A population from U. C. D. has been considered as F. aquilonius by its body length and tail length. Head 7-7.5 um wide, about 3.5 um high, stylet with rounded knobs; median bulb oval, posterior bulbar region spatulate with wide lumen ending abruptly, cardia region large; tail shorter than the vulva-anus distance, tail tip not effilated.  SEM shows a head similar to F. vulgaris but the four caphalic papillae are not visible.  F. aquilonius can be differentiated from F. vulgaris by its shorter tail, its slightly wider head and longer oesophagus. F. aquilonius can be differentiated from F. cylindricus by its shorter stylet (10-11 um vs. 12-13 um) and its lower c' value (c' = 7-9 vs. 10-15).

(Description- Raski and Geraert, 1986)