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Order Strepsiptera - Twisted-winged Insects

Polistes with Strepsiptera? - Xenos Strepsiptera - Halictophagus - male Xenos peckii  Northern Paper Wasp (Polistes fuscatus) with Strepsiptera - Xenos peckii stylopids - Xenos peckii - male Twisted-wing insect parasite on Isodontia? - Eupathocera auripedis Pseudoxenos erynnidis - Eupathocera erynnidis - female Eupathocera westwoodi? - Eupathocera westwoodi
Classification
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Hexapoda (Hexapods)
Class Insecta (Insects)
Order Strepsiptera (Twisted-winged Insects)
Synonyms and other taxonomic changes
Probably sister to beetles(1) and used to be included in beetles by some workers(2)
Explanation of Names
Strepsiptera Kirby, 1813
'twisted wing'
Numbers
~91 spp. in 14 genera of 6 families arranged in a single suborder (Stylopidia) in our area(3)(4)
~630 spp. in 43 genera of 15 families (10 extant) worldwide,(5)(4) arranged in 2 suborders, Mengenillidia (3 extant families) and Stylopidia (8 families)(6)(7)

Overview of our fauna of Strepsiptera
(* denotes taxa not yet in the guide)

Order Strepsiptera
Suborder Stylopidia
Family Corioxenidae
Subfamily *Corioxeninae *Floridoxenos (1 sp.), *Loania (1 sp.)
Subfamily Triozocerinae Triozocera (2 spp.)
Subfamily Halictophaginae Halictophagus (12 spp.)
Family *Elenchidae
Subfamily *Elenchinae *Elenchus (1 sp.)
Family *Myrmecolacidae *Caenocholax (1 sp.)
Family Stylopidae *Crawfordia (4 spp.), *Eurostylops (4 spp.), Halictoxenos (5 spp.), *Hylecthrus (1 sp.), Stylops (37 spp.)
Family Xenidae Eupathocera (6 spp.), Leionotoxenos (10 spp.), Xenos (6 spp.)
Size
0.5-4 mm(8)
Identification
Adult females are larviform/neotenic endoparasites; adult males are free-living, and their sole mission is to find and fertilize a female. They have reduced forewings and fan-shaped hind wings, branched antennae, and raspberry-like eyes, unique among living insects and somewhat similar to the eyes of trilobites.(7)
Key to families (adult males) by Kathirithamby & Taylor (2005).(3) See further taxonomic work by Pohl (2002) and Benda et al. (2022) on the overlooked separation of Stylopidae and Xenidae.
Range
worldwide
Suborder Stylopidia is cosmopolitan (2 families of 2 spp. each are restricted to the Neotropical and Oriental regions, respectively)
Suborder Mengenillidia is an Old World group(7), the recently discovered monotypic Bahiaxenidae is a "living fossil" from Brazil(5)
Food
obligate parasites of insects (hosts include members of 7 orders and 34 families)(7)
Life Cycle
Apart from the adult males, the only free-living stages are the viviparous 1st instar host-seeking larvae(7); the larvae hatch as free agents from eggs laid on flowers. When a suitable host visits the flower, the larva attach itself to it and become parasitic.(8)
Print References
Benda D., Pohl H., Nakase Y., Beutel R. & Straka J. (2022). A generic classification of Xenidae (Strepsiptera) based on the morphology of the female cephalothorax and male cephalotheca with a preliminary checklist of species. ZooKeys, 1093: 1-134. (Full Text)
Balzer, Z.S. & Davis, A.R. (2019). Description of the Adult Male of Stylops advarians Pierce (Strepsiptera: Stylopidae). Zootaxa 4674 (4): 496–500 (Full Text)
Bohart, R.M. (1936). A preliminary study of the genus Stylops in California (part 1) (Strepsiptera/Stylopidae). The Pan-Pacific Entomologist, 12: 9–19. (Full Text)
Bohart, R.M. (1941). A revision of the Strepsiptera with special reference to the species of North America. University of California Publications in Entomology, 7: 91–160.
Bohart, R.M. & Irwin, M.E. (1978). A Study of Stylopization in the Bee Genus Dufourea. The Pan-Pacific Entomologist, 54: 98–102. (Full Text)
Hoffmann, M., Gardein, H., Greil, H., & Erler, S. (2023). Anatomical, phenological and genetic aspects of the host-parasite relationship between Andrena vaga (Hymenoptera) and Stylops ater (Strepsiptera). Parasitology, 150(8), 744–753. (Full Text)
Kathirithamby, J. (1989). Review of the Order Strepsiptera. Systematic Entomology, 14: 41-92 (Full Text)
Nakase, Y. & Kato, M. (2021). Bee-Parasitic Strepsipterans (Strepsiptera: Stylopidae) Induce Their Hosts' Flower-Visiting Behavior Change. Journal of Insect Science, 21(5): 1–5 (Full Text)
Niehuis, Oliver et al. (2012). Genomic and Morphological Evidence Converge to Resolve the Enigma of Strepsiptera. Current Biology 22: 1309-1313. (Full Text)
Pierce, W.D. (1909). A monographic revision of the twisted winged insects comprising the order Strepsiptera Kirby. Bulletin of the United States National Museum, 66: 1–232. (Full Text)
Pierce, W.D. (1918). The comparative morphology of the order Strepsiptera: together with records and descriptions of insects. Proceedings of the United States National Museum, 54: 391–501. (Full Text)
Pohl, H. (2002). Phylogeny of the Strepsiptera based on morphological data of the first instar larvae. Zoologica Scripta, 31(1): 123-134. (Abstract from publisher's website)
Pohl, H., & Beutel, R.G. (2005). The phylogeny of Strepsiptera (Hexapoda). Cladistics, 21: 328-374 (Full Text)
Pohl, H., & Beutel, R.G. (2008). The evolution of Strepsiptera (Hexapoda). Zoology, 111(4):318-38.
Internet References
Fact sheet from Virginia Tech(9) (Web Archive) - taxonomy is out of date