Lewis & Short

ăsīlus, i, m., a gad-fly, horse-fly, usu. tabanus (cf. Plin. 11, 28, 34, § 100); Gr. οἶστρος (cf. Isid. Orig. 12, 8, 15), Verg. G. 3, 147 (cf. Hom. Od. 22, 300). (Even in Seneca’s time the word was antiquated; v. Sen. Ep. 58; cf. Plin. 11, 28, 34, § 100.)