The weeklong hunt for two criminally insane killers that began after they escaped at gunpoint from a psychiatric hospital ended quietly in the dark hours of yesterday morning as one of them emerged from a wooded area in Brooklyn, thirsty, incoherent and covered with ticks.

John Casablanca turned himself in to a pair of New York City police officers who had pulled over their patrol car to write a traffic ticket. Three hours later, his partner, Herbert Arnold, was arrested wandering the area at daybreak, a loaded pistol in his belt and $11.86 in his pocket.

The two men apparently contemplated their escape for months, authorities said, arranging for a gun to be smuggled in to them and grabbing a hospital aide as a hostage on the way out. But once they arrived back in the city, they had nowhere to go.

With long criminal records and histories of mental instability, the two fugitives set off a large-scale manhunt. More than 100 New York State Police troopers, city police officers and a bloodhound named Copperfield began watching the homes of their friends and relatives, cutting the men off from any means of support and tightening a net around the area where they were eventually found.

In the end, the officers who arrested the two yesterday described resigned, pitiful men -- dirty, tired, hungry -- with no fight in them.

The two men escaped May 28 from the Mid-Hudson Psychiatric Center in New Hampton, in Orange County, about 55 miles northwest of New York City, and apparently forced their hostage to drive them near Yankee Stadium, where they left the car.

The police said they knew little about what the two did all week. The men told the police that they hid out in a marshy area near the Belt Parkway in East New York and ate for the last time at a diner three days ago. There is no evidence they committed any crimes since their escape.

Mr. Arnold told police that they decided to split up early yesterday morning when Mr. Casablanca, 51, a former Mafia enforcer from the Bronx with three murders on his record, was becoming unstable because medication he was taking to treat a psychological condition was wearing off. Mr. Arnold, 50, a convicted murderer and rapist from Brooklyn, told police he advised Mr. Casablanca to turn himself in and planned to try to continue to elude police by himself. 'I Want to Give Myself Up'

At 3 A.M., Mr. Casablanca found police Lieut. Thomas Moran and his partner, Officer Gil Zimet, writing a ticket at the corner of Pennsylvania and Seaview Avenues, in a residential neighborhood in the East New York section. Lieutenant Moran said that even before he knew who Mr. Casablanca was, he knew the man had a problem. The officer said he was disheveled and confused.

" 'I want to give myself up,' " Lieutenant Moran quoted Mr. Casablanca as having told him. "He was a little dirty and babbling." The officers quickly realized who the man was. He offered no resistance as Officer Zimet handcuffed him. The officers took him to the 75th Precinct, where they gave him a drink of water.

Officers flooded the area and at 5:55 A.M., state police investigators spotted Mr. Arnold walking on Pennsylvania Avenue near Twin Pines Drive. Mr. Arnold, who was carrying the loaded Smith and Wesson .357 Magnum in his belt, put up no fight.

James L. Harney, the state police major who headed the hunt, attributed the peaceful conclusion to "around the clock surveillance which prevented the suspects from contacting sources in New York City." Previous Planning

Richard Bennett, the deputy director of the Mid-Hudson hospital, yesterday confirmed a report in the Times Herald Record of Middletown that the hospital learned in September that Mr. Arnold, Mr. Casablanca and two others were planning an escape. Mr. Bennett said talk by patients of escaping is not unusual at Mid-Hudson, and said that they stepped up searches of the four men after they were told of their plans.

"One of the staff members had been informed by a patient these two gentlemen and two other gentlemen were planning an escape where they would obtain a gun and go out through the front door," Mr. Bennett said. In November, he said, a woman arriving to visit Mr. Arnold was arrested when she was found to be carrying a knife that authorities believed she was planning to pass to him.

Mr. Bennett said Mr. Arnold was considered "someone who would try to introduce a weapon into the facility."

The two men followed their plans, taking as a hostage Robert Kamrowski, a treatment aide at the hospital. With Mr. Arnold holding a gun to Mr. Kamrowski's head, guards at the hospital -- none of whom carry guns -- followed procedures and opened the doors for the two patients and their hostage at about 5:30 P.M. last Friday. An Old Adversary

When Mr. Kamrowski reported that the men fled to New York City, the authorities predicted their movements, largely based on the knowledge of Vito R. Spano, a New York City police lieutenant, who knows Mr. Arnold the way a hunter knows his prey.

Lieutenant Spano tracked Mr. Arnold in 1991 for several months when Mr. Arnold and two others were wanted for abducting and raping a 16-year-old girl. The three surrendered after a seven-hour siege during which Mr. Arnold threatened to blow up his block.

Lieutenant Spano was familiar enough with Mr. Arnold to know he hated being called Herbert, preferring Tony or Anthony. He also knew that Mr. Arnold's family in the Sunset Park section of Brooklyn, was likely to protect him.

At home, Mr. Arnold is not considered a threat, nor is he viewed poorly in his neighborhood. "He was a nice person, he would play with the kids," Lillian Rivera, a neighbor on 46th Street said yesterday. 'A Time Bomb'

But Lieutenant Spano considered Mr. Arnold "a time bomb" whom he presumed would return to his neighborhood. Lieutenant Spano talked with Mr. Arnold's relatives by telephone, but they denied they had seen Mr. Arnold. The police flooded Brooklyn, receiving reports of possible sightings.

The presence of the police in Mr. Arnold's neighborhood apparently pushed the two men east into East New York and Canarsie. On Thursday, investigators searched the swampy areas along Jamaica Bay and Major Harney said he thought Copperfield picked up a scent of the two men.

But the investigators returned to state police headquarters in the Bronx, cold, tired and pulling ticks from their skin. The two men were still at large.

As the police were regrouping Thursday night, the partnership of Mr. Arnold and Mr. Casablanca was falling apart. Chilling Pasts

The two killers had chilling criminal records. Soon after being paroled for his first murder in 1981, Mr. Casablanca killed a bartender in Manhattan after an argument and then, a few days later, killed a friend who criticized him for killing the bartender. In both cases, Mr. Casablanca was found not guilty by reason of insanity. And Mr. Arnold, who was arrested for auto theft when he was 17, began his adult criminal record in 1965 when he was accused of raping a 13-year-old girl and killing her and her mother, crimes for which he was found mentally incompetent to stand trial.

But yesterday, the two surrendered quietly. Now the police hope they can help them discover how the gun was smuggled into the hospital to them.

SEX: M RACE: W TYPE: T MOTIVE: CE/PC

MO: Three contract murders; two killings in personal disputes.

DISPOSITION: Plea bargain to three counts of manslaughter, 1965 (paroled 1982); ruled insane on two counts, 1981.