1. There was no error in the denial of the defendant's motion to dismiss the indictments on the ground that his constitutional right to a speedy trial was denied. Applying the familiar "balancing process" articulated in Barker v. Wingo, 407 U.S. 514, 530-533 (1972), we find that the interval of twenty-one and one-half months between arrest and trial is sufficient to trigger further inquiry. Compare Commonwealth v. Campbell, 5 Mass. App. Ct. 571, 581-582 (1977). Much of the delay is attributable to the Commonwealth, although the defendant concedes that some of the delay is attributable to his own requests for continuances. Other continuances were granted, according to the docket entries, "by agreement" of the parties. The defendant failed to object to any of the continuances and does not contend that the Commonwealth intentionally sought to delay his trial. Thus, while evidence that the Commonwealth did not intentionally delay the trial does not excuse the delay, the reasons for the delay should be weighed less heavily than in a case of an intentional prosecutorial attempt to frustrate the defense. Commonwealth v. Burhoe, 3 Mass. App. Ct. 590, 594 (1975). Commonwealth v. Blaney, 5 Mass. App. Ct. 96, 99 (1977). Commonwealth v. Campbell, supra at 582-583. More significant is the fact that the defendant failed to file the present motion to dismiss until the day before trial *fn1 which suggests that he had not diligently sought a speedy trial. Compare Commonwealth v. Campbell, supra at 583 & n.12, with Commonwealth v. Beckett, 373 Mass. 329, 333 (1977). Finally, the defendant has failed to show actual prejudice resulting from the delay; "o prevail on the issue of prejudice . . . [a defendant] must demonstrate more than that a missing witness would have corroborated . . . professed innocence." Commonwealth v. Campbell, supra at 584. See Commonwealth v. Dabrieo, 370 Mass. 728, 738 (1976); Commonwealth v. Burhoe, supra at 595; Commonwealth v. Cooke, 4 Mass. App. Ct. 775 (1976). See also Commonwealth v. Anderson, ante 492, 498 (1978). 2. There was no error in the Judge's instruction that if, on the indictment charging unlawful possession of hypodermic needles and syringes (G. L. c. 94C, § 27), *fn2 the jury were to find that the defendant possessed the items, "it is incumbent upon the defendant to show that his possession is . . . ." An analogous situation was presented and decided adversely to the defendant in Commonwealth v. Jones, 372 Mass. 403 (1977). See Commonwealth v. Obshatkin, 2 Mass. App. Ct. 1, 3 (1974).