Iä! Nemestrinoidea! The Fat Flies of the World With Five Thousand Young!

Willi Hennig four years before his death

The advent of cladistics has both blessed entomology with greater understanding of the subjects it studies, and cursed it with taxonomic chaos; this is truer of some insect taxa than others, and especially true of the order Diptera (flies): none other than Willi Hennig (1913-1976)—he to blame for cladistics' creation—was first and foremost a dipterist. Therefore, it is to be expected that our understanding of fly phylogeny has been drastically affected in recent years by the innovations that Hennig set in motion, no less than his personal revisions to higher-level dipteran taxonomy. 

Specifically, the taxonomy of the lower Brachycera is now an exemplar of phylogenetic instability. These flies were once classified as a taxon called Orthorrhapha, but recognition that this group simply constituted all brachycerans that did not belong in the Cyclorrhapha put an end to its usage by dipterists; the orthorrhaphous superfamily Empidoidea is obviously sister to the Cyclorrhapha*, and together they constitute the Eremoneura. Beyond that, the phylogeny becomes (for lack of a better word) hairy: the Asiloidea may be either eremoneurans' adelphotaxon, or they may be paraphyletic—with the Bombyliidae and Mythicomyiidae their own clade (Bombylioidea), and/or with Apystomyia elinguis (translating more or less to "the fly of which nothing is known"; Marshall, 2012) constituting its own family and classified as the Eremoneura's sibling (Wiegmann et al., 2011).

An adult wormlion (probably Lampromyia sp.), photographed by Valter Jacinto

The remaining "Orthorrhapha" are no better understood: they consist of two clearly monophyletic infraorders (Tabanomorpha and Stratiomyomorpha; Woodley, 1989) plus an assortment of peculiar families whose phylogeny is up for grabs. Among them, for instance, are the Vermileonidae (wormlions): their maggots uniquely excavate pitfalls in friable substrates, catching prey a lá the notorious myrmeleontids, and are distinctively specialized to that end (Ludwig et al., 2001); adults are short-lived and feed on nectar (Devetak, 2008). Originally lumped in the Rhagionidae (snipe flies), they were justifiably elevated to familial rank (Nagatomi, 1977; Triplehorn and Johnson, 2005) and are now placed in their own infraorder (Vermileonomorpha; Griffiths, 1994), which may either be closer to the Asiloidea+Eremoneura clade or to the Tabanomorpha (possibly being even included in the latter)—the evidence is inconclusive.

The Nemestrinoidea, another group of lower Brachycera, are in a similar pickle. This (putative) superfamily contains two most peculiar taxa (Woodley, 1989): the Nemestrinidae and Acroceridae (formerly Cyrtidae). Their broader relationships are debatable, as is their relation to each other (Woodley et al., 2009): despite the many ecological parallels between the two, which will now be detailed.

A Virginian Ogcodes borealis, noticed by Scott Justis

The Acroceridae will come first; after all, I foreshadowed their appearance on this blog all the way back in January. Commonly known as "small-headed flies" (for obvious reasons), they are rendered visually distinctive by a bulky body (they were once dubbed "fat flies"—and it's a pity that name didn't stick; Wiedemann, 1830), disproportionately small holoptic† head, and singularly expansive calypters‡ (Schlinger, 1981): but within the bounds of this physique, the small-headed flies vary in appearance; there are circa 520 species described, and the family is fairly cosmopolitan (Winterton & Gillung, 2012) and absent from oceanic islands, with New Caledonia and New Zealand having one precinctive genus each (Paramonov, 1955; Gillung & Winterton, 2011). Ecologically, all acrocerids with habits known to science are larval parasitoids of spiders (Araneae), from which their other colloquial address ("spider flies"; Schlinger et al., 2013) is derived. There is an often-ignored report of Pterodontia flavipes larvae being circumstantially found on mites (both in the cohort Parasitengona) (Sferra, 1986), which may be corroborated by a similar association in Baltic amber (Kerr & Winterton, 2008); but in both cases phoresy is no less likely than parasitism.

Additionally, all acrocerid larvae seek out their hosts autonomously upon hatching (usually nocturnally; Cole, 1919); the hyperactive first instar is termed a planidium: the description and ontological purpose of which will be recalled by anyone who is familiar with mantidflies (with which nemestrinoids converge markedly). Rapid and broad dispersal is imperative; a number of acrocerid planidia bear a posterior duo of elongated setae (as seen at left)—these facilitate leaping (Cole, 1919; Bovey, 1936). Planidia of Acrocera, despite belonging to the type genus of the family, deviate from the norm by affixing themselves to a given substrate and awaiting some unfortunate spider's passage (Schlinger, 2003). 

A female Ogcodes eugonatus ovipositing on a clothesline; photographed by J. T. Layne in Missouri

Those of us remembering the likes of mantidflies and strepsipterans will know that another trait in this suite of adaptive characteristics is a vast amount of offspring per brood (compensating for the miniscule chance that any one planidium will successfully locate a host): observations confirming this include that of a female O. costatus, who was observed to lay 1,500 eggs within 2 hours; and P. flavipes has been seen to oviposit at a rate of 51 eggs per minute, and females of this species indeed strafe the leeward sides of protrusive objects (e.g., fenceposts, twigs) with their eggs, which adhere wherever they are fired due to their mucilaginous surfaces. Carpeting surfaces with eggs is the preeminent strategy among acrocerid mothers (Clausen, 1940): consequently, spiders that spend much of their time directly on substrates or vegetation are disproportionately afflicted with small-headed flies in general (Cady et al., 1993). (Small-headed flies are very catholic as far as host preference at the generic level and above; Larrivée & Borkent, 2009.)

A collected S. appendiculata, male, in lateral view

Upon locating suitable spiders, small-headed flies scurry up an available leg and manage an entry through any membranous articulation they can find (King, 1916); A. orbicula firmly seizes the host's exoskeleton with its mouthparts, then trepanning a miniscule hole: moulting on the exterior, the larva makes an entry through the previously-bored aperture rhizocephalan-style (Nielsen et al., 1999). Both modes of penetration are universally followed by movement to the host's abdomen. With the exception of Chile's Sphaerops appendiculata, all small-headed flies with known habits are endoparasitoids—respiring in the anaerobic environment of a spider's body by lodging their rearmost spiracles in the host's book lungs (Schlinger, 1987). Prolonged diapause (with a duration sometimes exceeding several years) may be entered depending on the host's youth (or lack thereof), before essentially all its innards are devoured in the maggot's fourth instar; prior to its demise, the said host often spins a sheltering web for the benefit of its parasitoid's eventual pupation (Montgomery, 1903).

Moegistorhynchus longirostris sipping from an iris (Lapeirousia anceps) (Zhang et al., 2012)

As adults, small-headed flies are often non-feeding; and their mouthparts are naturally vestigial. The nectarivores among them are equipped with proboscides of variable (but never-to-be-sneezed-at) length: as any ecologist would suspect, a number of these are confirmed pollinating specialists (Borkent & Schlinger, 2008)—but none have been ever more than implicated as primary pollinators (Grant & Grant, 1965; Potgieter et al., 1999). Tangle-veined flies (Nemestrinidae), on the other hand, mostly bear tremendously prolonged labia (Teskey, 1981); and those living in southern Africa (especially within the Cape Floristic Kingdom) comprise a keystone ecological guild as the sole pollinators (along with some Tabanidae) of a goodly amount of long-tubed flowers across four families (Pauw et al., 2009; Johnson, 2010). In fact, among them we find Moegistorhynchus longirostris, a fly which has the honor of having the most extreme proboscis-to-body ratio known among insects (4/1: Barraclough & Slotow, 2010; Karolyi et al., 2012). 

Atriadops sp., female, on an Australian windowscreen; photographed by Alexander Pierce

Living nemestrinids (around 300 species; Evenhuis, 1994) constitute five subfamilies (Atriadopsinae, Trichopsideinae, Nemestrininae, and the monogeneric Hirmoneurinae and Cyclopsideinae); with little recent taxonomic modification (Bernardi, 1973). Conversely, the subfamilial cladistics of the Acroceridae is debated—traditionally classified as the Philopotinae, Acrocerinae and Panopinae, with the latter sister to the remainder (Schlinger, 1987); a more recent phylogeny rendered "Acrocerinae" paraphyletic with respect to the philopotines, and placed Sphaerops and Acrocera as a basal clade of small-headed flies (Winterton et al., 2007).

Hirmoneura flavipes, caught in Arizona by Margarethe Brummermann

Tangle-veined flies are cosmopolitan hypermetamorphic parasitoids most diverse in subtropical arid regions (Narchuk, 2007), just as their spider-munching analogues; but subfamilies differ in terms of host preferences: those Atriadopsinae whose habits are known attack katydids (Tettigoniidae; Kanmyia, 1987), while the Hirmoneurinae are parasitoids of something completely different—beetles of the family Scarabaeidae (Richter, 1997). Again, the first larval instar is planidial (Riley, 1883), although the precise procedure of host location is sketchy; females oviposit in crevices on bark-bared wood, laying batches of 100 eggs or so (Stuardo, 1935): H. exotica unaccountably deposit their eggs in the tunnels of wood-boring bees (Brauer, 1883). In any event, hirmoneurine larvae only feed during their hosts' pupal stage; upon the beetle's inevitable decease, they extrude themselves ventrally and pupate standing above the remnant of their meal, weirdly erect (Clausen, 1940).    

Trichopsideines and nemestrinines plague grasshoppers (Acrididae) (Prescott, 1955) (the host[s] of Cyclopsidea are so far undiscovered). Planidia penetrate the host's integument where it is softest, adjacent to the abdominal spiracles; after wandering the tracheae for a few days, they cut a different hole in the trachea (in Trichopsidea clausa; York & Prescott, 1952) or directly on the host's exoskeleton near a spiracle (in T. flavopilosa and the nemestrinine Neorhynchocephalus) and press their rearmost spiracles to the newly-made pore, subsequently burrowing deep into the grasshopper's entrails: and a ropy umbilicus then gradually forms from the host tissue, maintaining the larva's connection to the orifice it previously chewed, thereby permitting respiration. Nemestrinids sometimes take these respiratory tubes to proportions unseen among other parasitoids that utilize the same principle—in T. flavopilosa and costata (Potgieter, 1929), the final-instar larva's tube is twice its bodily length. If and when the host moults, the larva must create an entirely new pore, or else suffocate (Prescott, 1961).

Wing of Neorhynchocephalus sp., pictured by Robert Behrstock and exhibitng typically nemestrinid venation

As their common name hints, tangle-veined flies' wing venation is unmistakably compressed, a fact made most obvious by the compound M4 oblique vein (Yeates, 1994; see left): this is an adaptation for nearly perpetual hovering (Grimaldi, 1999). Nemestrinids have a rich fossil record (largely in the form of their characteristic wings), extending as far back as the Early Jurassic (Ansorge & Mostovski, 2000); with an entire subfamily (Archinemestriinae) restricted to the Mesozoic (Mostovski, 1998): they become unusual after that geological era (Wedmann, 2007), but this is likely only due to a taphonomic bias against their preservation in amber; all Cenozoic species can be assigned to extant genera (Bequaert & Carpenter, 1936).

The fact that the earliest undoubted fossil of a small-headed fly only makes an appearance around 100 million years after the oldest known tangle-veined one (Mostovski, 1997; Grimaldi et al., 2002; Shi et al., 2012) could be a distinct strike against these insects' monophyly: chronological dissociation helped lead to the separation of the Mythicomyiidae from the Bombyliidae (Evenhuis, 2012). And thus are we led to the subject of nemestrinoid phylogeny. For many years, the Nemestrinoidea were regarded as closely akin to bee flies (Bombyliidae) merely by dint of the parasitoidal ontogeny they have in common (Hennig, 1973; Ovtshinnikova, 1998); but this treatment has been abandoned in favor of either situating the Nemestrinoidea near the Asiloidea (Woodley, 1989) or within the Tabanomorpha (Griffiths, 1994). Increasingly, Acroceridae and Nemestrinidae are phyletically dissociated, since they lack any unassailable synapomorphies (Woodley et al., 2009; Wiegmann et al., 2011).

A teneral stink fly (Coenomyia ferruginea) from Wisconsin, photographed by Marcie O'Connor

A different possibility is suggested by study of the extinct family Rhagionemestriidae, which lived in Late Jurassic-Early Cretaceous Eurasia (Nel, 2010). Originally classified as aberrant tangle-veined flies (Ussatchov, 1968) before being elevated to the rank of family (Nagatomi & Yang, 1998), they remain indisputably nemestrinoid in affiliation. Their wing venation bears a number of remarkable similarities to that of the Xylophagidae (stink flies, etc.; placed alone in the infraorder Xylophagomorpha): the resemblance is so strong that the xylophagids Exeretoneura and Heterostomus have been classified by some as extant Rhagionemestriidae (Mostovski & Martínez-Delclòs, 2000), with the enigmatic latter genus being placed in the same subfamily as the peculiar rhagionemestriid Sinonemestrius (once ranked as a family; Nagatomi & Yang, 1998): but this is an untenable position, especially given that the pupa of Heterostomus has turned out to be most similar to those of the Pelecorhynchidae (a tabanomorph family) (Coscarón et al., 2013). 

In any event, Sinonemestrius appears to provide us with a morphological link between the Xylophagidae and Nemestrinidae (Jarzembowski & Mostovski, 2000), at least. The Acroceridae, however, remain regrettably ambiguous.

Kudos to anyone who recognizes the literary reference made in this post's title...                                     

*An unranked clade, sometimes synonymized with the infraorder Muscomorpha. More accurately, that infraorder refers to the Cyclorrhapha and the former Asilomorpha. In any event, cyclorrhaphous flies are united by the circularity of the opening they make in their puparium upon eclosion.
†Almost entirely consisting of eyes.
‡One of two basal lobes in a fly's wing, absent in a goodly number of species.
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