The mosquito that sucked blood is the eyewitness that the criminal never expected

This article comes from the China Science Expo family

Science Academy (ID:kexuedayuan)

Everyone may have noticed a piece of news recently: The Gulou Branch of the Fuzhou Public Security Bureau successfully solved a burglary case by using the bloodstains left by mosquitoes that were slapped to death on the wall at the crime scene.

"Mosquito blood" on the wall at the crime scene

Image source: Xinhua News Agency

According to news reports, the police extracted the "mosquito blood" on the wall and sent it to the Criminal Investigation Brigade for DNA matching, and finally successfully matched it to a person with a criminal record - Chai. Until Chai was arrested, he probably didn't expect that it was the mosquito he killed with a slap that "betrayed" him. But it must be said that this case answered the questions in many people's minds: mosquitoes that have sucked blood can be used for DNA testing.

Part 1

Sherlock Holmes at the crime scene

In fact, this is not the first time that mosquitoes have acted as "eyewitnesses" at the crime scene. As early as more than 10 years ago, mosquitoes have been involved in the investigation of criminal cases. In 2006, a conference paper described a case that occurred on the island of Sicily in 2003: the reporter found a dead person near the beach in Sicily, with obvious marks of strangulation on the neck.

The police quickly identified a suspect, a businessman who was considered a strong suspect because his car had been in the area on the night of the murder. The suspect's residence was 8 kilometers away from the body dumping site. After searching his residence, the police did not find any other relevant biological evidence except for the dried blood of mosquitoes on the wall . In desperation, the technicians could only use moistened filter paper to obtain some blood on the wall, carefully collected the remains of the mosquito body, and left the suspect's home.

No one expected that these inconspicuous bloodstains would become the key to solving the case.

Beautiful Sicily beaches

Image source: veer gallery

Technicians obtained information on 15 genetic loci from the blood through PCR, and these genetic information completely matched the genetic information of the victim . Through this evidence, it can at least be confirmed that the victim appeared near the suspect's home.

Of course, some people may ask, why couldn't the mosquito sucked the victim's blood and then flew to the suspect's home, where it was slapped to death against the wall by the suspect?

At this time, entomologists came on the scene. Based on the characteristics of the mosquitoes, the entomologists determined the species of the mosquitoes. Under normal circumstances, the activity range of this type of mosquito is only a few hundred meters, and it is difficult for them to cross the area between the beach and the suspect's home. 8 kilometers is almost an unreachable distance for them.
Furthermore, during the search of the suspect's residence, the police also found a pair of sneakers and a set of clothes. The police extracted sand from the soles of the shoes, which was consistent with the sand on the beach where the body was found. In addition, fragments of marigold leaves were found on the clothes. This plant was found in the bushes near the body.

The businessman probably didn't expect that mosquitoes, sand, and marigolds would expose his crime of murder.

Part 2

A mirror that reveals the identity of the suspect

In fact, it is not just the mosquitoes at the crime scene. Even if they are thousands of miles away, as long as they have close contact with the suspects, mosquitoes may become the key to solving the case.

In 2006, a vicious criminal case occurred in Guazhou Town, Hanjiang District, Yangzhou City, Jiangsu Province, China. The case was eventually solved and was also related to mosquitoes.

This was a case of a body dumped in a taxi, and the deceased was a taxi driver. The police extracted biological evidence of the deceased and a suspected suspect in the taxi. The police immediately compared them in the DNA database, but no matching samples were found, and the identity of the suspect could not be confirmed.
The case came to a standstill. The police, unwilling to admit defeat, immediately launched a large number of investigations and visits, and locked in a suspect named Zhu Jianping. However, the subsequent results disappointed the police again. The suspect's ID card was fake, and there was no one named Zhu Jianping in Huangmei County, Hubei Province. But coincidentally, in Jiujiang City, Jiangxi Province, a place separated from Huangmei County, Hubei by a river, there was also a person named Zhu Jianping, who had a history of drug abuse.

When the police arrived in Jiujiang, they found that Zhu Jianping had disappeared. This made the police very anxious. They had no way to determine whether Zhu Jianping was the real murderer. What should they do if they missed the real murderer after further investigation?

As the case develops to this point, everyone must be curious. The process of the police solving the case is indeed very twists and turns, but does this have anything to do with mosquitoes?

Indeed, the mosquito was the key to solving the case in the end.

Just when everyone thought they were going to return empty-handed, the police found a mosquito on the mosquito net at Zhu Jianping's home, which made everyone very excited. It was through this mosquito that sucked Zhu Jianping's blood that the police obtained Zhu Jianping's DNA information. After comparison, it was determined that Zhu Jianping was the murderer of the driver . Within 9 days, this major case was solved, and this case solved with the help of mosquitoes has become a classic teaching case in the country.

Both of the above cases fully demonstrate the role of mosquitoes as "eyewitnesses" in solving cases, but technicians should also pay special attention to some precautions for the "testimony" of this special witness.

According to a paper published in the Journal of Law and Medicine, researchers pointed out that since the amount of mosquito blood that can be extracted is generally not large, different test kits should be used to repeat such trace samples to improve the accuracy of the test results.

Part 3

What kind of mosquito can help solve crimes?

Although the ability of mosquitoes to help solve crimes has been proven in many cases, this Sherlock Holmes is not omnipotent. After all, their main job is to suck blood and reproduce, and helping solve crimes is purely a helpless act after being swatted to death.

What kind of mosquitoes can be used as evidence to solve a case? We have to start with the principle of how mosquitoes solve cases: mosquitoes suck human blood, which contains human DNA. If a mosquito is killed, the blood it sucks is fixed on some media and can become evidence to solve a case.

Mosquito sucking blood

Image source: asianscientist.com

But all of this has a major prerequisite, which is that the DNA in the blood is complete enough to be used for genetic typing through PCR reaction to determine the identity of the blood owner.

So the question is, after a mosquito sucks blood, how long will it take for us to identify a person from the "blood" in its body?

In 2017, a study published in the journal Public Library of Science answered this question. According to scientists, within two days after a mosquito sucks blood, the "blood" in its body can be used for DNA identification to determine the identity of the person sucked.

Scientists conducted experiments on seven volunteers, letting female mosquitoes suck the volunteers' blood, and extracted blood from the mosquitoes after a certain period of time for testing. They measured the concentration of DNA in the blood and performed genotyping. The results showed that the DNA content in the mosquitoes' "blood" gradually decreased over time; similarly, the effect of genotyping identification became worse and worse. Within 12 hours, almost all 16 sites could be identified, but after 72 hours, there was basically no way to perform genotyping identification.

Genotyping performance gets worse over time

Image source: References

However, the police will not investigate cases based on just one piece of evidence. The DNA information provided by mosquitoes is only one of them. Please remember:

As long as a crime is committed, there will always be evidence left. The law is long and there is no loophole! It is true that "If you don't want to know, you should not do it yourself!"