Jaren Lockhart was stabbed through the heart with a single-edged knife by someone straddling her while a second person pinned her arms to the ground, prosecutors alleged Tuesday.

The Bourbon Street dancer’s last moments were re-enacted by prosecutors during the testimony of a forensic pathologist on the opening day of the second-degree murder trial for Terry Speaks, one of two accused killers.

The day also included opening statements, graphic autopsy photos and a last-minute decision by Speaks that he would no longer represent himself.

Prosecutors say Speaks and his alleged accomplice, Margaret Sanchez, brought Lockhart to their house in Kenner in the early morning hours of June 6, 2012, for a “private party” and then killed her, cut up her body and threw the pieces off the U.S. 90 bridge in Bay St. Louis, Mississippi.

Pieces of Lockhart’s body began washing up on the shores of Gulf Coast beaches the following day; not all of her body was recovered.

Tuesday’s testimony included autopsy photos of Lockhart’s dismembered corpse. Assistant District Attorney Doug Freese warned family members before the trial that the photos were “horrific” and among the worst he had seen in a career spanning some 300 cases.

Rows reserved for the victim’s family members were noticeably thin as the lights dimmed and the jury was shown photos of the pieces of Lockhart’s bloated, naked body. Her head was severed, her face was unrecognizable and her hair had started to come off as a result of being in the water for hours.

The photos did not have any apparent effect on Speaks, who at that point was still representing himself against charges that could send him to jail for the rest of his life.

Under questioning by Freese, forensic pathologist Dana Troxclair testified that portions of Lockhart’s torso had been carved out with a knife in places where she had tattoos. Her forearms, which had tattoo “sleeves,” were never recovered, and the flesh had been taken off the bone just above the elbows.

Looking at photos of a small stab wound in the center of Lockhart’s chest, Troxclair testified that a 4 1/2-inch blade was used to pierce her heart.

Authorities found no murder weapon, no knives or tools that were used to dismember Lockhart and no forensic evidence placing her in the car and apartment belonging to Sanchez and Speaks.

Freese asked Troxclair how it could be possible to stab someone in the heart and later dismember her body without producing a lot of blood.

Troxclair explained that if the knife was left in the wound, the skin could close up around it, and that there was still a lot of blood in Lockhart’s heart.

“There was probably some blood but not a lot,” she said.

As for the “disarticulation” — the cutting up of the body at the joints — Troxclair testified that once a person dies, he or she can be cut up without releasing a lot of blood because blood is not coursing through the body and the veins have collapsed.

Troxclair further testified that the bruising on Lockhart’s body could not have happened after her death and that bruises on her upper arms and throat were consistent with being hit or restrained while struggling.

This led to a demonstration by Freese and co-counsel Thomas Block, in which Block lay down on his back and Freese straddled him to demonstrate the stabbing and then went around to show how a second person would have held him down.

The demonstration drew no objection from Speaks, whose cross-examination consisted largely of asking Troxclair if Lockhart could have been killed with a scythe and cut up with a chainsaw. He also asked if the wounds indicated who committed the crime.

Troxclair answered “no” on each count.

Earlier Tuesday, Freese’s opening statement provided new details about the prosecution’s case against Speaks and Sanchez, although Sanchez will be tried at a later, unspecified date.

Freese painted Speaks and Sanchez as “small people who think very highly of themselves,” saying they cruelly exerted their power over a vulnerable, heroin-addicted mother who needed money.

Freese said Lockhart’s co-workers and other dancers will testify that Speaks and Sanchez were offering girls more than $500 to leave with them. He said one witness will tell the jury that she pointed the suspects to Lockhart, an act that still “haunts her to this day.”

Freese said testimony will show Speaks deleted pictures of him and Sanchez on the same beach where pieces of Lockhart’s body would later wash up.

He told jurors that a man across the street from Sanchez and Speaks’ house the day after Lockhart was stabbed to death saw Speaks standing at his open trunk with a garbage bag. He said the man will testify that Speaks — visibly startled when he noticed he was being watched — closed the trunk and hurried back into the house.

Other testimony will show that in the days after the killing, Speaks was so startled by a firm knock on a friend’s door that he ran over a glass table and broke it before jumping a fence, the prosecutor said.

Freese said that upon returning an hour later, Speaks said, “I wanted to get a Snickers.”

Freese told the jury that friends will testify that the couple’s house and car were typically filthy but were noticeably cleaner the day after the murder and that the couple spoke about the crime in code while Speaks was in prison in upstate New York on unrelated charges.

Speaks’ opening statement drew at least three objections — sustained by 24th Judicial District Court Judge Steven Grefer — for straying improperly into argumentation.

He began with a nod to Freese’s opening statement, saying, “I tell you, after hearing that opening argument I’m ready to convict myself.”

Speaks asked the jury to consider six points, beginning with each of the letters spelling the word “assume.”

In considering the evidence, he reminded the jury to ask itself a basic question: “Does it actually link me, Terry, to the crimes I’m being charged with?”

Just before 6 p.m., Grefer told the court that Speaks had decided he would exercise his right to have attorneys represent him.

His attorneys — public defenders John Benz and Brad Scott — had been in court, advising Speaks on points of law while he cross-examined witnesses and made objections at his own discretion.

Grefer called the jury in to tell them of the change and dismiss them for the day, giving Benz and Scott an extra couple of hours to prepare to take the reins.

The trial is expected to continue through the week.