746 Hymenoptera in America North of Mexico 



ries that include the most host species for chalcidoids include Lepidoptera, Homoptera, Diptera, 

 Coleoptera, and Hymenoptera. Chalcidoids generally have failed to adapt to the nymphal stage 

 of paurometabolous insects. The host spectrum of chalcidoids is being expanded constantly by 

 more comprehensive biological studies of other insects. Given the diversity of habits, host as- 

 sociations, and stages attacked, it seems reasonable to conclude that any insect potentially in- 

 cludes several niches where a chalcidoid can develop. 



All known Hymenoptera develop parthenogenetically and chalcidoids demonstrate three 

 types: arrhenotoky, thelytoky, and deuterotoky. Arrhenotoky is the most common type of 

 parthenogenesis among chalcidoids. Uninseminated arrhenotokous females deposit haploid eggs 

 that develop into hemizygous males. Inseminated arrhenotokous females produce female off- 

 spring from fertilized eggs and males from unfertilized eggs. Arrhenotoky is a mechanism 

 whereby lethal and deliterious genes can be relatively rapidly eliminated from a population and 

 superior genotypes can be relatively rapidly selected. 



Thelytoky is parthenogenesis in which males are unknown or rare and females produce 

 females by various asexual mechanisms. Cytologically, diploidy is maintained by apomixsis and 

 automixsis. Apomixsis (ameiotic thelytoky) is characterized by an absence of meiosis, and 

 chromosome number is not reduced. Automixis (meiotic thelytoky) has reduction divisions, and 

 diploidy is maintained in several ways. Rossler and DeBach (1973) review the methods of main- 

 taining a constant chromosome number. 



Thelytoky is common among parasitic Hymenoptera, but the extent of thelytoky in the Chalci- 

 doidea is not known because our knowledge of their biology is limited. Many species are known 

 from the original description only, and many species have been described from the female sex 

 only. Thelytoky may be more common than now realized. In the rather well known genus 

 Aphytis, DeBach (1969) records that about 30% of the species are thelytokous. 



The evolutionary significance of thelytoky is an issue of debate. Traditional views hold that 

 thelytoky is an "evolutionary blind alley". However, Rossler and DeBach (1972) have shown that 

 at least one species of thelytokous chalcidoid has females that are capable of sexual reproduc- 

 tion. 



Deuterotoky is parthenogenesis in which unfertilized eggs develop into both sexes. The 

 cytological mechanism of deuterotoky has not been examined in chalcidoids. This form of 

 parthenogenesis is common in some other animals, and has been reported in some species of 

 chalcidoids (Doutt, 1959). 



The cytogenetics of the Hymenoptera have been reviewed by Crozier (1975). That paper 

 points to a lack of knowledge developed about chalcidoid karyotypes and cytological phenomena. 



Hymenoptera are haplodiploid and this has been confused with sex determination. The cor- 

 relation between males being haploid and females being diploid is positive and strong, but 

 haploidy and diploidy in themselves do not determine sex. Diploid males are known to occur 

 (Whiting, 1945). Several theories have been advanced to explain sex determination in the 

 Hymenoptera, but in no instance has one theory proven adequate to explain determination in all 

 groups (Whiting, 1940, 1943; daCunha and Kerr, 1957; Slobodchikoff and Daly, 1971). Crozier 

 (1975) suggests that any general theory should accommodate the multiple allele case with as lit- 

 tle modification as possible. 



Polyembryony is a cytological phenomenon in which a single egg develops into many in- 

 dividual progeny. Among Hymenoptera the process occurs in the Platygastridae 

 (Proctotrupoidea) and copidosomatine Encyrtidae (Silvestri, 1906; Leiby, 1922, 1926). 



Sex ratio in many species of animals approximates unity. In arrhenotokous chalcidoids the sex 

 ratio usually is female biased and fluctuates between 60 and 80 percent female. Numerous fac- 

 tors have been implicated in the determination of sex ratio including size, stage, or species of 

 host (Abdelrahman, 1974 a,b; Avidov and Podoler, 1968; Clausen, 1940 a), rate of oviposition 

 (Abdelrahman, 1974 b), egg orientation (King, 1961), genetic factors (Wilkes, 1964), differential 

 mortality (Roberts, 1933; Flanders, 1937; Abdelrahman, 1974 a), density fluctuations (Flanders, 

 1956), nutrition (Flanders, 1965; Moran et al., 1969), and many others. This Hst could be 

 lengthened substantially and its only limitation now is lack of research. 



Mayr (1969) defines sibling species as "pairs or groups of closely related species which are 

 reproductively isolated but morphologically identical or nearly so." Recent studies of chalcidoids 

 have demonstrated that this group has many sibling species complexes (Hafez and Doutt, 1954; 

 Claridge and Askew, 1960; DeBach, 1959, 1960, 1969; Khasimuddin and DeBach, 1976, a,b,c; Rao 

 and DeBach, 1969 a,b,c). These complexes suggest that chalcidoids are in an active state of 



