Lewis & Short

spĕcŭlātor (also written spĭcŭl-), ōris, m. [speculor], in milit. lang., a looker-out, spy, scout, explorer, éclaireur (syn. explorator).

  1. I. Lit.: speculator, quem mittimus ante, ut respiciat quae volumus, Varr. L. L. 6, § 82 Müll.: repentinus, Cic. Verr. 2, 5, 63, § 164; Caes. B. G. 2, 11; 5, 49 fin.; Cic. N. D. 2, 56, 140; Sall. J. 101, 1; 106, 2; Liv. 3, 40, 13; 22, 33, 1; 28, 2, 2 al.
    These scouts formed a special division in each legion, Auct. B. Hisp. 13; Tac. H. 1, 25; Inscr. Orell. 1222; 3518 al.
    Under the emperors they were employed as special adjutants, messengers, and body-guards of a general, Suet. Calig. 44; Tac. H. 2, 73; Suet. Aug. 74; id. Claud. 35; id. Galb. 18; id. Oth. 5; Tac. H. 1, 24 Lips.; 2, 11; 2, 33; Sen. Ira, 1, 16, 15; id. Ben. 3, 25; Vulg. Marc. 6, 27 (where the Greek has also σπεκουλάτωρ) al.
  2. II. Transf., in gen., a searcher, explorer, investigator, examiner: physicus, id est speculator venatorque naturae, Cic. N. D. 1, 30, 83; id. Div. in Caecil. 16, 51: ad has excipiendas voces speculator ex convivis Persei missus, Liv. 40, 7: Cleonis fuisse publice praepositos χαλαζοφύλακας, speculatores futurae grandinis, Sen. Q. N. 4, 6, 1: quo tu matutinus speculator amicae? Prop. 2, 29 (3, 27), 31: apes consumptis in proximo floribus speculatores ad pabula ulteriora mittunt, Plin. 11, 8, 8, § 19: Fabius cautā speculator mente futuri, Sil. 1, 679.