Why do flying insects always haunt the kitchen and bathroom? The reason behind this is…

Sometimes there are some small flying insects on the walls of the bathroom at home. I believe everyone is familiar with the picture below.

Even though the doors and windows are closed, you can still find these small flying insects. Image source: pinterest

This small flying insect belongs to the family of moth flies , specifically, it is either the white-spotted moth fly or the star-spotted moth fly.

You will find that they usually appear in humid places and are not easy to kill. When you see them, they stay on the wall for a long time without moving. Just when you want to slap them, they will immediately jump to the other side of the wall. They are very annoying.

Adult moth flies. Image credit: Lyle J. Buss, UF/IFAS

Not only that, it also carries a bunch of pathogens. Scientists caught some moth flies and tested them, and found E. coli and Salmonella. Obviously, moth flies are pests to humans, just like mosquitoes and flies.

What’s annoying is that as long as you close the doors and windows, mosquitoes and flies will basically not fly in anymore. But it’s different with moth flies. No matter how tightly you close the doors and windows, it’s useless because they fly up from the sewer.

If you don’t believe me, you can put some tape on the top of the sewer pipe and take it off after a day. You will probably find flies stuck on it. Image source: Reference [2]

01

Why do flies like to stay in sewers?

Because when washing dishes, all kinds of tiny kitchen waste will be flushed into the sewer. In this process, some leftovers and some organic matter will inevitably remain on the pipe wall, forming a sticky "nutrient film" (biofilm), which becomes the best food for moth larvae.

Moreover, moth flies need to attach their eggs to a moist surface, and the surface of the sewer pipe wall is the most likely place for moth flies to breed. Generally speaking, moth flies will lay 15 to 40 eggs at a time, and sometimes there will be hundreds of eggs.

The eggs of moth flies are translucent and very small, usually no more than 1 mm in length.

The life cycle of a moth fly. Image source: Semantic Scholar

02

Then the question comes, when we flush

Why don't these little flying insects get flushed away?

A scientist (Nathan B. Speirs) was curious about this phenomenon and did some research. But you may not believe it, his research area was not entomology, but fluid mechanics.

In order to better understand the body surface and hair structure of moth flies, the first thing he did after obtaining the experimental samples was to examine the morphological characteristics of the moth flies using an electron microscope.

After repeated magnification of the electron microscope lens, the details of the moth's wings became clearer and clearer. Image source: Reference [3]

By repeatedly magnifying the electron microscope lens, we can clearly see that the wings of the moth fly are covered with dense barbs, each barb is about 4.28 microns apart. There are also fine hairs on the edges of the wings, and each hair has a barb structure when magnified.

The hairs on the edge of the wings and the barbs on the hairs, Image source: Reference 3

The same barbed structure can be found on their leg hair:

The barbed structure on the leg hairs of moth flies is clearly visible. Image source: Reference 3

03

What is the use of the densely packed barbs all over the body of the moth fly?

What is certain is that the barbs are not used to defend against potential predators, as the micron-sized barbs are unlikely to hurt anyone.

The actual function of the barbs is to make the surface of the moth fly super hydrophobic. Hydrophobicity and hydrophilicity are opposites. "Hydrophilic" simply means having an affinity with water and attracting each other, while "hydrophobic" means repelling water.

The characteristic of lotus leaves that they remain unstained despite growing out of mud is related to the hydrophobic structures on the lotus leaves. Specifically, these hydrophobic structures are the large number of micron-sized waxy microemulsion structures on the surface of the lotus leaves.

Micron-scale waxy microemulsion structure on the surface of lotus leaves. Image source: Barthlott

Each papilla is distributed with a large number of nanoscale fine branched structures. Image source: Barthlott

In addition, there are many waxy micro-tubes on the lotus leaves:

Waxy microtubules on lotus leaves. Image source: Barthlott

It is these densely packed micro-protrusions on the lotus leaf that make the water droplets become almost spherical the moment they touch the lotus leaf. They can roll in all directions, taking away the dust on the surface of the lotus leaf without leaving any trace.

The difference between a normal surface (left) and a hydrophobic surface (middle and right). When a water droplet touches a surface, the tilt angle is greater than 90 degrees, which is called a hydrophobic structure, and greater than 150 degrees is called a super-hydrophobic structure. Image source: Wikipedia

The same principle applies to the micron-sized barbs on the body of the moth fly, but having a micro-protruding structure alone is not enough, the material must also keep up.

The surfaces of hydrophobic materials have one thing in common, which is low surface energy.

If the surface energy of a material is very high, it means that it has stronger chemical bonds on its surface, so it is easier to attract liquid to wet the surface. If the surface energy is low, it is not easy to wet the surface. This is the principle of hydrophobicity of low surface energy materials.

The pictures show non-wetting (contact angle of 180°), wetting (contact angle <90°) and complete wetting (contact angle of 0°). (The contact angle is the angle connecting the solid-liquid interface and the liquid-gas interface), Image source: Wikipedia

The fine hairs on the surface of moth flies are mainly composed of chitin, a substance with low surface energy, so it is highly hydrophobic and only soluble in specific solvents.

After calculation, it was found that the hydrophobic structure on the surface of the moth fly can produce a contact angle of 175° when it comes into contact with water. This is not only a hydrophobic structure, but also a super hydrophobic structure.

To verify this, scientists conducted two small experiments:

What happens when a drop of water is placed on a fly. Image source: Reference [3]

First, place a drop of water on the fly. You can see that a contact angle of 175° is created. The water will not stick to it but will just turn into droplets and roll off.

Although the surface of moth flies is hydrophobic, it is not oleophobic. As mentioned earlier, chitin is soluble in specific solvents, so the second small experiment is to put a drop of silicone oil on the body of moth flies, which will immediately soak into the body of moth flies.

A drop of silicone oil is placed on a moth fly. Image source: Reference [3]

The interaction between water droplets and the hairs on the body of the moth fly can be observed more clearly through an electron microscope. Image source: Reference [3]

This is why moth flies are not afraid of water, because their body surface is a super hydrophobic structure.

04

When moth flies face water

Is it completely invincible?

It's not completely invincible either.

Scientists used three forms of water to attack the flies.

The first is water droplets. If the flies are continuously dripped with water droplets, they can almost always keep their bodies dry and escape from the dangerous situation of being hit by the water droplets.

When moth flies encounter water droplets, they do not experience much pressure. Image source: Reference [3]

The second is water mist, which is composed of many water droplets with a diameter of less than 4.5 microns. When these tiny water droplets come into contact with the surface of moth flies, they have almost no effect on them. They flap their wings to shake off the water droplets and fly away.

Water mist has almost no effect on moth flies. Image source: Reference [3]

The third is water flow. When the water flow hits the moth flies, it will submerge them in an instant, but they will soon float to the surface and escape. However, if the water flow hits them for a long enough time, allowing them to soak in the water for more than 5 hours, they may drown. In the experiment, scientists used water flow to hit the moth flies for 5 hours, and 2 out of 4 were drowned.

Although the fly was blown below the water surface for a moment, it soon floated to the surface and flew away. Image source: Reference [3]

This shows that moth flies have strong adaptability when facing various forms of water threats. In reality, it is not practical to run a tap for several hours, which is why moth flies on the walls of sewer pipes will not be washed away.

05

How can we eradicate these little flying insects?

If you want to eradicate these annoying little flies, you have to remove their source of nutrition, which is the "nutrient film" on the wall of the sewer pipe . The easiest way to do this is to scrape it off with a brush and then rinse it with freshly boiled water (pour it into the sewer 1 to 2 times a day for 1 week).

Illustration of how to remove small flying insects. Image source: Treehugger

The most important thing is to remember to clean your drains regularly, don’t be lazy!

References:

[1]https://wiki.nus.edu.sg/display/TAX/Clogmia+albipunctata+-+Bathroom+fly

[2] Griffith T, Gillett-Kaufman J. Drain Fly Psychoda spp.(Insecta: Diptera: Psychodidae): EENY716/IN1226, 10/2018[J]. EDIS, 2018, 2018(6)

[3] Speirs NB, Mahadik GA, Thoroddsen S T. How drain flies manage to almost never get washed away[J]. Scientific Reports, 2020,10(1):17829.

Author: Su Chengyu

This article is produced by Science Popularization China and supervised by China Science Popularization Expo

The cover image and the images in this article are from the copyright library

Reproduction of image content is not authorized