Evanioidea comprises three extant families, Aulacidae, Gasteruptiidae, and Evaniidae, and several fossil taxa of uncertain placement. Some systematists have uncovered morphological (Naumann 1991, Whitfield et al. 1989, Gauld and Bolton 1988, Gibson 1985, Carlson 1979, Crosskey 1951), biological (Gauld and Bolton 1988, Crosskey 1951, Bradley 1908), and molecular (Dowton and Austin 2001, Dowton et al. 1997) evidence questioning the monophyly of Evanioidea, but no formal higher level taxonomic alterations have been made since Hedicke (1939).
Deans et al. (2004) and Mason (1993) noted that several amber fossils (Fig. 3) reveal intermediate forms between Aulacidae and Evaniidae, lending credit to the idea that Evanioidea are monophyletic, and Jennings et al. (2004) noted that Hyptiogastrites electrinus Cockerell from Burmese amber (Fig. 4) exhibits characteristics of both Gasteruptiidae and Aulacidae.
Figure 3. Evaniid fossil in Lebanese amber with some characteristics of the family Aulacidae. Image copyright © 2004 Andrew R. Deans.
Basibuyuk et al. (2002), Jennings et al. (2004), and Nel et al. (2004) provide excellent summaries of Evanioidea in the the fossil record. The phylogenetic structure at the top of the page was estimated by Basibuyuk et al. (2002). The stem group taxa (Evaniops, Evanigaster, Cletoclistogastrinae, Praeaulacinae, and Anomopterellinae) all belong to the fossil family Praeaulacidae, which are found in the Jurassic andLower Cretaceous of Kazakhstan, East Siberia, Mongolia andAustralia (Rasnitsyn 1972). Baissinae (at least 30 spp.; includes Manlayinae) has variably been placed in Gasteruptiidae, Aulacidae, or its own family (Baissidae) (Nel et al. 2004), though the latest estimation of relationships places Baissinae sister to extant Aulacidae and Gasteruptiidae (Basibuyuk et al. 2002).
Evanioidea are represented by close to 100 fossil species. Our understanding of how these taxa relate to extant groups remains unstable due to the incomplete preservation of most fossils and the lack of phylogenetic characters therein. Future efforts in this research area are needed and will undoubtedly shed light on the evolution of this group of insects.
Are Evanioidea monophyletic?
The most apparent evidence suggesting a non-monophyletic Evanioidea are the differences in host biology for the three families. Evaniids oviposit into cockroach oothecae buried in substrate, loose in leaf litter, or attached to female cockroaches. Gasteruptiids are predator-inquilines that lay eggs inside the cells of solitary bees and wasps nesting in plant stems or in underground nests, with the subsequent larvae developing on the food stores and/or nest inhabitants (Jennings and Austin 2004). Aulacids, considered by some hymenopterists to be the most ancestral of the three families, are endoparasitic on wood-boring xiphydriid sawflies and cerambycid and buprestid beetles (Jennings and Austin 2003). Many hymenopterists hypothesize an easy transition from parasitizing wood-boring larval hosts to "parasitizing" nest cells within plant stems but not to "parasitizing" cockroach egg cases (Whitfield 1998). Gasteruptiidae and Aulacidae also share more synapomorphies with each other than with Evaniidae and have frequently been combined into one family (Gasteruptiidae; see Townes 1950). These facts, combined with evidence that species in several other distantly related lineages (Chalcidoidea, Cynipoidea, Ichneumonoidea) independently evolved dorsally articulated metasomas, have led people to question the monophyly of Evanioidea.
Evanioidea share two possible apomorphies: the dorsal articulation of the metasoma to the mesosoma (Fig. 1) and the loss of all functional metasomal spiracles (breathing holes) except on the seventh segment (Figs. 1 and 2).
Figure 1. Diagram of a female Evanioid (Gasteruptiidae) showing the dorsal articulation of the metasoma to the mesosoma (after Goulet and Huber 1993).
Figure 2. Evanioid (Evaniidae) metasoma (female) showing reduction of spiracles except for on the seventh segment. Image copyright © 2004 Andrew R. Deans.
Evolution and Systematics
Discussion of Phylogenetic Relationships
Despite conflicting evidence several recent phylogenetic and other studies predict an Evaniidae + Gasteruptiidae clade. Ronquist et al. (1999), using morphological data, recovered these two families as sister taxa with low support. Quicke et al. (1994) found some similarities between the ovipositor in Aulacidae and Gasteruptiidae, but not Evaniidae. Both families have a medial thickening of the ventral wall of the upper valve, but the Gasteruptiidae have a mid-dorsal longitudinal ridge that is absent in aulacids. Evaniidae differ in that their ovipositor is dorso-ventrally compressed rather than diverging, a character that may be functionally linked to parasitising blattodean oothecae.
Dowton and Austin (2001) recovered Evaniidae + Gasteruptiidae using evidence from three genes (Fig. 5 in Dowton and Austin 2001) and three genes plus morphological characters (Figs. 7,9,13 in Dowton and Austin 2001). However, their placement of Evaniidae depended on the tree building methods and models employed. Sister-group relationships also resulted between ensign wasps (Evaniidae) and other taxa, mainly Evaniidae + Heloridae (Figs. 2,3,6 in Dowton and Austin 2001) and Evaniidae + Ceraphronoidea (Figs. 4,10 in Dowton and Austin 2001). Unpublished reanalyses using Dowton and Austin's data (Deans and Whitfield 2003, using Bayesian methods) recovered a monophyletic Evanioidea 100% of the time, but the relationships between families differed depending on the assumptions made.