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Chalcidoid or chalcid wasps are one of the most diverse groups of Hymenoptera (bees, ants, wasps) numerically, structurally, and biologically. They range in size from the smallest insect known, Dicopomorpha echmepterygis Mockford (1997), at about 130 microns (0.13 mm), to over 30 mm, including bizarre as well as beautiful winged and wingless forms. About 22,000 valid species have been described in about 2,100 genera world-wide, but these numbers represent only a fraction of true chalcid diversity and estimates of 60,000 to 100,000 species world-wide do not seem unreasonable. There are over 2,600 described species in over 700 genera in North America. Illustrated keys to the families and genera of chalcids known from North America are given in Annotated Keys to the Genera of Nearctic Chalcidoidea (Hymenoptera) by Gibson et al. (1997). Other important references to the chalcid taxonomic literature, with emphasis on the North American fauna, are given in the sections below.
Literature on chalcid wasps is vast, but there are two vital resources for any serious student of chalcidology, biological control specialist, or anyone else who needs information on the literature, names, distribution or hosts of world chalcids. The Universal Chalcidoidea Database by John Noyes provides an online version of his earlier CD-Rom version of the database (Noyes 2002). Although the CD-Rom and Internet databases are not official publications under the International Code of Zoological Nomenclature, they are unique and invaluable. They database about 31,000 world chalcid species names as well as over 550,000 entries to some aspect of the taxonomy, biology, morphology and distribution of all valid species. Each data entry is validated by being linked to one or more of over 40,000 original chalcid literature references published from 1758 through 2003. Both the CD and Internet versions also contain over 350 chalcid images of species. The Internet version additionally contains an online illustrated key to the 19 recognized world chalcid families and family information, including diagnostic features, subfamily classification, biological attributes and major references. Another extensive source of online information concerning the systematics and biology of chalcids can be found on the Chalcidoidea subsite of the Systematic Entomology Laboratory web site. The Chalcidoidea section consists of an online version of Grissell and Schauff (1997b) (excluding the pictorial key to families) and PDF documents of all 24 issues of Chalcid Forum, an international newsletter dedicated to promoting communication among chalcid workers. The SEL site also contains information on equipment and methods used to collect, rear and preserve chalcids for study under Collecting Chalcidoidea: Why and How. Because of their small size and often extreme fragility, special techniques are required to preserve specimens for microscopy. A detailed, step by step procedure for mounting minute chalcids on microscope slides is online as part of Collecting and Preserving Chalcidoids. Another site with instructions for preparing slides of chalcids is Encarsia of Australia. Comprehensive information on the morphology of chalcids and the terms used by taxonomists to describe chalcid species is found in Chalcid Wasps: Introduction to Glossary of Positional and Morphological Terms. A final online resource for information about chalcids is Nomina Nearctica, which provides a checklist of valid names of described species of Chalcidoidea and other Hymenoptera in America north of Mexico based on alphabetical lists of genera and species.
Within the United States, training in systematics is available at several universities. The Departments of Entomology at Texas A&M University in College Station and at the University of Riverside, California, currently have several students studying chalcids and other groups of parasitic Hymenoptera. Information on the people and programs can be found under the Parasitic Hymenoptera Research Laboratory at Texas A&M and under Research on Chalcidoid Systematics at the University of Riverside. A directory of world chalcidologists is available at http://iris.biosci.ohio-state.edu/newsletters/cmen.html.
The most comprehensive review of chalcid biology and ecology is by Bendel-Janssen (1977). Grissell and Schauff (1997a) state that the host range of chalcids is thought to exceed that of all other insect groups except for the order Diptera. Members are known to attack hosts in about 340 families of 15 insect orders (Blattaria, Coleoptera, Diptera, Hemiptera, Homoptera, Hymenoptera, Lepidoptera, Mantodea, Neuroptera, Odonata, Orthoptera, Psocoptera, Siphonaptera, Strepsiptera and Thysanoptera), as well as egg sacs of spiders (Araneae), ticks and gall-forming mites (Acari), cocoons of pseudoscorpions (Pseudoscorpiones), and gall-forming Anguinidae (Nematoda). In addition, phytophagous chalcids are known from six families. Agaoninae (Agaonidae) are exclusively phytophagous within the ovarioles of figs (Ficus), but are beneficial as the obligate pollinators of figs. Some members of five other families (Eulophidae, Eurytomidae, Pteromalidae, Tanaostigmatidae and Torymidae) are seed-feeders or gall-formers on plants, though for many chalcids reared from galls it is not known whether they are primary gall-formers, or inquilines or parasitoids in the galls. John Noyes provides a concise summary of known chalcid biology by family in Chalcid Forum 24. Possibly as a consequence of their host diversity, chalcids demonstrate all but 2 of 15 feeding types that have been defined for insects, including parasitism of all host life stages from egg to adult, as internal or external parasitoids, as primary or hyperparasitoids, and with their eggs laid in, on or away from the host (Grissell and Schauff 1997a). Some chalcids of the family Aphelinidae even parasitize the opposite sex of their own species (Woolley 1997).
Like other Hymenoptera, most chalcids have a haploid-diploid mechanism of sex determination in which fertilized (diploid) eggs normally develop into females and unfertilized (haploid) eggs normally develop into males (arrhentokous development). However, males are unknown or are very rare for some species and in these species females produce females asexually (thelytokous development). This latter condition appears often to be the result of the presence of maternally inherited cytoplasmic microorganisms, bacteria in the genus Wolbachia. Heat or such antibiotics as tetracycline can kill the bacterium and turn a thelytokous strain into an arrhentokous strain (Stouthamer et al. 1990). It has also been shown that reproductive isolation between closely related species is at least sometimes the result of the presence of Wolbachia or of the species possessing different bacterial 'types'. Viable hybrid offspring can be produced if the species are 'cured' of their endosymbiotic bacterium (Breeuwer and Werren 1993, Breeuwer and Werren 1995, Perrotminnot et al. 1996).
Because of their small size and structural diversity it is difficult to give easily visible and reliable features that differentiate chalcids from all other insects. Those chalcids that are fully winged are like other Hymenoptera in having two sets of membranous wings, with the forewings much larger than the hind wings. More importantly, the forewing does not have any areas (cells) entirely enclosed by veins and there is at most a single vein complex along the leading edge of the wing. This vein usually branches apically [figure] so that the vein complex typically consists of a submarginal, marginal, postmarginal, and stigmal vein. Unfortunately, some other parasitic microhymenoptera, most notably the family Scelionidae (Platygastroidea), have a forewing venation that is very similar to typical chalcids and consequently they are often mistaken for chalcids. Many chalcids are quite easily distinguished by having a distinct metallic colouration, which almost all other microhymenoptera with a similar venation lack. However, many chalcids, particularly apterous and brachypterous ones, are non-metallic and somewhat more arcane features are needed to differentiate these from other microhymenoptera. Most chalcids have the pronotum more or less distinctly separated from the base of the forewing because there is an intervening sclerite, the prepectus, between the pronotum and mesopleuron [SEM]. Although often difficult to see, chalcids also have a unique placement of the mesothoracic spiracle — it is located between the pronotum and the lateral margin of the mesoscutum, often above the anterodorsal angle of the prepectus but at least separate from the anterodorsal margin of the mesopleuron (mesepisternum) [SEM]. Other microhymenoptera have the spiracle lower down, either between the posterior margin of the pronotum and the anterior margin of the mesopleuron or on the pronotum itself in this same relative position. Furthermore, other microhymenoptera with a forewing venation similar to chalcids have the posterolateral angle of the pronotum extending back to touch the more or less oval sclerite (tegula) that covers the base of the forewing. As a result of this structural difference, most chalcids have the pronotum moveable relative to the mesothorax, whereas the pronotum is rigidly associated with the mesothorax in other microhymenoptera. Finally, almost all chalcids also have ridge-like, longitudinal sensilla on the flagellum of the antenna [SEM].
Gibson (1986a) proposed that Chalcidoidea, including the family Mymaridae, is monophyletic based on three shared derived features (synapomorphies) that are listed above as distinguishing features:
- an externally visible prepectus [SEM]
- mesothoracic spiracle at exposed lateral margin of mesoscutum[SEM], and
- flagellum with uniquely structured longitudinal (multiporous plate) sensilla [SEM]
However, Gibson and Huber (2000) subsequently discovered that individuals of the family Rotoitidae have a linear prepectus that normally is concealed under the posterolateral margin of the pronotum. This is the hypothesized groundplan (primitive) structure for Apocrita (Gibson 1999) and would invalidate the first hypothesis if the rotoitid-like structure is the groundplan state for the superfamily and not an autapomorphic (unique) secondary reduction of Rotoitidae alone.
Gibson (1986a) also proposed that Chalcidoidea was the sister group of the family Mymarommatidae based on three other features shared by the two taxa:
- unique loss of two of the three ancestral (plesiomorphic) sites of origin for a mesothoracic muscle, the mesotrochanteral depressor
- origin of an anteriorly projecting phragma as the site of origin for all or part of the remaining portion of the mesotrochanteral muscle, the mesotergal-mesotrochanteral (t2-tr2) muscle, and
- loss of the basal ring from the male genital capsule
Since then most authors have classified mymarommatids separately as their own superfamily, the Mymarommatoidea.
Quicke et al. (1994) also provided evidence that chalcids other than Mymaridae (and Mymarommatidae) have a structure of the female ovipositor that differs from all other Hymenoptera — the fused second valvulae have asymmetrical dorsolateral portions that overlap medially to a greater or lesser extent [SEM]. Furthermore, they have a transversely striated band of notal membrane, the laminated bridge near the base of the second valvulae. Quicke et al. proposed that both the unique structure of the second valvulae and the laminated bridge were synapomorphies uniting all chalcids except mymarids. This would support an hypothesis that Mymaridae is the sister group of all other Chalcidoidea. Gibson and Huber (2000) subsequently showed that females of at least one of two known genera of Rotoitidae have an ovipositor structure that is intermediate between the mymarid-like and other chalcid-like structures. The rotoitid ovipositor has about the basal half of the second valvulae structured like mymarids and most other Hymenoptera, but the apical half with asymmetrical overlapping portions similar to other chalcids. This intermediate structure could be evidence that Rotoitidae is the second-most basal lineage of Chalcidoidea after Mymaridae.
Heraty et al. (1997) also showed that Chalcidoidea, excluding Mymarommatidae, have a unique structure of another mesothoracic muscle, the furcal-laterophragmal muscle. Unlike other Hymenoptera, chalcids have this muscle attached along the entire length of the laterophragmal apodeme and this is additional evidence for excluding Mymarommatidae from Chalcidoidea. Heraty et al. further showed that structure of the furcal-laterophragmal muscle was somewhat different in mymarids than in other chalcids. Different hypotheses for evolution of the particular mymarid structure are possible, but the structure could be interpreted as an intermediate condition in a single transformation series between that of the plesiomorphic condition possessed by mymarommatids and the most apomorphic condition possessed by most other chalcids. If so, this is additional evidence that Mymaridae is the sister group of all other Chalcidoidea.
Finally, Basibuyuk and Quicke (1995) found features of the antennal cleaning organ of the front leg that might be unique for chalcids. They stated that most chalcids have a protibial comb [SEM] and that mymarids have "an indication of this structure". Only ichneumonoids were said to have somewhat similar setae of unknown function. Basibuyuk and Quicke also stated that the setae of the basitarsal comb [SEM] are uniquely structured in Chalcidoidea. The setae were said to be "distinctly flattened" in chalcids and cylindrical in the rest of the Hymenoptera, though the state in mymarommatids was not stated explicitly. These features need to be examined more comprehensively throughout chalcids to determine whether different states might indicate phylogenetic relationships within the superfamily.
Though recent studies appear to support hypotheses that Mymarommatidae is the sister group of the Chalcidoidea, relationships between Mymarommatidae + Chalcidoidea and other apocritan Hymenoptera are more controversial. Rasnitsyn (1988) proposed that Chalcidoidea is the sister group of the Platygastroidea (Platygastridae + Scelionidae). Ronquist et al. (1999) reanalysed Rasnitsyn's 1988 data using cladistic parsimony analysis and, perhaps not surprisingly, retrieved a (Chalcidoidea + Mymarommatidae) + (Platygastridae + Scelionidae) sister-group relationship from most analyses. Much more unexpected was that Ceraphronoidea was also indicated to be closely related to Chalcidoidea + Mymarommatidae, either as the sister group of the clade (Chalcidoidea + Mymarommatidae) + (Platygastridae + Scelionidae) or as the sister group of Platygastridae + Scelionidae. They suggested that these indicated relationships might reflect parallel reductions correlated with small body size, particularly in the large number of wing venational characters used in the analysis.
Comparatively recently within systematics, molecular evidence has been used to supplement more traditional morphological studies for analysing phylogenetic relationships. A Chalcidoidea + Platygastroidea sister-group relationship was supported through analysis of the mitochondrial 16S rRNA gene by Dowton et al. (1997), and this was also a commonly retrieved relationship in the much more comprehensive study of Dowton and Austin (2001). They analysed three gene regions for many more apocritan taxa, plus morphology based primarily on Ronquist et al. (1999). Maximum parsimony analysis of just the molecular data with all character transformations weighted equally indicated Chalcidoidea as the sister group of Monomachidae + (Diapriidae + Maamingidae), with this group part of an unresolved trichotomy comprised of Proctotrupidae + Vanhorniidae and Cynipoidea + Platygastroidea. Simultaneous analysis of the molecular and unordered morphological data resulted in a Cynipoidea + Chalcidoidea sister-group relationship, which formed an unresolved polychotomy with most other apocritan groups. However, almost all other analyses of the molecular evidence alone or simultaneous analysis of the molecular evidence and morphology retrieved a Chalcidoidea + Platygastroidea sister-group relationship, which inevitably comprised the terminal clade within Apocrita. Relationships of this clade with other apocritans varied considerably within the different analyses and Dowton and Austin (2001) should be examined for specifics. Based simply on morphology, Gibson (1999) proposed that Platygastroidea forms a monophyletic group with Pelecinidae + Proctotrupidae + Vanhorniidae, and that Chalcidoidea + Mymarommatoidea likely represent a relatively basal clade with uncertain relationships in Apocrita.
If anything, higher level relationships within Chalcidoidea are even less well resolved than are relationships of Chalcidoidea with other Hymenoptera. A comprehensive historical review of chalcid higher classification was given by Boucek (1988b). Historically, family classification has been based primarily on external morphology rather than phylogenetic relationships. Because of this, and because of the extreme structural diversity that characterizes the group, there have been a comparatively large number of families recognized within the superfamily as well as instability in the number of families recognized. Anywhere from 9 to 24 families have been recognized since about 1950, with 19 or 20 families generally being recognized at present. However, monophyly of many if not most of the families is in doubt. Chalcid families often seem to intergrade into each other, with 'family level' features sometimes working for only one sex, not being possessed by all members of the family, or being possessed by some members of other families. Consequently, at least some families appear to be more taxa of convenience than monophyletic evolutionary lineages. As one might suspect, the classification of some subfamilies to one family or another is controversial and there is even uncertainty about the proper family classification of some genera.
Although still at what might be called an embryonic stage, molecular analyses have now started to provide independent evidence to test and refine family concepts that are widely acknowledged to be less than satisfactory. Rasplus et al. (1998) concluded from analysis of the D1 and D2 regions of the 28S rRNA gene that Agaonidae sensu Boucek (1988a) is not monophyletic. They redefined the family to include only the pollinating Agaoninae sensu Boucek (1988a), reassigning some subfamilies to the Pteromalidae and excluding other subfamilies that they left unassigned to family. More recently, Gauthier et al. (2000) analysed the D2 region of the 28S rDNA gene and concluded that the family Elasmidae deserved only tribal status within the subfamily Eulophinae of Eulophidae. As for the previous study, they also excluded some taxa from Eulophidae and left them unassigned to family. Campbell et al. (2000) provided the first comprehensive analysis of chalcid subfamily and family relationships based on analysis of the D2 region of 28S rDNA. They included 85 taxa, representing 18 families and 32 subfamilies, and concluded that their analysis placed 80% of the taxa (including outgroup taxa) "into some form of realistic grouping (generic or family group taxon) based on morphological evidence". Of 12 families with more than one taxon represented, only three were indicated as monophyletic (Eucharitidae, Mymaridae and Trichogrammatidae), though Eulophidae was monophyletic with the inclusion of Elasmus (= Elasmidae). More unrealistic or intriguing results included none of the three genera of Eunotinae (Pteromalidae) grouping together, and neither Eupelminae nor Calosotine (Eupelmidae) being indicated as monophyletic or showing any affinities with Cleonyminae, Tanaostigmatidae or Encyrtidae. Interestingly, their analysis indicated Mymaridae as the sister group of all other Chalcidoidea, whereas most analyses of Dowton and Austin (2001) retrieved Mymaridae as an apical clade within Chalcidoidea. Resolving the relationships of Mymaridae with other Chalcidoidea is critical for establishing the ancestral life history of the group. Traditionally, chalcids have been considered as most likely evolving from some ectoparasitoid of wood-boring beetles. If Mymarommatoidea is the sister group of Chalcidoidea and if Mymaridae is truly the sister group of all other Chalcidoidea then parsimony would indicate the immediate ancestor was an endoparasitic egg parasitoid Dowton and Austin (2001).
There is no doubt that molecular analysis of chalcid relationships is an exciting new frontier that holds promising rewards for our understanding of the evolution of the group. However, such analyses seem likely to add to the instability of chalcid classification, at least in the short term, and by themselves seem unlikely to provide a fully resolved pattern of chalcid relationships. In order to fully resolve the evolutionary history of the Chalcidoidea it will also be necessary to develop a comprehensive, accurate, morphological-based character matrix for the superfamily. This is an extremely complex and daunting task considering the enormous diversity of the group, but must be done in order to advance chalcid classification beyond the level of personal preference. Although no cladistic hypotheses of family relationships based on analysis of morphology have considered the Chalcidoidea in its entirety, Noyes (1990) presented a tree diagram illustrating one set of potential chalcid family relationships (see reproduction in "A word on chalcidoid classification" or dendrogram 1 in Heraty et al. 1997). Gibson et al. (1999) reviewed current concepts of chalcid phylogenetics and classification, and this paper should be consulted for a comprehensive list of relevant publications. However, some of the more major molecular or morphological analyses that have investigated monophyly and relationships of chalcid families or subfamilies through explicit character state analysis are listed below. References are given under the primary family or families investigated though the papers often include discussion of other families.
- Rasplus et al. (1998)
- Shaffee and Rizvi (1990), Heraty et al. (1997), LaSalle et al. (1997)
- Chalcididae and/or Leucospidae
- Delavare 1988, Wijesekara 1997, Delvare 1999
- Encyrtidae, Eupelmidae, and/or Tanaostigmatidae
- LaSalle and Noyes (1985), Gibson (1986b), LaSalle (1987), Gibson (1989), Gibson (1995)
- Eucharitidae and/or Perilampidae
- Heraty and Darling (1984), Darling (1988), Heraty (1989), Darling and Miller (1990), Darling (1992), Heraty (1994), Heraty (2000), Heraty (2002)
- Eulophidae and Elasmidae
- Gauthier et al. (2000), Gumovsky (2002)
- Schauff (1984), Gibson (1986a)
- Dzhanokmen (2000), Torok and Abraham (2002), Gibson (2003)
- Woolley (1988)
- Grissell (1995)
Listed below in alphabetical order are the 19 families of Chalcidoidea that are generally accepted. Five of the family names are linked to other Internet pages developed by specialists that provide some type of information on the respective families. The relatively few references listed as resources are not intended to be exhaustive. Individuals interested in the North American fauna should refer to the references listed under each family in Gibson et al. (1997). Individuals interested in fauna from other regions should also check references included in works listed under Regional Keys to Families. The references listed below are primarily family-level studies or faunal studies from regions other than North America, but only English language publications.
- Ramirez (1970), Ramirez (1974), Wiebes (1982)
- Yasnosh (1976), Hayat and Verma (1980), Hayat (1983), Yasnosh (1983), Hayat (1994), Hayat (1998)
- Narendran (1989), Delvare and Boucek (1992), Wijesekara (1997)
- Tachikawa (1963), Trjapitzin (1973a), Trjapitzin (1973b), Trjapitzin and Gordh (1978a), Trjapitzin and Gordh (1978b), Prinsloo and Annecke (1979), Noyes (1980), Tachikawa (1981), Noyes and Hayat (1984), Noyes (1988), Dahms and Gordh (1997), Noyes (2000)
- Heraty (1985), Heraty (1994), Heraty (2002)
- Eulophidae (including Elasmidae)
- Burks (1965), Graham (1987), Graham (1991), Hansson (2002)
- Gibson (1989), Gibson (1995)
- Burks (1971), Stage and Snelling (1986), Zerova (1988), Narendran (1994)
- Boucek (1974)
- Annecke and Doutt (1961), Subba Rao and Hayat (1983), Schauff (1984), Huber (1986), Viggiani (1988), Noyes and Valentine (1989a), Yoshimoto (1990)
- Boucek (1986), Hanson (1992), Narendran (1999)
- Boucek (1978), Darling 1996
- Graham (1969), Boucek and Rasplus (1991), Gibson (2003), Sureshan (2003)
- Boucek and Noyes (1987), Gibson and Huber (2000)
- Woolley (1986), Woolley (1988)
- LaSalle (1987)
- Boucek and Askew (1968), Sugonjaev (1971)
- Narendran (1994), Grissell (1995), Grissell (1999)
- Doutt and Viggiani (1968), Yousuf and Shafee (1986a), Yousuf and Shafee (1986b), Yousuf and Shafee (1987), Pinto (1998), Viggiani (2002)
It is extremely difficult to briefly characterize or present a key to chalcid families on a world basis, particularly with current uncertainty and controversy over membership of some families. Gibson (1993) provided a world key that attempted to include exceptional forms, but because of this the key is quite complicated and it is recommended that individuals normally try to identify families using relevant regional keys. An online key to the families of Chalcidoidea and Mymarommatoidea is given by Noyes (2003). For North America, an excellent pictorial key to families and common subfamilies was given by Grissell and Schauff (1997b); a more traditional dichotomous key was also given by Grissell and Schauff (1997a). The following publications that include keys to chalcid families are listed in alphabetical order by area covered; those with an asterisk also include keys to genera.
Australasian region — * Boucek (1988a)
(plus keys to genera of 14 of 21 families)
Australia — Riek (1970); Naumann (1991)
Britain — Gauld and Bolton (1988)
Canada — Yoshimoto (1984)
Costa Rica — * Hanson and LaSalle (1995)
Cuba — * Alayo and Hernandez (1978)
Ethiopian region — Prinsloo (1980)
Europe — * Peck, Boucek, and Hoffer (1964); Graham (1969) (plus key to genera of Pteromalidae)
European USSR — * Nikolskaya (1952); * Medvedev (1978)
Nearctic region — Grissell and Schauff (1997a) in * Gibson et al. (1997); Grissell and Schauff (1997b)
New Zealand — * Noyes and Valentine (1989b) (plus keys to genera of 12 of 16 families)
Oriental region — * Subba Rao and Hayat (1985)
Tropical Africa and America — Delvare and Aberlenc (1989)
World — Gibson (1993); Noyes (2003)
Catalogs are one of the most important resources for taxonomists
and biologists alike because they are the means by which all
information published on species from 1758 to the present is or can
be organized. Catalogs list the described taxa known from a
particular region, provide the currently accepted valid name of the
taxon, list the synonyms of those valid names, and usually provide
additional information and/or references to information on the
distribution and biology of the taxa. The CD-Rom catalog of Noyes (2002) and the online catalog of
(2003) basically supersede previously published printed
catalogs. However, listed below are some of the more recent
published catalogs for Chalcidoidea; catalogs for individual
families or genera are not listed.Canada and United States —
Peck (1963), Gordh
et al. (1979)
Central and South America — De Santis (1967), De Santis (1979), De Santis (1980), De Santis (1983), De Santis (1989), De Santis and Fidalgo (1994)
Cuba — Alayo (1970)
India — Subba Rao and Hayat (1986)
Philippines — Baltazar (1966)
Sweden — Hedqvist (2003)