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Strepsiptera
aka: Twisted Wing Parasites
aka: The Stylops

The order Strepsiptera, or twisted wing parasites, belongs to the class Insecta, phylum Arthropoda. It is a very small, cosmopolitan order of bizarre parasitoids containing about 624 named species of minute endopterygote insects with nine extant families. They parasitize thirty-four families and several orders of Insecta, including Blattodea (cockroaches), Diptera (flies), Homoptera (leafhoppers), Mantodea (mantises), Hemiptera (true bugs), Orthoptera (grasshoppers and crickets), Zygentoma (silverfish and firebrats), and Hymenoptera (ants, bees, and wasps). Three species of strepsiptera (halictophagids) have been reported from Arkansas hosts (all leafhoppers, Cicadellidae).

Strepsiptera have two major groups: the Stylopidia and Mengenillidia. The former, which has endoparasitic females with multiple genital openings, includes seven families: the Bohartillidae (one extant and two fossil species), Corioxenidae (forty-six extant and one fossil species), Elenchidae (twenty-seven extant and one fossil species), Halictophagidae (140 species), Lychnocolacidae (twenty-three species), Myrmecolacidae (eighty extant and fourteen fossil species), Stylopidae (148 extant and one fossil species), and Xenidae (113 species). The latter group includes two extant families: Bahiaxenidae (one species) and Mengenillidae (sixteen species). The sole member of the family Bahiaxenidae is Bahiaxenos relictus, described in 2009 from relictual sand dunes associated with the Rio São Francisco in Bahia, Brazil. It is considered a “living fossil” and is the most basal living strepsipteran.

The family Stylopidae is the largest family, with about seventy species in the United States and Canada. Many members are parasites of bees, with some infecting wasps. They have four-segmented tarsi and four- to six-segmented antennae, with the third segment having a lateral process. Strepsiptera of the family Halictophagidae make up the second-largest family, with about fourteen North American species. It includes members that are parasites of leafhoppers, planthoppers, treehoppers, and pygmy mole crickets, and they possess three-segmented tarsi and seven-segmented antennae, with lateral processes (flabella) from the third and fourth segments. However, this character varies widely; for example, those in genus Halictophagus have flabella on segments three to six, whereas Tridactylophagus has flabella only on segment three. The family Corioxenidae includes four species in the United States. Examples include Triozocera texana and T. vernalis, parasites of shield bugs (cydnids) that occur in the southern United States and south into Brazil, and Loania canadensis, which parasitizes true bugs (cymids) from Canada to the United States. The family Elenchidae includes Elenchus koebelei, a parasite of planthoppers. They have two-segmented tarsi and four-segmented antennae, with the third segment having a lateral process.

The phylogeny of these insects was a mystery for many years; however, most investigators, based on the insects’ forewing characters and similarity in parasitic life history, consider them related to coleopterans (beetles) of the families Meloidae (blister beetles) and Ripiphoridae (wedge-shaped beetles). Other biologists suggest that, due to recent molecular analyses, strepsipterans are more closely related to flies, although this conclusion is not universally accepted. Study of these insects’ evolutionary position has been problematic due to difficulties in phylogenetic analyses.

The earliest known strepsipteran fossil is that of Cretostylops engeli, discovered in middle Cretaceous (about 100 million years ago) amber from Myanmar (formerly Burma). This finding strongly suggests that their life cycles evolved during an age when dinosaurs flourished.

Modern-day scientists who have provided a great deal of information on the order are Jerry L. Cook of Sam Houston State University in Texas, Teiji Kifune of Fukuoka University in Japan, Ragnar Kinzelbach of the University of Rostock in Germany, and Hans Pohl of Friedrich-Schiller-Universität Jena in Germany.

The life history of parasitic species in this order is complex and involves hypermetamorphosis (a method of development in which the larval insect passes through numerous instars, each markedly diverse from the rest in morphology). The sexes exhibit extreme sexual dimorphism, with free-living males that are very short-lived (usually less than five hours of life) and totally endoparasitic females (except in one family). They possess club-shaped front wings, as well as large and fanlike hind wings, eyes, legs, and elongate antennae; they superficially look like flies. Their mouthparts cannot be used for feeding and are modified into sensory structures. Interestingly, adult males possess eyes unlike those of any other insect, although they resemble the schizochroal eyes found in some trilobites. Rather than having a normal insect compound eye consisting of up to thousands of ommatidia (optical units) that each produce a pixel of the entire image, the strepsipteran eyes consist of only a few dozen “eyelets” that each produce a complete image. Adult females of the family Mengenillidae are also free-living and possess a distinct head, segmented antennae, chewing mouthparts, compound eyes, rudimentary legs, and a single genital opening. All other females are not known to leave their hosts and are neotenic in form, lacking eyes, legs, and wings. Mengenillid males have strong mandibles, a distinct labrum, and more than five antennal segments.

Reproduction in strepsipterans is unique among the Arthropoda. After emerging, males locate virgin females via release of a pheromone. The male mates with the female, and she never leaves the host. The female produces up to several thousand tiny larvae (termed triungulin larvae), which escape from the brood opening on her head (protruding outside the host body) and go into the soil or on vegetation. These larvae have legs (which lack a trochanter, the leg segment that forms the articulation between the basal coxa and the femur), and they actively search out new hosts. In one major group, the Stylopidia, the female’s anterior region protrudes out of the host body, and the male mates by rupturing the female’s brood canal opening, which lies between the head and prothorax. Sperm passes through the opening in a process called hypodermic insemination. From this union, the offspring consume their mother from the inside in a manner known as hemocelous viviparity.

The Myrmecolacidae is an exceptional family with a complex life cycle, in which males and females attack dimorphic insect hosts. The males parasitize ants (Hymenoptera: Formicidae), while the females infect Orthoptera and Mantodea. Interestingly, they can cause their ant hosts to linger on the tips of grass blades, increasing both the chance of females being located by the male parasites and a chance for a good position for male emergence. Eggs of these parasites hatch inside the female, and the larvae can move around freely within the female’s hemocoel, which is unique to these insects. The female possesses a brood canal that communicates with the outside, through which the larvae escapes. The larvae are very energetic, as they only have a limited amount of time to find a host before they exhaust their food reserves. These first instar larvae have simple, single-lens eyes (called stemmata), and once they latch on to a host, they enter by secreting enzymes that soften the host’s cuticle, usually in the abdominal region. Some species have been reported to enter the eggs of hosts. For example, larvae of Stichotrema dallatorreanurn from Papua New Guinea have been reported to enter the foot of their orthopteran host. Once they get inside, they undergo hypermetamorphosis and become a legless larval form, which is less motile. They induce the host to produce a bag-like structure, inside which they feed and grow. This structure, made from host-derived tissue, protects them from the immunity of the host.

Although females directly become neotenic adults, larval males produce pupae after their last molt. The color and shape of the host’s abdomen may be changed, and the host usually becomes sterile. The parasites undergo pupation and eventually become adults. Adult males emerge from the host bodies, while females remain inside and can occupy up to ninety percent of the abdominal volume of their hosts.

Xenos vesperum (family Stylopidae) infects a social neotropical vespid (paper) wasp, Polistes carnifex. These obligate parasites parasitize developing wasp larvae in the nest and are present within the abdomen of female wasps once they hatch. They remain there until they push through the cuticle and pupate (males) or release infective first-instar larvae onto flowers (females). Foraging wasps transport these larvae back to their nests.

The use of strepsipterans in control of nuisance or pest insects such as leafhoppers and cockroaches has been suggested. For example, the inoculation of a pest population with the corresponding parasitoid has been found to aid in minimizing the negative impact of these pests. As a means of avoiding using pesticides for pest control, this method could be potentially useful in agriculture.

To date, there have been three species of strepsipterans (halictophagids) reported from Arkansas hosts (all leafhoppers, Cicadellidae): Halictophagus bidentatus from an unknown locality in the state, H. omani from unspecified locales in Howard (type locality) and Lawrence counties, and H. oncometopiae from Siloam Springs in Benton County (type locality). In the near future, there may also be discoveries of a Xenos species that parasitizes paper wasps and Elechus that infects planthoppers (Delphacidae), among others from the state.

For those who may be interested in the study of strepsiptera, the best method for collection is to obtain parasitized hosts such as leafhoppers, planthoppers, bees, wasps, and other insects, and to rear the parasites. Most often, the infected hosts possess a distended abdomen, and one end of the parasite may protrude from between two of the host abdominal segments. Once collected, they should be preserved in seventy percent isopropanol or ethanol and studied as slide-mounted specimens using light microscopy or stereomicroscopy.

For additional information:
Beani, Laura. “Crazy Wasps: When Parasites Manipulate the Polistes Phenotype.” Annales Zoologici Fennici 43 (2006): 564–574.

Bohart, R. M. “New Species of Halictophagus with a Key to the Genus in North America (Strepsiptera: Halictophagidae).” Annals of the Entomological Society of America 34 (1943): 341–359.

———. “A Revision of the Strepsiptera with Special Reference to the Species of North America.” University of California Publications in Entomology 7 (1941): 91–160.

Bonneton, F., F. G. Brunet, Jeyaraney Kathirithamby, and V. Laudet. “The Rapid Divergence of the Ecdysone Receptor is a Synapomorphy for Mecopterida that Clarifies the Strepsiptera Problem.” Insect Molecular Biology 15 (2006): 351–362.

Buschbeck, E. K., B. Ehmer, and R. R. Hoy. “The Unusual Visual System of the Strepsiptera: External Eye and Neuropils.” Journal of Comparative Physiology A 189 (2003): 617–630.

Cook, Jerry L. “Review of the Biology of Parasitic Insects in the Order Strepsiptera.” Comparative Parasitology 81 (2014): 134–151.

Dallai, R., Laura Beani, Jeyaraney Kathirithamby, P. Lupetti, and B. A. Afzelius. “New Findings on Sperm Ultrastructure of Xenos vesparum (Rossi) (Strepsiptera, Insecta).” Tissue and Cell 35 (2003): 19–27.

Grimaldi, D., Jeyaraney Kathirithamby, and V. Schawaroch. “Strepsiptera and Triungula in Cretaceous Amber.” Insect Systematics & Evolution 36 (2005): 1–20.

Huelsenbeck, John P. “Systematic Bias in Phylogenetic Analysis: Is the Strepsiptera Problem Solved?” Systematic Biology 47 (1998): 519–537.

Hughes, D. P., Laura Beani, S. Turillazzi, and Jeyaraney Kathirithamby. “Prevalence of the Parasite Strepsiptera in Polistes as Detected by Dissection of Immatures.” Insectes Sociaux 50 (2003): 62–68.

Kathirithamby, Jeyaraney. “Host-Parasitoid Associations in Strepsiptera.” Annual Review of Entomology 54 (2009): 227–249.

———. “Morphology of the Female Myrmecolacidae (Strepsiptera) Including the Apron, and an Associated Structure Analogous to the Peritrophic Matrix.” Zoological Journal of the Linnean Society 128 (2000): 269–287.

Kathirithamby, Jeyaraney, and M. S. Engel. “A Revised Key to the Living and Fossil Families of Strepsiptera with the Description of a New Family Cretostylopidae.” Journal of the Kansas Entomological Society 87 (2014): 385–388.

Kathirithamby, Jeyaraney, M. Hrabar, J. A. Delagdo, F. Collantes, S. Dötteri, D. Windsor, and G. Gries. “We Do Not Select Nor Are We Choosy: Reproductive Biology of Strepsiptera (Insecta).” Biological Journal of the Linnean Society 116 (2015): 221–238.

Kathirithamby, Jeyaraney, and S. Johnston. “The Discovery After 94 years of the Elusive Female of a Myrmecolacid (Strepsiptera), and the Cryptic Species of Caenocholax fenyesi Pierce Sensu Lato.” Proceedings of the Royal Society of London B 271 (2004): S5–S8.

Kathirithamby, Jeyaraney, Larry D. Ross, and J. Spencer Johnston. “Masquerading as Self? Endoparasitic Strepsiptera (Insecta) Enclose Themselves in Host-Derived Epidermal Bag.” Proceedings of the National Academy of Sciences 100 (2003): 7655–7659.

Kathirithamby, Jeyaraney, and S. J. Taylor. “A New Species of Halictophagus (Insecta: Strepsiptera: Halictophagidae) from Texas, and a Checklist of Strepsiptera from the United States and Canada.” Zootaxa 1056 (2005): 1–18.

McMahon, D. P., A. Hayward, and Jeyaraney Kathirithamby. “The First Molecular Phylogeny of Strepsiptera (Insecta) Reveals an Early Burst of Molecular Evolution Correlated with the Transition of Endoparasitism.” PLoS One 6 (2011): e21206.

———. “Mitochondrial Genome of Mengenilla australiensis (Strepsiptera).” BMC Genomics 10 (2009): 603.

Niehuis, O., G. Hartig, S. Grath, Hans Pohl, J. Lehmann, H. Tafer, A. Donath, V. Krauss, C. Eisenhardt, J. Hertel, M. Petersen, C. Mayer, K. Meusemann, R. S. Peters, P. F. Stadler, Rolf G. Beutel, E. Bornberg-Bauer, D. D. McKenna, and B. Misof. “Genomic and Morphological Evidence Converge to Resolve the Enigma of Strepsiptera.” Current Biology 22 (2012): 1309–1313.

Pohl, Hans, Rolf G. Beutel, and Ragnar Kinzelbach. “Protoxenidae Fam. Nov. (Insecta, Strepsiptera) from Baltic Amber-A ‘Missing Link’ in Strepsipteran Phylogeny.” Zoologica Scripta 34 (2005): 57–69.

Triplehorn, Charles, and Norman F. Johnson. Borror’s Introduction to the Study of Insects. 7th ed. Philadelphia: Saunders College Publishing, 2004.

Whiting, Michael F. “Long-Branch Distraction and the Strepsiptera.” Systematic Biology 47 (1998): 134–138.

———. Strepsiptera. In Encyclopedia of Insects, edited by V. H. Resh and R. T. Cardé. New York: Academic Press, 2003.

Whiting, Michael F., James C. Carpenter, Quentin D. Wheeler, and Ward C. Wheeler. “The Strepsiptera Problem: Phylogeny of the Holometabolous Insect Orders Inferred from 18S and 28S Ribosomal DNA Sequences and Morphology.” Systematic Biology 46 (1997): 1–68.

Chris T. McAllister
Eastern Oklahoma State College

Last Updated 2/17/2018

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