The Adephaga is the second largest suborder of beetles, with over 40,000 known species. Most members are predacious (the Greek word "adephagos" means "gluttonous"). The two living families with terrestrial members, Carabidae and Trachypachidae, are occasionally called the Geadephaga; the remaining, aquatic families are the Hydradephaga. The majority of species in the suborder belong to the family Carabidae.
Adephagans diverged from their sister group in the late Permian, with the most recent common ancestor of living adephagans probably existing in the early Triassic, around 240 million years ago (Ponomarenko, 1977; Erwin, 1979). Both aquatic and terrestrial representatives of the suborder appear in the fossil record in the late Triassic, with a Jurassic fauna consisting of trachypachid, carabid, gyrinid, and haliplid-like forms (Ponomarenko, 1977). The familial and tribal diversification of the group spans the Mesozoic period, with a few tribes radiating explosively in the Tertiary (e.g., members of the carabid subfamily Harpalinae, Erwin, 1985).
Adephagans are diverse in diet and structure. Most are general predators, although algal feeders (Haliplidae), seed feeders (many harpaline carabids), fungal feeders (rhysodines), specialist predators on snails (licinine and cychrine carabids), and ectoparasitoids of other insects (brachinine and lebiine carabids) or millipedes (peleciine carabids), occur. Many lineages have gone down, into caves, while others have gone up, into the rain forest canopy or alpine habitats. The body forms of some have become highly modified structurally for life in unusual habitats (e.g., gyrinids at the air-water interface, paussine carabids in ants' nests, rhysodines in heartwood). Some are ovoviparous (pseudomorphine carabids, Liebherr and Kavanaugh, 1985). A variety of chemical defense mechanisms have evolved in the group, including the explosive discharge of bombardier beetles (Aneschansley et al., 1969).
Members of the suborder have the following properties: Adults with notopleural sutures visible on prothorax, with six visible abdominal sterna, the first three fused and divided by hind coxae; pygidial defense glands; testes tubular, coiled, consisting of a single follicle; ovaries polytrophic. Larvae with fused labrum and no mandibular molae.
Evolution and Systematics
Discussion of Phylogenetic Relationships
Family-level relationships within Adephagans have been extensively examined, but no consensus has been reached (for some recent reviews, see Kavanaugh, 1986; Beutel, 1993). There has been detailed examination of head structure (Beutel, 1986, 1989a), thoracic sclerites and musculature (e.g., Bell, 1964, 1966, 1967, 1982; Nichols, 1985; Kavanaugh, 1986; Beutel, 1986, 1988, 1989b, 1990, 1992b) , female abdominal structure (Bils, 1976; Burmeister, 1976, 1980, 1990a, 1990b), ventral nerve cord (Heath and Evans, 1990), digestive system (Yahiro, 1990), wing venation (Ward, 1979), larvae (e.g., Liebherr and Ball, 1990; Bousquet and Smetana, 1991; Beutel, 1991a, 1991b, 1992a, 1992c, 1993; Arndt, 1993), chromosomes (Serrano, 1981b; Serrano and Yadav, 1984), defensive glands (Forsyth, 1972; Kanehisa and Murase, 1977), and defensive secretions (e.g., Kanehisa and Murase, 1977; Moore, 1979; Kanehisa and Kawazu, 1985).
The following three phylogenetic hypotheses give an indication of the controversies surrounding the phylogeny of adephagans. This first tree is from Baehr (1979):
this second from Kavanaugh (1986):
and this tree from Beutel (1995):
Much of the attention has focused on a few controversial groups:
- Trachypachidae: These are considered as either the sister group of Carabidae (e.g., Kavanaugh, 1986), or as the sister group to all or part of Hydradephaga (Bell, 1966; Hammond, 1979; Ward, 1979; Roughley, 1981; Evans, 1982; Arndt, 1993; Deuve, 1993). Bell (1982) believes that "understanding Trachypachus is the key to understanding the Adephaga." Superficially, these terrestrial beetles are very much like carabids, and have frequently been placed in the same family (Lindroth, 1961). However, trachypachids have glabrous antennae, immobilized hind coxae, and medial binding patches on their wings, and in this way are like dytiscimorph water beetles. If the adephagan ancestor was aquatic or semi-aquatic, then trachypachids may represent an intermediate stage, with several morphological remnants (e.g., glabrous antennae) of an aquatic life (Kavanaugh, 1986; Ponomarenko, 1977). According to this view, later lineages lost other indications of an aquatic ancestry, and diversified into modern carabids. In contrast, if the adephagan ancestor was terrestrial, then features linking trachypachids and dytiscimorphs are derived, indicating a close relationship (Roughley, 1981; Bell, 1982).
- Omophronini: This tribe is generally thought to be within Carabidae (many authors including Baehr, 1979; Beutel, 1993; Erwin, 1985), but has been proposed to be related to Hydradephaga and trachypachids (Nichols, 1985; Ruhnau, 1986; Deuve, 1993).
- Cicindelitae (tiger beetles, here included within Carabidae): These may be a separate clade from carabids (Bils, 1976; Regenfuss, 1975), or they may belong within Carabidae, related to carabines (Deuve, 1993; Kavanaugh pers. comm.), hiletines (Ward, 1979), or loricerines (Arndt, 1993).
- Rhysodini: Wrinkled bark beetles may be highly derived carabids, related to scaritines (Bell, 1967; Forsyth, 1972; Beutel, 1990, 1992a) or psydrites (Erwin, 1985) or they may lie outside of carabids (Arndt, 1993; Deuve, 1988; Beutel, 1993). In the Tree of Life project they are placed within carabids.
The critical questions about phylogeny of adephagan families revolve around the placement of the root of the adephagan clade, and the nature of the ancestor. Was the ancestral adephagan aquatic or terrestrial? Is the root next to Gyrinidae (Beutel and Roughley, 1994)? Is the root within terrestrial carabids, with hydradephagans arising within Carabidae (Bils, 1976; Nichols, 1985)? The long independent history of the other suborders of beetles has obscured morphological clues, making the information from outgroups suspect, and the position of the root questionable. For example, the possible sister group to the adephagans, the suborder Myxophaga (Kukalová-Peck and Lawrence, 1993), consists of minute or small aquatic (hygropetric) or interstitial beetles. The reduction in size and specialized habitat of these beetles might be associated with enough correlated, derived modifications to cast doubt upon use of their structural features in outgroup analysis. Hopefully the molecular sequence data currently being gathered will help clarify these issues.