Discussion of Phylogenetic Relationships
Compared to the other four large orders of insects (Hemiptera, Hymenoptera, Diptera, and Lepidoptera), the phylogenetic relationships of the major lineages of beetles are relatively poorly known. Only recently has some of the morphological data been examined phylogenetically (e.g., Beutel, 1997; Beutel and Haas, 2000), and molecular sequence information is only now being gathered.
There are several competing hypotheses regarding subordinal relationships. The two most widely discussed differ most strikingly in their placement of the suborder Polyphaga: this suborder is either the sister group of Myxophaga (Crowson, 1960, 1981; Machatschke, 1962; Klausnitzer, 1975; Beutel, 1997; Beutel and Haas, 2000), or the sister group of all remaining beetles (Lawrence and Newton, 1982; Kukalová-Peck and Lawrence, 1993), as shown in the following two figures:
Left: "Polyphaga+Myxophaga" hypothesis, right: "Basal Polyphaga" hypothesis of relationships among suborders of beetles.
Evidence for a close relationship of Polyphaga to Myxophaga includes the shared reduction in the number of larval leg articles (Crowson, 1960, 1981). Klausnitzer (1975) further considered the Adephaga as sister to Myxophaga + Polyphaga, based on completely sclerotized elytra, reduced number of crossveins in the hind wings, and folded (as opposed to rolled) hind wings of those three suborders.
Evidence for the alternative hypothesis, that Polyphaga is the sister group to remaining beetles, is based primarily on characters of wing structure, and on the loss of the cervical sclerites in the three suborders other than Polyphaga (Lawrence and Newton, 1982; Kukalová-Peck and Lawrence, 1993).
Recent cladistic analyses of some of the morphological data (Beutel, 1997; Beutel and Haas, 2000) supports the Polyphaga + Myxophaga hypothesis.
The composition of the clade Coleoptera is not in dispute, with the exception of the twisted-wing parasites, Strepsiptera. These odd insects have been regarded as related to the beetle families Rhipiphoridae and Meloidae, with which they share first instar larvae that are active, host-seeking triungulins and later instar larvae that are endoparasites of other insects (Crowson, 1981), or as the sister group of beetles (e.g. Kukulová-Peck and Lawrence, 1993), or more distantly related to insects (see further discussion in Strepsiptera).
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