Thursday, December 20, 2012

The Most Innocent Man on Death Row: Blow Flies


Of censuring Rome? Rather a ditch in Egypt
Be gentle grave unto me! rather on Nilus' mud
Lay me stark naked, and let water-flies
Blow me into abhorring! rather make
My country's high pyramids my gibbet,
And hang me up in chains!
                             -- William Shakespeare, Antony and Cleopatra

Blow Flies to the Rescue
Blow flies were so named for the effect their maggots have on decaying meat. Blow flies are known also as bluebottles, greenbottles, cluster flies, and carrion flies. They are frequently the first insects to invade a dead body, since they can smell carrion from ten miles distant, and since they can travel faster than their land-based counterparts. Upon arrival, the female deposits her eggs onto the body, sometimes within minutes of death, but only during daylight, and only if the air is not too cold. The eggs soon hatch and begin their predictable, temperature-dependent progression through their larval stages. So consistent is the development that blow fly larvae can provide an estimate of how long a body has been dead.

More traditional time-of-death estimation techniques become increasingly unreliable after three days postmortem. Blow fly larvae, on the other hand, can provide reasonably accurate date-of-death estimates for up to four weeks postmortem. Any such entomological estimate, however, would require that the specific species be identified and the ambient temperature history be well known. An accurate estimate would require also that the entomological evidence be properly preserved.

While blow flies have long been associated with death, they saved Larry Swearingen's life.

At least they did so for the time being. A day before Swearingen's first scheduled execution, the Texas Court of Criminal Appeals granted him stay to consider new defense claims that blow flies proved he was in jail when Melissa Trotter's body was deposited in the Sam Houston National Forest.

Sometime after his standard appeals were denied, Swearingen's appellate team turned their attention to the previously ignored insect evidence. In her autopsy report, Dr. Carter noted "postmortem insect and animal activity" and that the "oral cavity contained fly larvae." Swearingen's defense team attempted to secure the entomological evidence from the State. The State opposed the release of such evidence. At a post-conviction hearing, the State argued:
Specifically, the insects, obviously, the State assumes this stuff is going to get examined and corroborate the evidence at trial. In the unlikely event it shows a few days less than 24 days, 18 days or something, the question is, will the Court then be able to find that there’s a reasonable probability the outcome of the trial would have been different with that information. And based on all the other facts in the case, I don’t think you can say that, because we still know, based on other facts, Larry took her with him that day, Larry killed her that day, because she still had food contents, what was presumably food in her stomach.
Despite the State's babbling argument, the State court agreed with the State prosecutors.  Later, however, the Federal court disagreed with the State court, and Swearingen was allowed to see the evidence. The State provided a petri dish that contained numerous desiccated pupae, larvae, and insect parts. Forensic entomologist Dr. Dael Morris described the dish and its contents.
Exhibit 218 consisted of 11, 3 1/2", standard plastic petri dishes loosely containing "scrapings" collected from Melissa's articles of clothing during autopsy and subsequent forensic examinations. It appears that insect material from each item of clothing was dislodged onto the catch papers and subsequently funneled into the petris. As expected, the contents of each petri largely consisted of a dried hodge-podge of dehydrated insect larvae and other insects and insect parts, fibres and other non-insect materials. I examined all of the exhibits for insect evidence, including the articles of clothing, inside and out, and sorted through the insect material contained in the petris. Representative insect samples were submitted to the University of West Virginia laboratory for DNA analysis in order to determine species. Species determination is critical in the final analyses since developmental periods vary between different species of calliphorids. 
Exhibit 218 petri dishes contained dried remains of insects including many dehydrated fly larvae (maggots) of various ages. The plastic petris were of the variety that cannot hold preservative fluids. Indeed, the plastic petris are the variety designed to allow air to enter to facilitate the growth of microbiological organisms. Live insect specimens placed in them would have continued to develop until they were either eaten by the predatory insects, ants and beetles, also collected into the same containers, or until they eventually succumbed to suffocation once exhibits were sealed into plastic bags. Continued development in the containers was expected since there was no indication in any of the documentation read in advance, especially the pathology report, that any insect specimens had been killed and preserved. 
Hundreds of dehydrated maggots were sorted in exhibit 218, including Calliphoridae, Piophilidae and Spaeroceridae, There were few signs of fly development beyond the third instar larval stage. A single blow fly adult had evidently emerged in the petris to be partially consumed by insect predators. Seen in three fragments, the head, a partial thorax and a partial abdomen it was identified as Cynomyopsis cadaverina (Diptera: Calliphoridae) through classic taxonomic treatment of what was left of the head. 
The fragmented remains of a single calliphorid pupal case from which the adult fly evidently emerged was also seen. This was not a surprise since a single newly formed calliphorid pupa was observed in the photographic evidence. There were no other signs of pupae. This newly formed pupa was the key specimen in determining the age of first colonizing flies.
Though Dr. Dael Morris does not so explicitly state, her analysis was conservative in that it tended to estimate the earliest possible date of infestation. Since the flies were stored in a fashion that allowed them to continue their development, some of their growth could have occurred after the body was discovered.

As an example, assume a scenario in which the body was placed in the forest in the morning, was colonized within minutes, and was discovered in the afternoon. Assume further that the entomological evidence was stored in a manner that allowed development for seven days, but no longer. Assume finally that an analysis conducted years later determined that the oldest of the pupae was seven days old. In that case, the analyst would be able to declare only that the body had been deposited in the forest no sooner than seven days prior to discovery. That would be a conservative estimate.

While such a finding would be technically correct, given its careful wording, it would seriously overstate how long the body had been in the forest before discovery. With that qualification in mind, consider Dr. Morris' conclusion.
Results of DNA analysis, taxonomic examinations and autopsy and death scene photographic evidence indicate that Cynomyopsis cadaverina was the first fly species to colonize Melissa Trotter's remains. ... death scene temperatures were not favorable for blow fly egg laying until December 18th, and this date is consistent with the stage of development of C. cadaverina in this case.
Even given its conservative nature, Dr. Morris' entomological analysis indicated that Larry Swearingen was in jail when Melissa Trotter's body was placed in Sam Houston National Forest.

Dr. James Arends, a consulting entomologist, substantiated Dr. Morris' analysis and conclusion. I excerpt from his report.
Melissa Trotter's body according to the autopsy was recovered in January 2, 1999, from the San Jacinto National Forest, located in Southeast Texas, near Conroe, Texas. Dr. Carter did not provide a date of death in her autopsy report, but apparently testified that fungal growth and insect activity, in her opinion, indicated that Ms. Trotter had been dead for 25 days, which put her death exactly on the December 8, 1998, the date she was last scene by state witnesses. 
The autopsy report contains several remarkable observations that indicate a date of death later than December 11, 1998. ... By autopsy, there is also a remarkable absence of animal activity, which is unlikely in a corpse exposed for 22 days in the environment where it was found. Finally, Ms. Morris identified a single species of blow fly, C. Cadeverina, from the insect material collected from the evidence in this case, whereas a body exposed outdoors for the length of time that the State maintains would in all probability exhibit colonization by several species. ... 
It is my forensic entomological opinion, based on the information that I have been presented, that the temperature conditions for December 1998 and the developmental stages of fly larvae and pupae reported by Ms. Morris that Ms. Trotter's body was exposed and colonized by blow flies after December 11, 1998.
Dr. Arends repeatedly references December 11 because that was the date that Larry Swearingen was jailed. If Melissa Trotter died after December 11, then Larry Swearingen could not be guilty of her murder.

Dr. Arends dated his report as "this 19th day of January, 2007." Larry Swearingen was scheduled for execution five days hence. Four days hence, one day before his execution, the Texas Court of Criminal Appeals granted Larry Swearingen a stay.

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