Today was a fun day – we had a a lot of fun imaging mosquitoes and larvae under Foldscope. Most of that effort was focused on identifying mosquitoes (a post on the same coming soon); but I focused on imaging a mosquito larvae (3rd instar larvae of Culex pippens).

It’s always fun to see live organisms – specially since they reveal the inner working. As Yogi Bera once said, you can learn a lot by just watching.

So, I wondered – how do mosquito larvae breadth under water. As it turns out; I am told – they carry a snorkeling tube around all the time. It’s attached to the 8th body segment sticking out. So it was time to pull out my Foldscope and find out – how do mosquitoes actually breadth.

You can see in the tone of my voice; how excited I am. The big tube is the siphon tube with the Tracheal tracks (I think) running down. I found the siphon attached to the air-water interface. Now; when I move my hands – I was able to displace the water and thus push the larvae deeper into the water. This detaches the siphon.. As seen clearly in the video.

Now, when I relax the tension, the interface comes back and as soon as the siphon is close enough; it’s just latches on and “snaps” back on the interface.

Now, one clue comes from the fact – that when the siphon detaches; a small air bubble is seen that remains attached to the siphon tip. This is a tell tale sign that the tip is hydrophobic (a classical word used to determine that the tip does not wet). Another way to think of this is to compare to energetics. By covered with water; it’s in a high energy state – and by making contact with air-water interface; the total energy of the system is lowered.

Watch it snapping on and off several times in the video. I was quiet surprised by how fast it snaps back in; with almost no hysteresis. We can write down the energetics of this system and also calculate what weight of a mosquito larvae can a clean water surface really support.

Now this explains why you put a drop of oil to kill mosquito larvae in your back yard. The oil film cuts this interface latching and thus cutting the air supply completely.

Now; go find a mosquito larvae around you and share how they look like.

Cheers

Manu