How does a fly unstick it’s foot? An adhesive ON/OFF switch!!

Nov 20, 2014 • 12:49 AM UTC

China Basin, San Francisco, CA 94107, USA

140x Magnification

Microorganisms

Manu Prakash

I am a faculty at Stanford and run the Prakash Lab at Department of Bioengineering at Stanford University. Foldscope community is at the heart of our Frugal Science movement - and I can not tell you how proud I am of this community and grassroots movement. Find our work here: http://prakashlab.stanford.edu

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Lying on my bed, I have often wondered how does a fly hang upside down. Although it’s small in size (so small forces could balance its weight); this force needs to not only turn ON, but also be turned OFF. Otherwise the fly would stay stuck at one place all the time.
Now, let’s think about this in terms of energy. If it takes equal energy to break all the bonds that are used for adhesion; it would be tremendous waste of energy taking every step. This is similar to you walking on a newly made tar road (which is extremely sticky; please don’t try that – not only will you loose your shoes, it’s also not good for the road).
So I decided to catch a fly, mount it live in Foldscope and actually watch this process live. Sounds like a fun project.
Here is the fly I caught – with spotted wings. If you could identify this fly; please leave a comment down below.

Figure: live fly mounted between clear scotch tape. The tape created a little bubble around the fly, confining but not squashing the fly. The fly is still capable of moving its limbs.
Methods:
1. I took a ziplock bag and caught a fly from the garden.
2. I often use a piece of ice to cool down the fly, so I can mount it carefully without it flying away. You can also use other objects that have been chilled in the freezer for a few minutes. You can also put the ziplock bag in the fridge for 30seconds (or as long as it takes to immobilize the insect – why does chilling down the brain immobilize insects???).
3. Once it is immobilized, I mount the fly between two transparent scotch tape on the Foldscope plastic slide.

4. I imaged this fly with my 140X foldscope and collected data on my iPhone 5 using the standard magnet couplers.
Results and observations:
1. Firstly, I was thrilled to see adhesive pad structures at the base of the legs. It’s beautiful to see how microstructures are organized to form two adhesive pads. What’s also visible clearly is hair like structures completely covering these pads. Hair might increase the surface area of the pads significantly – that increasing adhesion.
What is even more amazing is the fact that side view of the adhesive pads shows the arrangement of the two hooks used by flies to also attach to rough surfaces. The pads cover the bottom and the hooks curl around the pad to still give access to the rough surface. Thus these two completely distinct mechanism provide means to adhere to a vast variety of surfaces.
To me, this also explains why these hooks are curved at the first place. The access to the surface is only possible if the hooks are curved – that’s a speculation currently. I should check that on other species.

2. Now for the fun part – I turned the video mode on and zoomed in to see the foot while it was attached. When I started imaging, by chance, the fly pulled its foot out and I was clearly able to visualize how flies “unstick” it’s legs individually.
https://www.youtube.com/watch?v=8_2yVbF70Mc&feature=youtu.be
What’s important to note in the video above is the fact that flies use the process of “curling” to unstick from the substrate. It’s well known that peeling an adhesive requires lower force; since the entire force is concentrated at a line interface. It’s beautiful to actually see this real time. Secondly you notice how fast this process is.
Next steps
Next, I will collect several different species of flies and collect the same data sets for all of them. I am very curious if the speed of peeling is the same in all of them.
Note: I know of another organism that uses peeling as a means to un-adhere from surfaces – it’s a Gecko. So parallels exist between this observation and those looked in Gecko. Also; I have not done a reference search to see if this has been seen before – I will do some reference library search to see what other people have observed.