Key to the Mezirinae of Illinois

1 Fourth, fifth, and sixth abdominal sternites with narrow, sharp, transverse ventral ridge behind anterior margin; third antennomere at most only slightly longer than second ( Neuroctenus).................................................. 2

- Ventral ridge absent behind anterior margin of fourth, fifth, and sixth abdominal sternites; third antennomere conspicuously longer than second................................................................................... 3

2 (1) Seventh connexival segment of male with short, transverse fold; male paratergites reaching four-fifths length of large pygophore; third to seventh connexival segments of female with fold along outer border; female paratergites large, rounded, and reaching tip of tricuspidate ninth segment; abdomen broadly ovate.......................... Neuroctenus pseudonymus

- Connexiva without folds in both sexes; male paratergites smaller, reaching only three-fourths length of posteriorly rounded pygophore; female paraterites subtriangular, not reaching tip of notched ninth segment; abdomen more-or-less parallel-sided..................................................................................... Neuroctenus simplex

3 (1') Base of pronotum in front of scutellum trisinuate; ventral spiracles nearer to lateral margins than to anterior or posterior margins of segments; size small, length 4 mm or less ( Nannium)....................................... Nannium pusio

- Base of pronotum in front of scutellum more-or-less straight; ventral spiracles equidistant from lateral, anterior, and posterior margins of segments; size larger, usually 5 mm or more ( Mezira)............................................... 4

4 (3') Larger species, length 7 mm or more; scutellum with median carina distinct; head and pronotum sparsely granulated................................................................................................ Mezira lobata

- Smaller species, length not greater than 6.5 mm; scutellum with median carina obsolete or faint; head and pronotum densely granulated...........................................................................................5

5 (4') Body with long, thick, erect setae; parameres with caudal margin more-or-less straight between apical lobe and median lobe of hypophysis............................................................................. Mezira granulata

- Body with short, thin, appressed setae, each seta curving around associated tubercle; parameres with caudal margin more-orless sinuate between apical lobe and median lobe of hypophysis........................................ Mezira sayi