Still, they have a degree of notoriety among entomologists, as was shown when I struck up a conversation with a fellow student at Ohio State University's Stone Laboratory some time ago, and happened to mention twisted-winged parasites.
"Yeah," my peer said, "those things are f*cking awesome."
|A male member of Corioxenidae; photographed in Texas by Mike Quinn|
|Female of Callipharixenos muiri (Kathirithamby, 1989)|
|A male stylopid (lower left) humpin' an unseen female in a bee's butt; photographed by Colin Boyd|
|A Tunisian female of Mengenilla moldrzyki (Mengenillidae); Pohl et al., 2012|
|A male Caenocholax fenyesi (Myrmecolacidae) eclosing from an ant (Solenopsis invicta); Cook et al., 2005|
If you are familiar with the various insects that begin their lives as planidia, you will know that this initial ontogenetic period invariably corresponds with hypermetamorphosis (complete metamorphosis with subdivision of the larval stage). Hence you could correctly surmise that twisted-winged parasites are hypermetamorphic too (Osswald et al., 2010): once a planidium has penetrated its host (often following the host's molt—the exoskeleton is then more pliant) and plunged into the fluid-filled interior, subsequent instars take a sedentary foot-lacking grub-like shape; a protective bag of exuviae gradually surrounds the larva, since each time it molts it doesn't shed its cuticle. One species masquerades as a part of its katydid host by enclosing itself in a sack of tissue derived from the host's epidermis, thereby deflecting the katydid's immune reaction (Kathirithamby et al., 2003).
|Stylops melittae pupae protruding from a Czech wasp; picture credit given to Josef Dvořák
|Ripiphorus vierecki (Ripiphoridae), male, photographed by Margarethe Brummermann in Arizona|
But these characteristics do not really form a convincing argument for kinship between the two: many unrelated male insects have extravagantly feathery antennae; hypermetamorphosis has arisen independently in insects several times; and beetle elytra, however ineffectually stunted they happen to be in Ripiphoridae, are functionally dissimilar to the strepsipteran halteres (Pix et al., 1993). Still, the ad hoc placement stood until molecular phylogeny came along in the 1990s and introduced a rival hypothesis. Analyses using genetics (Chalwatzis et al., 1996; Whiting et al., 1997) indicated that the Strepsiptera were a sister-group to the true flies (Diptera), a view not without precedent (Newman, 1864; Pierce, 1918). Both taxa have a wing-pair modified into halteres—but the gyroscopic stabilizers originate from the metathorax in flies, as opposed to the mesothoracic ones in twisted-winged parasites: a big evolutionary jump, in developmental terms; but not in the realm of impossibility with the aid of homeotic mutation (Whiting & Wheeler, 1994).
The two allegedly comprised a clade dubbed "Halteria". This theory, while popular, was not without controversy; an argument was made that the grouping of Strepsiptera and Diptera together was an artifact due to a cladistic phenomenon called "long-branch attraction" (Carmean & Crespi, 1995; Huelsenbeck, 1997), which I will not deign to explain (since I really have no idea what it is): thus it seemed that Halteria was artificial, and strepsipterans' descent remained up in the air. A slew of new (and reputedly improved) morphological/molecular phylogenies (Beutel et al., 2010; McKenna & Farrell, 2010) have now strongly pointed towards twisted-winged parasites' being beetles' closest kin (although not beetles themselves, as some have persisted in suggesting) (Niehuis et al., 2012). For the time being, thus, we may say that beetles are twisted-winged parasites' nearest relatives.
*Emergence from the pupa.
†The internal bodily fluid that bathes the organs of arthropods, analogous to vertebrate blood and lymph.
‡ Larva-like but sexually mature.
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