Eighteen-year-old Debbie has chalked up her first criminal conviction. An Arlington judge sentenced her to six months in jail and a $500 fine for prostitution.

Those who know say that's a bit rough for a first offense, but that isn't the point of this story. Nor is it to evoke sympathy for Debbie, a 7th-grade dropout who got busted, she says, on her very first "date."

The interesting thing about Debbie is how she came to be busted.

It started one mid-March evening when Marie, head of the After Dark Out Call Service, listed in the D.C. Yellow Pages under "massage," got a call from a man who described himself as an athletic scout. He said he wanted a girl sent to his motel "in the Crystal City area" of Arlington.

"We don't send people to Virginia," Marie said she told the caller. "You have to come here." He did.

Marie said the man asked about the possibility of specific sex acts; she told him that kind of talk was illegal. "You can't come in here and solicit prostitution," she said she told him. "Well, will I have a good time?" the man insisted. Marie told him she was sure there wouldn't be any problem.

The man paid Marie $230 for three hours. He then drove Debbie to an Arlington motel where he already had a room. "He started to unzip me, and I told him I'd do it," Debbie said. "I got buck naked, and he stripped down to his shorts. Then he went into the bathroom, and two guys came out and told me I was under arrest."

"I never took any money," she said. "In fact, they took the $10 Marie had given me for cab fare back to the District." She is now free on appeal bond.

I'm willing to assume that Debbie was a prostitute. Still, it seems to me the cops came short of proving it. In fact, the policeman himself seems to have violated the law: first by soliciting in the District, where his status was that of civilian, not cop, and second by transporting Debbie into Virginia for immoral purposes--an apparent violation of D.C. and Virginia law as well as the federal Mann Act.

But even if the case against Debbie was legally solid, the question remains: what was the point?

As with Abscam, it's easy to be so disgusted with the immorality of the crooks that you overlook the immorality, the techniques and the rationale of the cops. What is Arlington's interest in busting D.C. prostitutes --assuming that is what Debbie was? Her lawyer says it is part of a vendetta the Arlington authorities have against Marie, who is on probation there on a pandering conviction. The lawyer says she was told by a prosecutor that the charges against Debbie would be dropped if she would agree to furnish evidence against Marie.

That might be sufficient explanation, except for the fact that Debbie is not the only non-Virginian who has been enticed--or brought--into the county by undercover policemen and arrested for prostitution. In one case, two Baltimore County women were lured to Arlington by police who agreed to pay their $55 cab fare. Again: what was the point?

Arlington authorities say that the idea is to keep outside pimps and madams from sending their girls into Arlington. It has also been noted that the two detectives who dreamed up the technique have compiled an arrest record calculated to impress their superiors.

I keep thinking of the cartoon kid (was it Dennis the Menace?) letting flies in through an open screen door in order to kill them. The kid saw it as a way of smashing a lot of flies. Intelligent grownups are expected to be more interested in whether it makes the house more sanitary.