How to recognize a Eulophid
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The only near-universal criterion for recognition of Nearctic Eulophids to family is a combination of leg characters: All tarsi 4-segmented, and protibial spur short and straight (1a).
1a-b: Eulophid protarsus (left), and Pteromalid protarsus (right). tbs=protibial spur.
This combination of characters separates Nearctic Eulophids from all other Chalcidoidea except the unplaced genus Cales, which is included in this key. The two Eulophid exceptions to this combination of characters are: the Oriental Entedonine genus Trisecodes Delvare & LaSalle, with 3 tarsal segments, which is separated from Trichogrammatids only by subfamily and generic characters; and the Tetrastichine genus Crataepus, which has a large, forked protibial spur (2a) the form of which is unique and serves well as an identification character. Idioporus LaSalle & Polaszek is a Neotropical genus with 4-segmented tarsi, and is currently classified as a Eunotine Pteromalid. In Idioporus, the protibial spur is long and curved though not as stout as in other Pteromalids. Tetracampids have a short, straight protibial spur like that of Eulophids, but the tarsi are 5-segmented except for most males, which have 4-segmented mesotarsi. Many Tetracampines strongly resemble Eulophines of the Elachertus genus-group.
2a: Crataepus protibial spur.
I have encountered specimens of many other Chalcidoid families misidentified as Eulophids by otherwise authoritative experts, but for the most part this has been due to misapplying generic characters that happen to be shared by species in those other families. This is especially common when a single criterion for family placement is being used or if the Eulophid genus involved is essentially monothetic, such as Hemiptarsenus. It should be noted that it is normally very difficult to count tarsal segments in small insects like Chalcidoids. This can be partially alleviated by using backlighting (focusing the light on the stage under the specimen) on a white background to highlight separations between segments. The protibial spur is usually much easier to see, and should be regarded as the primary character for beginners to assess. It can be difficult to assess as well, however, if both forelegs are buried in glue or folded under the body.
Image credits: 1a-b: Grissell & Schauff (1997). 2a: Schauff, et al. (1997).