This Insect of the Week is a little different; this week we will be looking at an order instead of an individual species. The name Strepsiptera means “twisted wing” (Greek strepsi, “twisted”; pteron, “wing”) which describes only one of the interesting anomalies specific to this Order. Strepsiptera are very unique morphologically and behaviorally making them quite fascinating. They are obligate endoparasites and can be found parasitizing seven orders and 34 families of insects, from silverfish to flies.

The first species, Xenos vesparum, was described in 1793 by Italian entomologist Pietro Rossi who placed the newly described species in Hymenoptera near Ichneumonoidea. In 1802 William Kirby described Stylops mellittae found parasitizing an Andrenid bee but failed to associate it with Rossi’s species. However, in 1813 Kirby recognized the anomalies that connected these two species and proposed a new Order, Strepsiptera. This new order shares unique characteristics with different orders (sperm pump, Diptera; triungulin stage, Coleoptera) making it difficult determine who it is most closely related to, commonly referred to as the Strepsiptera problem. By the end of the 19th century Stepsiptera had been placed between Odonata (dragonflies/damsel flies) and Ephemeroptera (mayflies), Hymenoptera (bees, wasps and ants) and Lepidoptera (butterflies and moths), and Siphonaptera (fleas) and Diptera (flies). In the 20th century three main hypotheses emerged; placement within Coleoptera (beetles), sister to Coleoptera, or placement outside of Holometabolous insects. The most recent phylogenetic analyses support the hypothesis that Strepsiptera is most closely related to Coleoptera.

Strepsiptera is a morphologically distinct order and extremely sexually dimorphic. The males and females share no external morphological characters. The fore wing is reduced and looks similar to the halteres of a fly and the hind wings that look like beautiful white fans that twist in flight. The eyes have unusually large lenses that give the eye a berry-like resemblance. Buschbeck et al. refers to these structures as eyelets, and believe that each one of the lenses represents a simple eye that is capable of processing a portion of the visual field, suggesting that the aggregation of simple eyes is distinctly different than previously described compound eyes.
The male’s mouthparts are covered in setae and have been repurposed to function solely as sensory organs to aid in locating a female. The female is larviform (neotenic) throughout adulthood, lacking such characters as eyes, antennae, legs, wings and external genitalia and live as endoparasites found under the abdominal terga of the insects they parasitize. This is not without exception, the females in the family Mengenillidae are free-living and have eyes, mouthparts, antennae, legs and a ventral genital opening but lack wings and look nothing similar to the winged male.

The schematic below clearly illustrates the first instar, also known as a triungulin, and the developmental split between the males and females. This is called hypermetamorphosis! The males upon eclosing have 5-6 hours to find a female before they die. This is the reason for all the enhanced sensory structures. The female is tucked under the abdominal terga of the insect she is parasitizing with her cephalothorax poking out. Once a male finds a female he inseminates her by injecting his sperm directly into her cephalothorax. Untypical of most insect, the female gives birth to live young and not just 2 or 3 but hundreds. The young exit the mother through an external brood canal as first instar larvae (triungulin) to find a new host. Once the host is found they will tuck themselves under the cuticle and go through the final larval instars and this is where the males and females split in development. The male will continue to a pupal stage and become a winged adult and the female will remain equivalent to a 4th instar larva and remain in the host for the duration of her life.
Once the host is found they will tuck themselves under the cuticle and go through the final larval instars and this is where the males and females split in development. The male will continue to a pupal stage and become a winged adult and the female will remain equivalent to a 4th instar larva and remain in the host for the duration of her life.
Collect and Preserve
The females can be found below the cuticle of other collected specimens and should be preserved in 80% ethanol. The males can be found at light traps and should be double mounted with wings spread. Extra care should be taken as their wings are very delicate.

Strepsiptera can be found worldwide. Of the three specimens in our holdings, there are two from Berkeley, California and one from Clayton, North Carolina.

• For a more detailed overview of Strepsiptera visit the Tree of Life page authored by Jeyaraney Kathirithamby.
• Host-Parasitoid Associations in Strepsiptera authored by Jeyaraney Kathirithamby gives a great description of the relationship between Strepsiptera and their hosts.
• For more photos of insects parasitized by Strepsiptera visit Bug Guide.
• Whiting M.F., J.C. Carpenter, Q.D. Wheeler, W.D. Wheeler. The Strepsiptera problem: phylogeny of the holometabolous insect orders inferred from 18S and 28S ribosomal DNA sequences and morphology. Syst Biol (1997) 46 (1): 1-68. doi: 10.1093/sysbio/46.1.1
• Kirby, William. Strepsiptera, a new Order of Insects proposed; and the Characters of the Order, with those of its Genera, laid down. Transactions of The Linnean Society of London. (1815) 11: 86-122. http://biostor.org/reference/5958
• Buschbeck, E.K., B. Ehmer, R.R. Hoy. The unusual visual system of the Strepsiptera: external eye and neuropils. Journal of Comparative Physiology A (2003) 189: 617-630. DOI 10.1007/s00359-003-0443-x
  Photo Credits
• Images of Strepsiptera eyes: Buschbeck, E.K., B. Ehmer, R.R. Hoy. The unusual visual system of the Strepsiptera: external eye and neuropils. Journal of Comparative Physiology A (2003) 189: 617-630. DOI 10.1007/s00359-003-0443-x
• Life Cycle and male head SEM: Kathirithamby, Jeyaraney. Host-Parasitoid Associations in Strepsiptera. Annu. Rev. Entomol. (2009) 54: 227-49. doi: 10.1146/annurev.ento.54.110807.090525
• All other images: Beani, Laura. Crazy wasps: when parasites manipulate the Polistes phenotype. Ann. Zool. Fennici (2006) 43: 564-574.