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Why Do Flies Rub Their ‘Hands’?

Last Updated: 05.02.23


Just like we’ve talked about in our latest article, flies’ behavior might seem weird to us. Maybe one of the strangest things about flies is how they rub their hands. But they are doing more than this. Apparently, they are cleaning themselves, checking for injuries or preparing to fly. 


Rubbing habit

It seems that every time a fly is resting, it rubs its ‘hands’. The rubbing behavior is present in many fly species, but it could be the most obvious in house flies’ behavior as they are a common presence during warm seasons.

Flies rub their limbs in order to keep them clean. Even though it may seem paradoxical given almost every fly species has an attraction for filth and grime, cleaning is one of their most time-consuming activities.

Through the grooming process, flies get rid of chemical and physical sediments and clean the smell receptors. Having clean receptors plays a big role in flying, searching for food, finding mating partners and just about anything that a fly does during its life.



Flies as an example of good hygiene

When a fly lands close to you, pay attention to it. It doesn’t just rub its first set of legs. It also cleans its head, wings and even rubs its hind legs together. The cleaning process is so thorough and exhaustive that the University of Arizona recommended using flies as an example to teach kindergarten kids about proper hygiene. 

This cleaning behavior has been observed in other insects species as well. Fully developed antennae (not present in flies) seem to be the biggest priority for insects when it comes to cleaning, according to the North Carolina University.


Cleaning behavior

A fly that got covered in dust could spend more than 15-20 minutes cleaning the grit from its small body. Because the fly can use only its legs to complete this task, it needs an efficient plan. 

Researchers have noticed that flies clean themselves following a sequence of cleaning movements. They always start cleaning the head first before getting to the body parts with lower priority. 


How studying the flies might helps us

New research showed the neural pathways responsible for controlling the cleaning of each body part are organized hierarchically. So the cleaning moves with a higher priority are executed first and as the cleaning process continues, the lower priority movements are integrated.

According to the research team’s leader, new information about behavior patterns will help scientists identify the neural circuits that are responsible for grooming in fruit flies. The study was conducted on fruit flies so the team could better understand how insects conduct progressions of moves to build up more complex behaviors.

Even if grooming is a deep-seated movement sequence, getting to know the logic behind it better can possibly give information about other types of sequences. Using the fly’s grooming process as a starting point, scientists could better understand how people put words together to form a sentence or how a person executes complex tasks such as dancing.


How the experiment was conducted

For activities with this level of complexity, it is of great importance that every step is executed in a particular order. By covering flies in dust and monitoring how they dealt with the mess, Andrew Seeds, a postdoctoral researcher in Simpson’s lab, had observed that flies clean themselves using a predictable order of cleaning moves. 

Apparently, flies select a body part to clean and after they cleaned that body part completely, a new decision about what they should clean next is made. This string of decisions leads to the development of a sequence. 

To test this notion, Seeds conceived an experiment in which he was able to trigger various segments of the cleaning sequence and observe how the fly will carry out its moves. The common trigger for cleaning was the existence of a speck of dust on one of the fly’s body parts. 

However, scientists had a genetic tool that allowed them to control separate movements of the cleaning sequence.

For example, when a fly was fully covered in dust and the scientists activated the abdominal cleaning, the fly would clean its head and then the abdomen. However, it would ignore the rest of its dust-covered body and just keep on cleaning the abdomen.



How the cleaning process takes place

Seeds covered a fruit fly in dust and waited for its reaction. The fly started to clean itself by removing the dust from its eyes to regain its most important sense. Next, it used its hind legs to clean the abdomen, wings and lastly the thorax. Frequently, the fly took a break from cleaning to get rid of the dust accumulated on the legs that it was using to perform the cleaning.

Even though this seemed to be the preferred sequence, Seeds observed some flies using another pattern. This proves the cleaning sequence doesn’t have a precise order and it’s more flexible. 

This represented a clue that grooming is not a string of moves in which every movement precisely causes the next one. Scientists believe the early movements in the cleaning process are able to suppress later movements in most of the cases.


They prepare to fly

The movement of rubbing their legs together helps them to neurologically prepare themselves for flight. Through the preening process, the fly makes sure the small hairs on the legs and thorax are aligned. 

Furthermore, the hairs act as proprioceptors and give information about every limb’s position. This way, it has a better initial lift and flying performance.


Checking for injuries

Through the rubbing movements, the fly gets precise information about the current state and preparation of each of its limbs. Also, it might remind it of any particular injury or damage. You might see flies doing this before going close to other flies as their interaction might result in a fight. 


Rinsing the taste sensors

There is a reason why flies keep walking all over your food. They taste it through their feet. You might have seen flies walking on food, stopping periodically and rubbing their front and hind limbs. Since flies can’t eat solid food, they are looking for a spot where the food started to rot and it’s easier to consume. 

Before they start looking for a new source of food, they have to clean their sensors in order to increase their accuracy. Even if they find an overripe fruit, they still can’t eat it instantly. Flies must liquefy their food first and they do this by vomiting saliva kept in the stomach. 

Besides being gross, this might be harmful to you as flies are carrying a lot of unwanted pathogens from their filthy feeding and breeding environment.



To smell fresh

Although it might seem strange for an insect attracted to garbage bins and animal feces, a fly should not have a strong scent. Even if it just ate some watermelon, the fly will want to get rid of that smell. This is because of its natural predators which could pick up that scent and hunt it down. So good hygiene is important for the fly’s survival.


So it can walk on every surface

How many times did you see a fly on the room’s ceiling and wonder how it can walk on there without falling? The fly manages to do this not because of suction cups or adhesive areas on its feet but rather through a large number of small hairs on the fly’s body.


It helps them dissipate heat

During the very warm days, you will see flies continuously looking for some shade so they can survive the high temperatures. However, flies have another way of getting through the hot days. 

They use the hairs on their body to dissipate heat. Hairs provide an additional surface to get rid of the heat. As they are rubbing their legs, flies make sure the hairs are properly settled so the process of dissipating heat is optimized.


To protect themselves

The grooming process expands as flies can be observed crossing their legs over the head. By doing this, they are clearing the hairs placed on their eyes. Scientists have found out that the fibers placed on the fly’s eyes can deflect almost 90 percent of airflow away from the surface of the eyes.  

Apparently, the hairs placed on the fly’s head are the most sensitive and important so the fly takes extra care of them compared to the hairs placed on its limbs.



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