Friday, March 15, 2013

Patriarchy and Other Eccentricities of the Zoraptera

With only 34 living speciesall in a single genus(Zorotypus) in one family (Zorotypidae), and only the extinct Xenozorotypus to keep this lone genus company, the taxon Zoraptera must surely be the least diverse of all the living insect orders (Engel, 2003); and (I would argue) probably the most obscure, only having been noticed by science exactly a hundred years past (Silvestri, 1913). Oh, Notoptera (icecrawlers and rockcrawlers) might be able to compete–or the Embioptera (webspinners) could perhaps give zorapterans a run for their money: but both of these taxa possess a vernacular name, something that the Zoraptera essentially lack. A few sources dub them "angel-flies", but that denominator is almost completely disused for a good reason: it is absolutely without any basis that I can see in either form or behavior. I mean, if zorapterans had any resemblance to the cherubim and seraphim, I would expect them to appear on greeting cards more often.

Zorotypus sp. photographed by Graham Montgomery in Texas
But what are these insects? They are little pantropical things, 3 mm. long or less, with short cerci, 2-segmented tarsi, large palpi and beady 8/9-segmented antennae; if winged rather than otherwise (more about this dimorphism later), their wingspan never exceeds 7 mm (Engel, 2005). Extant zorapterans universally conform to this chassis with fidelity (they're congeners, after all). Naturally inhabiting the nooks and crannies of rotting wood that has reached a certain level of dampness (Wilson, 1959) (moist sawdust is a favorite man-made habitat), they feed mostly upon fungi, but may also exhibit some carnivorousness (Choe, 1997). The habitat required by zorapterans is a fleeting one, and colonizing new frontiers outside of their preferred microclimate is out of the question for the blind, wingless individuals that constitute the majority of a given population; this necessitates a generation of sighted alates* that disperse when the log that is a zorapteran's whole world turns sour.

A drone of Mastotermes darwiniensis
Subsequent to their arrival at a suitable habitat for their offspring, the alates deliberately shed their wings (designed to be comfortably snapped off at the bases at will) and–deepening these insects' superficial resemblance to the Isoptera (termites)–establish a new colony: for zorapterans are social, albeit to a very limited degree in comparison to even the most primitive termites (Mastotermitidae), which are eusocial. Depending on the species, colonies may be hundreds strong: but the zorapteran super-organisms are primitive, with only a rudiment of discrimination by the members of one against those from another; consequently, the boundaries between colonies in the wild are fluid.
A depiction of a Chulpex, painted by the illustrious Wayne Barlowe
More pertinent to the definition of the zorapteran degree of "sociality" (in the sociobiological sense) is the fact that among these insects there is no distinctly sterile caste in contrast to a reproductive one: the alate morph is more akin to the "alternating generations" mechanism observed in heteroecious aphids. The colony is organized hierarchically nevertheless–at the top of the heap sits a male under which is ranked a chain of subordinates; status is determined by duration of membership in the colony (better referred to as a harem), and maintained by continuous bullying (Choe, 1994). This makes the Zoraptera exhibit some of the few (perhaps the only) arthropod societies to be definably patriarchal. In fact, the only example of a patriarchy among exoskeleton-bearing organisms that I can think of at the moment is confined to science fiction: the eusocial aliens called the Chulpex, described in Avram Davidson's Masters of the Maze (1965) (Summers & Barlowe, 1979).

Line drawing of the wings of an alate Zorotypus snyderi (Richards & Davies, 1977)
In addition to being the most obscure (and certainly the least diverse) extant order belonging to the class Insecta, the Zoraptera is also perhaps the most phylogenetically inscrutable: a grand total of 10 different phylogenetic hypotheses were proposed regarding them in the first 89 years of our discovering their existence (Engel & Grimaldi, 2002). Only the Strepsiptera (twisted-winged parasites) approach the "angel-flies" in terms of their degree of cladistic dispute (Kristensen, 1991): but strepsipterans, at least, demand human attention by their striking weirdness (see "Twisted-Winged Parasites Are Friggin' Awesome"); whereas zorapterans are discreet things, and so apt to be ignored. The only incontrovertible clue we have to the ordinal kinship of the Zoraptera is its members' hemimetabolous development (i.e., incomplete metamorphosis), which would exclude them from the great clade called Exopterygota. Beyond that, their placement is up for grabs, what with their specialized wing venation, which precludes any closeness to the termites they often live alongside (compare the wings of the M. darwiniensis previously shown with those of the Z. snyderi directly above).

Four hypotheses as to zorapteran phylogeny remain in the running today: first, that they comprise the adelphotaxon of the clade Paraneoptera (Hennig, 1953; Beutel & Weide, 2005), which contains the orders Hemiptera (true bugs), lice–both free-living and parasitic (Psocodea), and Thysanoptera (thrips). The other three competing theories all place zorapterans in the Polyneoptera, which includes such familiar insects as cockroaches (Blattaria) and crickets (Orthoptera: Gryllidae). One posits that the Zoraptera are most akin to the sadly-misnamed earwigs (order Dermaptera) (Jarvis et al., 2005; Terry & Whiting, 2005). Another (Boudreaux, 1979; Smithers, 1991; Kukalová-Peck & Peck, 1993; Wheeler et al., 2001; Yoshizawa & Johnson, 2005; Wang et al., 2013) proposes that the superorder Dictyoptera (a well-supported group consisting of the orders Blattaria, Mantodea, and Isoptera) is the zorapteran sister-group.

Webspinner sexes compared - Oligotoma nigra - male - female
Male (left) and female (right) webspinners (Oligotoma nigra) from New Mexico, photographed by Jim Mclarin
A fourth postulated cousin to the Zoraptera would be the webspinners (Embioptera), mentioned in the inaugural paragraph of this post and often united with the former in a clade called Mystroptera (Rafael & Engel, 2006). Webspinners are so named for their sheltering in labyrinthine silken galleries, which the tubular insects construct with peculiar silk glands on their forelegs: some species have a grand total of 300 spigots per individual (Engel & Grimaldi, 2006); there is a propensity among webspinners for running backwards down their hackled socks just as unerringly as they run forwards (thanks to their sensitive posterior cerci; Hoell et al., 1998). Gregarious like the Zoraptera, webspinner aggregations consist of a foundress and her descendants, the nymphs raised on a diet of masticated leaf litter (Imms, 1931); the short-lived males are winged in some species. Interestingly, those webspinners males that are alate are able to fold their wings forward over their bodies without injury (Ross, 2009).

Image of Pentastomida
Male (right) and female (left) Armillifer sp. tongue worms (Porocephalidae)
The Mystroptera is supported by various lines of inquiry (Minet & Bourgoin, 1986; Grimaldi & Engel, 2005; Yoshizawa, 2007)albeit no more than any of the other three prevalent zorapteran phylogeniesalthough some analyses (Terry & Whiting, 2005) support Phasmatodea (stick insects) as webspinners' closest kin; zorapteran spermatozoa are one clue that hints at  zorapteran kinship with the webspinners (Dallai et al., 2011). Not that this amounts to an unassailable decision in the Mystroptera theory's favor: but it so happens that the study of sperm morphology has already cleared up the systematics of a number of oddball Animalia, such as the members of the class Pentastomida. These vermicular endoparasites of tetrapods' respiratory tracts have been classified as flatworms (Platyhelminthes; Fröhlich, 1789), nematodes (Diesing, 1850), annelids (Heymons, 1935), mites (Leuckart, 1860), myriapods (Osche, 1963), crustaceans (van Beneden, 1849), or as a phylum of their own (Self, 1969; Waloszek et al., 2006; Almeida et al., 2008); but comparison of their spermatozoa and those of fish lice (order Branchiura) was the first real hint that the so-called "tongue worms", despite all appearances, were degenerate crustaceans (Wingstrand, 1972). Subsequent molecular analyses corroborated this (Abele et al., 1989; Lavrov et al., 2004; Møller et al., 2008), and the placement of the Pentastomida as siblings (or even descendants) of the decidedly crustacean fish lice (comprising the clade Ichthyostraca; Zrzavý, 2001) is now accepted by many carcinologists (Martin & Davis, 2001). Thus the cladistic saying: "When all else fails, cherchez la sperme."||

Farewell, and beware the ides of March.

*Members of a winged morph.
†"Truly social."
||No, I do not speak French. 

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