaseous effluvia is the byproduct of this process (Miyazaki, 2012). As a result, the Ohmu need not be pegged as the descendants of a factual organism, nor even conform to the bounds of plausibility under randomized evolutionary circumstances: indeed, it is probable that the Ohmu genome is a pastiche from multiple taxonomic sources. It only remains to determine which, exactly...

Nausicaa lifts an ohmu shell.
Nausicaä with molted Ohmu ocellus
But first an anatomical overview is in order. The Ohmu are clearly arthropodan in nature, what with their jointed appendages and chitinous exoskeleton; this is confirmed in both film and manga by the portrayal of exuviae, demonstrating that the creatures engage in ecdysis§ (Miyazaki, 2012) and is corroborated by the blue coloration of their blood (vol. 1, p. 207), which indicates that their circulatory metalloproteins are hemocyanins (as is typical of arthropods; Jaenicke et al., 2012). The trunk consists of a frontal prosoma consisting of four visible segments, the posterior two of which bear ommatidia: six on the anterior and eight on the posterior. The trunk's remainder (which we may term an opisthosoma) bears at least seven apparent tergites in the case of what is explicitly denoted as a juvenile (p. 202), with up to thirteen in mature individuals (p. 207). Even the smallest observed Ohmu in either film or manga reaches behemoth dimensions for an arthropod at a meter in length (p. 128). While not impossible—the largest extant terrestrial arthropod has a comparable leg-span (Drew et al., 2010) (if a lesser body mass)—this size is outlandishly dwarfed by adult Ohmu, which could conservatively be estimated at fifty meters in length (p. 126).  

Impression of an Ohmu by Billy Frolov
Considering both this observation and the size disparity between these two age groups, we can deduce that Ohmu add new opisthosomal segments with growth: that is, their ontogeny can be classed as anamorphosis, which is observable in such unrelated arthropods as proturans and trilobites (Minelli et al., 2003; Fusco, 2005). Interestingly, the aforementioned juvenile individual appears later in the manga, significantly grown (yet still immature) and with an identical number of tergites (vol. 2, p. 225): this hints at a sort of inverse hemianamorphosis (Enghoff et al., 1993), with segment addition commencing after the first few instars. Intraspecific variation is also not impossible, if myriapods are any model (Fusco, 2005). Tangentially, it is worth noting here that the film version of Nausicaä of the Valley of the Wind reveals that ambulating Ohmu move via peristalsis: that is, they inch. This is not only atypical of arthropods (one Cambrian xenusiana member of the clade Panarthropoda||may have engaged in peristaltic movement; Vintaned et al., 2011), but is also mechanically peculiar, seeing as Ohmu do not burrow.

Ventral view of Lepidurus apus (Notostraca: Triopsidae) by Ricardo Fernández
The trunk's anterior is equipped with a suite of prolapsed, deflexed appendages, ambulatory much like those visible at the edge of each pleurite; their number is unclear, but I would venture a guess that there are at least six pairs in evidence. The number of legs per apparent segment is also obscure, but it can be pinned down as three pairs (vol. 2, p. 224): a count that is odd both in the numerical and usual senses. This can most readily be attributed to fusion of multiple segments in the Ohmu embryo, opening the possibility of decoupling in the number of ventrites relative to tergites, which occurs most famously in notostracans (Minelli & Fusco, 2004) but is widespread. We rarely get a good look at an Ohmu underside, but when we do, it appears that the trunk's ventrites neatly correspond to each opisthosomal tergite, whereas the prosoma possesses a single ventrite to its four tergites (pp. 133, 226). These views reveal additional anatomical strangeness in the form of a row of short locomotory appendages at the rear rim of each ventrite, running the trunk's breadth (Miyazaki, 2012). It is unclear if these are legs, but even if that is their identity I can think of no analogy among actual Arthropoda. There is a great deal of embryological funkiness going on here.

Ohmu inspects a wounded Nausicaä with sensory tendrils
Lastly, it would behoove us to consider Ohmu internal anatomy. Since Nausicaä is at one point protectively swallowed by a mature individual (vol. 2, p. 151), we are given a glimpse of a foregut with a papilla-covered lumen, but the creatures' viscera are otherwise ignored; this is unfortunate, since knowledge of the whys and wherefores of their respiration could help explain their implausibly humongous size. The most peculiar revealed aspect of an Ohmu interior would be the golden tendrils that can be everted from the anterior for manipulatory purposes, reaching up to a length equal to that of the prosoma (vol. 1, pp. 204, 207). Their number is, again, unclear (there are at least five pairs); but these appendages' greatest import lies in the fact that they are unsegmented. In an adult, said appendages are about four centimeters in diameter right down to their blunt tip—unlike most sensory extremities among the arthropods, these do not taper. Gripping capability is provided by an abrasive surface and transparent cuticular bullae (p. 110). These pasta-like tentacles are so bizarrely un-arthropodan that it is tempting to posit that their place in the Ohmu genome is derived from some organism completely outside that phylum; but I will imminently detail two ways in which their provenance could plausibly be arthropodan (Miyazaki, 2012).

millipedes (Sphaerotheriida)?
Sphaerotheriid millipede, an oniscomorph
With that, it is at long last appropriate to speculate on the phylogenomic origins of the Ohmu. Their plumply sclerotized demeanor causes them to be often likened to pillbugs (Armadillidiidae) by fans, but this comparison breaks down immediately upon even the most cursory of examinations: compared to "roly-polies", Ohmu have far too many legs, simple—as opposed to compound—eyes, and are conspicuously lacking in antennae. Along with their apparently uniramous legs, this latter absence strongly indicates that Ohmu heritage has little of the Crustacea in it (if any at all); comparisons to trilobites are inaccurate for selfsame reasons (Hughes, 2003). 

 photo Termitodesmusceylonicus.jpg
Two Termitodesmus ceylonicus (Glomeridesmida: Termitodesmidae) captured by Rowland Shelley
Their undifferentiated tagmosis and multitude of legs puts one more in mind of myriapods: specifically oniscomorphan millipedes, which have a number of apparent segments comparable to that of the Ohmu (Racheboeuf et al., 2004) and multiple leg pairs on each somite; but since Ohmu are presumably unable to protectively enroll (what with their prominent spines and ellipsoid outline), I venture that their closest topological and behavioral cousins among the Myriapoda would in fact be the glomeridesmidans (sole extant members of the superorder Limacomorpha), which are likewise incapable of volvation (Blanke & Wesener, 2014). 

Depiction of assorted eurypterids by the dean of paleoartists, Charles R. Knight
Beyond pentazonian millipedes (constituted by the Oniscomorpha and Limacomorpha; Enghoff, 1990), Ohmu might also be comparable to the Merostomata: both by dint of their absent antennae and in a more superficial fashion by the prosoma/opisthosoma dichotomy introduced earlier. This taxon is of course now considered invalid (Schultz, 2007), at least in its traditional sense (Garwood & Dunlop, 2014), although the "merostomoid" mien of an Ohmu is undeniable. Nevertheless, the fact that the Ohmu prosoma is not fused into a cephalon sets them apart from the misnamed "sea scorpions" (Eurypterida) and their purported kin.

Cambrian lobopodians, from Dzik 2011 fig3
Chronodendrogram of the paraphyletic lobopodian class Xenusia (Dzik, 2011)
Finally, we must jury-rig a hypothesis as to the genetic derivation of the Ohmu sensory tendrils. As they are unsegmented, it is tempting to deny that their transgenic origin lies among the Arthropoda; but two alternative possibilities occur to me: first, that the twining appendages are an atavism, homologous with lobopodians' extremities; second, that they represent extensions of a labrum. The former of these two postulates proceeds from a recognition that "arthropodization" (the acquisition of exoskeletal joints on ancestrally unsegmented limbs) occurred in at least two lineages within the Panarthropoda (Ma et al., 2013), and moreover that said clade covers a variable morphological continuum from the completely articulated arthropods themselves to taxa that are not armored in any way, shape or form (Edgecombe, 2009), suggesting that the presence or absence of jointed limbs has some evolutionary plasticity (note also that some analyses situate the lobopodian Onychophora within the Arthropoda; Koenemann et al., 2010): the Ohmu might have secondarily lost segmentation in their manipulatory appendages, which in this scenario are homologous with one or several pairs of cephalic appendages in other arthropods—albeit multiplied a number of times that is beyond belief.

small wasp - Pseudochalcura gibbosa
Pseudochalcura gibbosa (Eucharitidae), photographed by Kim Fleming
But these tendrils' lack of segmentation also brings to mind the labrum, a lobe situated anterior to the mouth in hexapods, crustaceans and other taxa (Racheboeuf et al., 2008; Posnien et al., 2009) that acts as an "upper lip" in feeding. Whether or not this appendage is a muscular extension of the head, or a modified trunk appendage (as are all other cephalic limbs in the Arthropoda) is a matter of ongoing embryological debate, but this contention is immaterial to its posited presence on the Ohmu head. That titanic pseudo-arthrosphaerid's labrum under this hypothesis would be digitate after the fashion of perilampid and eucharitid wasps (Darling, 1988), but exaggerated to an unparalleled degree.

Pissed Ohmu pursuing Yupa Miralda
As an appendix to this discussion, I should bring up what can be inferred from our primary sources regarding Ohmu ecology and behavior. We are granted little information as to the latter, other than that the creatures are sentient and engage in some degree of brood care (given their behavior in vol. 1); reproduction is completely ignored, although if they resemble the majority of arthropods, Ohmu are both dioecious and viviparous. Diet is at least partly fungivorous (vol. 2, p. iii). The ommatidia turn from the usual azure to crimson when an individual is angered: I know of no biochemical mechanism that would enable this, and it must be regarded as narrative license, humanizing a species that has nothing in the way of anthropomorphism.
Nausicaä on Ohmu with epizoic growth
Their ecological role as a keystone organism in the Sea of Corruption is fleshed out to a much more pronounced degree: zoochory of the Sea's flora via these arthropods (vol. 2, p. 143) is essential to its spread, and fungal and botanical growth will sprout outright on the surface of aged or dying individuals (p. 181). Epizoic associations of suchlike intimacy seldom occur in arthropods, but have been documented on weevil species (Gradstein et al., 1984), a harvestman (Machado & Vidal, 2002), and a millipede (Martínez-Torres et al., 2011); the latter exemplar supports 10 separate species of bryophytes classified in half that many families. Although commensal cuticular mosaics consisting of fungi in the likeness of those observed on the Ohmu—as opposed to bryophyteshave never been reported on arthropods (to my knowledge), epizoic communities of this sort are far from inconceivable.

In sum total, the only impossibility of Miyazaki's conception of the Ohmu is physiological: namely, their aforementioned gigantism. The largest known terrestrial arthropod in geological history was merely two meters in length, and only permitted to evolve such colossal dimensions by dint of the atmosphere it inhabited (14% more oxygenated than our contemporary air; Beerling, 2007): while the hydrosphere in Nausicaä's era is laden with pollutants, it is uncertain what residual effects humanity's ecological ruination has had upon atmospheric composition. Using Occam's Razor (or at least so I think), I propose that it is most likely that Ohmu respiration and circulation was genetically modified to facilitate their size.

...And now that we have exhausted the possibilities for intelligent discussion of the Ohmu, I consider this post finished.           

*A clade consisting of the Abelisauroidea and Ceratosauridae (Pol & Rauhut, 2012). 
A ballpark estimate that remains in the realm of fanon, based upon the prologue's statement that industrial civilization collapsed roughly a millennium after its beginning, and that the story is set a millennium after that (Miyazaki, 2012); this is summed, of course, with a dating of the Industrial Revolution as commencing c. 1800 A.D.
‡Molted exoskeletons.
||The most inclusive group constituting the common ancestor of the phyla Arthropoda, Onychophora, and Tardigrada and all its descendants.
A morphological grade referring to primitive panarthropods too basal to lie within any of that clade's extant phyla.   

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