This is a family of relatively few species, but of distinct morphological habit. The distinctness of the family is, however, somewhat less appparent when one looks at several generic assignments. The genus Signiphora Ashmead (1880) was placed with the Encyrtidae while the related genus Thysanus Walker (1840) was placed as an aphelinine in the Eulophidae (Quezada, et al. 1973). The 2 genera were even synonymized for a while (resulting in the problem of whether to call the family Thysanidae based on the oldest genus or Signiphoridae based on the oldest family-group name; both names are used). The recent Hymenoptera catalog (Gordh 1979) and Riek (1970) treat Signiphorinae as a subfamily of Encyrtidae, but we treat it as a distinct family. Woolley (1988) revised the phylogeny and classification of the family at the generic level and is revising the New World species. Subba Rao (1974) provided a key to world genera, and Quezada et al. (1973) gave further characters to distinguish the genera Thysanus and Signiphora. A new key to Nearctic genera has been published by Woolley (in Gibson, et al. 1997).

STATISTICS: Number of world species: about 75 (21 Nearctic); number of world genera: 4 (3 Nearctic).

BIOLOGY: At one time signiphorids were thought to be mostly hyperparasitic on other Hymenoptera through scales, whiteflies, psyllids, and Diptera (e.g. Tachinidae). Recent studies, however, have shown that some species are primary parasites of diaspine scale insects.

DISTINGUISHING CHARACTERS: Signiphorids are fairly distinctive in the following characters: enlarged, undivided antennal club (funicle segments appearing at most like ring segments), scutellum and metanotum transverse, propodeum with triangular median area (might be mistaken for scutellum), body strongly dorsoventrally flattened, and gaster broadly attached to the mesosoma (i.e. no apparent petiole and narrowly joining with propodeum). There are no other chalcidoids which have this habitus and combination of distinctive characters.

COLLECTING: This family is not often collected, but rearing scale insects may produce some specimens. Sweeping scale infested shrubs may also be productive.

DISTRIBUTION: According to Nikol'skaya (1963) Signiphoridae are found mostly in the American tropics and subtropics. Woolley (1988) stated that at least some species of most genera are cosmopolitan.