What are strepsipterans?

Strepsipterans (twisted-wing parasites) are an odd group of insects whose phylogenetic relationships have long been a mystery. Females are wingless, and, in all families except Mengenillidae, are highly modified and reduced, without appendages, and never leave the host. Males have many derived features as well, including reduced forewings, fan-shaped hind wings, antennae with processes, and unusual, raspberry-like eyes. A more complete description is provided by Kinzelbach (1990).

In recent years, four phylogenetic placements of these insects have been discussed:

1. sister group to the rest of Endopterygota. Two features of strepsipterans are plesiomorphic within endopterygotes, with the alternative and derived character states suggesting that the remainder of endopterygotes may form a clade. See Kristensen (1991, 1995), Whiting et al. (1997). These plesiomorphic traits in strepsipterans are:
   • wing buds in second instar larvae
   • greater similarity between larval and adult eyes than in the remainder of endopterygotes

2. within the beetle suborder Polyphaga, perhaps as sister to part or all of the family Rhipiphoridae. Rhipiphorids have a number of derived features in common with strepsipterans, in particular:
   • active, host-seeking first instar larva, with hypermetamorphosis into endoparastic later-instar larvae.
   • antennae flabellate, similar to strepsipteran males
   • reduced forewings in some genera

3. sister to Coleoptera. Strepsipterans lack apomorphies of Polyphaga, suggesting that the previous hypothesis is untenable. They do, however, share at least one derived feature with Coleoptera as a whole:
   • leathery forewing; hind wing provides thrust for flight.
   • venational characters (Kukalova-Peck and Lawrence, 1993; disputed by Whiting and Kathirithamby, 1995).

4. sister to Diptera. This hypothesis is based on both morphological and molecular evidence, and will be the primary subject of this session.

• In 1994, Whiting and Wheeler published a short note in Nature, indicating that a phylogenetic analysis they conducted of 18S ribosomal DNA suggested that Strepsiptera were related to Diptera. No data or details of the analysis were included in the note (the data and analysis would eventually appear in Whiting et al., 1997). They suggested at the time that the reduced, haltere-like forewing of strepsipteran males may be homologous to the hindwing halteres of dipterans, and that they came to be on different thoracic segments through a homeotic mutation.
• Carmean and Crespi (1995), in response, provided an analysis of 13 18S ribosomal DNA sequences, and showed that the branches leading to Diptera and Strepsiptera were both very long. They suggested that the grouping of these taxa in a parsimony analysis of the data might be artifactual, caused by long-branch attraction (Felsenstein, 1978).
• Chalwatzis et al. (1996) analyzed 18S rDNA of 19 insect species, and similarly found a grouping of strepsipterans plus dipterans, even though they used distance methods (with several distance measures) that would be affected by long-branch attraction under different conditions than parsimony methods.
• Whiting et al. (1997) presented their complete analysis, using parsimony methods, of 85 18S rDNA sequences and 52 28S rDNA sequences, as well as morphological data. This analysis also supported a grouping of streps plus dipterans.
• Hueslenbeck (1997) showed that the data set of 13 18S ribosomal DNA sequences used by Carmean and Crespi (1995), when analyzed with maximum likelihood methods, yielded a tree with strepsipterans as sister to Coleoptera. He suggested that long branch attraction is the cause for the placement of strepsipterans with Diptera when parsimony methods are used.

Required papers

• Whiting, M.F., J.C. Carpenter, Q.D. Wheeler and W.C. Wheeler. 1997. The Strepsiptera problem: Phylogeny of the holometabolous insect orders inferred from 18S and 28S ribosomal DNA sequences and morphology. Systematic Biology, 46:1-68.
• Huelsenbeck, J.P. 1997. Is the Felsenstein Zone a fly trap? Systematic Biology, 46:69-74.

Full references list

• Boudreaux, H.B. 1987. Arthropod Phylogeny, with Special Reference to Insects. Robert E. Krieger Publishing Company, Malabar, Florida.
• Carmean, C. and B.J. Crespi. 1995. Do long branches attract flies? Nature 373: 666.
• Carmean, D., L.S. Kimsey and M.L. Berbee. 1992. 18s rDNA sequences and the holometabolous insects. Molec. Phylogen. and Evol. 1(4): 270-278.
• Crowson, R.A. 1981. The Biology of the Coleoptera. Academic Press, New York.
• Felsenstein, J. 1978. Cases in which parsimony or compatibility methods will be positively misleading. Syst. Zool., 27:401-410.
• Hennig, W. 1981. Insect Phylogeny. John Wiley and Sons, New York.
• Kathirithamby, J. 1989. Review of the order Strepsiptera. Syst. Ento. 14: 41-92.
• Kathirithamby, J. Strepsiptera. 1991. Pp. 684-695 in The Insects of Australia. Cornell University Press. Ithaca, New York.
• Kinzelbach, R. 1990. The systematic position of Strepsiptera (Insecta). Amer. Ento. 36: 292-303.
• Kinzelbach, R. and H. Lutz. 1985. Stylopid larva from the Eocene-a spotlight on the phylogeny of the stylopids (Strepsiptera). Ann. Ento. Soc. Amer. 78(5): 600-602.
• Kinzelbach, R. and H. Pohl. 1994. The fossil Strepsiptera. Ann. Ento. Soc. Amer. 87(1): 59-70.
• Kristensen, N.P. 1991. Phylogeny of extant hexapods. Pp. 125-140 in The Insects of Australia. Cornell University Press. Ithaca, New York.
• Kristensen, N.P. 1995. Forty years' insect phylogenetics. Zool. Beitr. N.F. 36 (1): 83-124.
• Kukalova-Peck, J. and J.F. Lawrence. 1993. Evolution of the hind wing in Coleoptera. Can. Ent. 125: 181-258.
• Lawrence, J.F. and A.F. Newton, Jr. 1995. Families and subfamilies of Coleoptera (with selected genera, notes, references and data on family-group names). Pp. 779-1006 in Biology, Phylogeny, and Classification of Coleoptera, Papers Celebrating the 80th Birthday of Roy A. Crowson. Muzeum i Instytut Zoologii PAN, Warszawa.
• Pierce, W.D. 1964. The Strepsiptera are a true order, unrelated to Coleoptera. Ann. Ento. Soc. Amer. 57: 603-605.
• Whiting, M.F. and J. Kathirithamby. 1995. Strepsiptera do not share hind wing venational synapomorphies with Coleoptera: a reply to Kukalova-Peck and Lawrence. Journ. N.Y. Ento. Soc. 103(1): 1-14.
• Whiting, M.F. and W.C. Wheeler. 1994. Insect homeotic transformation. Nature 368: 696.