The tribuli are small engines with four spikes, one fixed in the ground, the three others erect or adverse, (Procopius, Gothic. l. iii. c. 24. Just. Lipsius, Poliorcetwv, l. v. c. 3.) The metaphor was borrowed from the tribuli, (land-caltrops,) an herb with a prickly fruit, common in Italy. (Martin, ad Virgil. Georgic. i. 153 vol. ii. p. 33.)