Donald S. Caruthers Jr., a former assistant Arlington Commonwealth's attorney, was appointed yesterday as special prosecutor for the trial of Joseph N. Martin, a former Northern Virginia insurance salesman accused of hiring a man to kill an Arlington couple in 1977.

The appointment of Caruthers, 38, a Fairfax County resident who maintains law offices in Arlington and Fairfax, was made by Arlington Circuit Court Judge Charles S. Russell Jr.

Russell's action followed a request last week by the county's cheif prosecutor, William S. Burroughs Jr., that a special prosecutor be appointed to handle the case, scheduled for trial October 22.

In a letter to Russell, Burroughs, who is running for reelection November 6, reversed an earlier position of his office, saying that because he might be a significant witness against Martin, a special prosecutor should be appointed.

Two weeks earlier, one of Burroughs' assistants repeatedly assured Russell that Burroughs would not be "a major witness" against Martin. At that time Russell said that if Burroughs "is going to testify to any matter . . . of substance then, in my view, he cannot prosecute nor can any member of the commonwealth attorney's staff."

Caruthers said yesterday he has handled few criminal cases since entering private practice in 1971 after servrng three years as an assistant prosecutor. "I enjoyed my work as a prosecutor," said Caruthers, who worked with Burroughs when both were assistant Arlington prosecutors. "I expect the case to be very exciting."

Martin, 28, a Las Vegas resident free on $50,000 bond, has pleaded innocent to charges that he hired Richard Lee Earman to kill Alan Foreman and Donna Shoemaker in return for $5,000. Earman, who was acquitted of murder charges after an eight-day trial, pleaded guilty to murder conspiracy charges and testified at a court hearing that he killed the couple at Martin's request.