How does the enclosed insect-cocoon jump 400 times its own length ?

  • #1
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Main Question or Discussion Point

How does the "enclosed" insect-cocoon jump 400 times its own length!!!!?

Hi!

NOTE! this is not about the "mexican jumping beans" which barely move or roll, they never jump but only move a little bit and tumble.

There are several insect species that have remarkable jumping cocoons. Its the larva inside that is responsible for the extreme jumping action obviously. The thing is, that these small 1mm, 3mm and 6mm cocoons jump 20times, 116 times and even 400 times their own length!!!

Now How in this world do the cocoon jump this much??
Shouldnt it be impossible to move according to the physics law since there is always "action equals an opposite and equal reaction". The cocoon is "enclosed" and yet it jumps 100 times its own size!

It is possible of course to jump an enclosed ball,we are talking about ONLY small jumps, rolling, and maybe a maximum jump height of the ball/cocoon size.

Entomoogists say that it is the "abrupt action of the larva inside" that is responsible! But Seriously, however strong the larva may be.. it shouldn't jump several times its own length(ha?). The reason is simply that once the larva´s "head" hits the upper wall of cocoon (and cocoon starts to jump), the lower part of the cocoon will also jump of course and hit the lower part of the larva which should halt any big jumps...

I am really interested in how physically/ mechanically the cocoon jump 100 times its own length simply by jerking itself inside..

Your answers/discussion is highly appreciated
Regards
leviterande
 

Answers and Replies

  • #2
A.T.
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You mean these?

https://www.youtube.com/watch?v=pm5rXMuXZx8

Here a Nature article from 1896:

http://www.nature.com/nature/journal/v55/n1412/abs/055065a0.html

The cocoon is "enclosed" and yet it jumps 100 times its own size!
You are confusing "isolated" and "enclosed". If the cocoon is initially interacting with something (ground, thread) it can transfer momentum to it, and get the opposite momentum to jump. It could not propel itself if it was floating in space.

The reason is simply that once the larva´s "head" hits the upper wall of cocoon (and cocoon starts to jump), the lower part of the cocoon will also jump of course and hit the lower part of the larva which should halt any big jumps...
Once the cocoon lifts off, the larva+cocoon are in free fall. Their joint center of mass follows a parable, regardless of how they interact with each other. So it doesn't matter if the lower part of the cocoon hits the lower part of the larva.
 
  • #3
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Yes, there are several different wasps and moths that have that jumping behaviour.

the one in the video you showed is indeed a parasite of caterpillars and the wasp name is "Scirtetes robustus" ( Hymenoptera: ichneumonidae: Campopleginae)

"You are confusing "isolated" and "enclosed". If the cocoon is initially interacting with something (ground, thread) it can transfer momentum to it, and get the opposite momentum to jump. It could not propel itself if it was floating in space."

Ok, you are correct, but lets just analyze this even deeper:)

imagine everything is in slow motion now:

1-In order to jump, the larva FIRST HITS the lower cocoon part touching the ground. right?

2-after that(imaging everything in slow motion, bear with me please :)) the larva is starting to jump inside the cocoon(while the cocoon is still on the ground). the larva´s lower part is now "off" the lower part of the cocoon, and momentary the larva is in the middle of the jump towards the upper wall.

3- The larva´s head finally explosively hit the upper wall and without any doubt the cocoon starts to "lift" off the ground.


4- ALRIGHT here is where I think it gets tricky to understand: as the cocoon lifts of the ground the upper wall is bounced off the larva´s head (there is some space between larvas head and upper wall?) and the lower part is now hitting the lower larva´s body which should cancel any big jumps. Yes indeed it will jump but only 1 times its length only. Please remember by no means I am certain but I am trying to imagine how this works...
 
  • #4
A.T.
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1-In order to jump, the larva FIRST HITS the lower cocoon part touching the ground. right?

2-after that(imaging everything in slow motion, bear with me please :)) the larva is starting to jump inside the cocoon(while the cocoon is still on the ground). the larva´s lower part is now "off" the lower part of the cocoon, and momentary the larva is in the middle of the jump towards the upper wall.

3- The larva´s head finally explosively hit the upper wall and without any doubt the cocoon starts to "lift" off the ground.
Good so far. It should be noted that in point 3 the larva will transfer some of it's momentum to the cocoon. So it will slow down a bit, but not much if the cocoon is much lighter than the larva.

4- ALRIGHT here is where I think it gets tricky to understand: as the cocoon lifts of the ground the upper wall is bounced off the larva´s head (there is some space between larvas head and upper wall?)
This doesn't need to happen if the head somehow sticks to the upper wall. In this case the moving larva just takes the cocoon with it. But's lets assume the head and upper wall separate again:

...and the lower part is now hitting the lower larva´s body which should cancel any big jumps.
Why should it cancel anything? You don't provide any reason for this. You just say it should. After lifting of the ground the net center of mass is affected only by gravity. The trajectory of the net center of mass is not affected by the larva's bouncing around within the cocoon.

The momentum transfer in the collision of head against upper wall is much, much greater than in the later collision with the lower wall. That is due to the fact that the velocity difference is much smaller in the later case. The cocoon will not stop moving when the lower wall hits the larva, just slow down a bit. But the net center of mass is not affected by those bounces.
 
  • #5
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Thanx for the reply.
btw are you experienced in the jumping cocoon field or just general physics?

Anyway if I have very strong jumping legs and jump inside an 8ft ball I could jump 800 meters in this case , right? or half a mile.
SO if I build small strong plastic ball and Install a very powerful hydraulic mechanism inside it, it should jump 100 times its length, right?

Furthermore If I take a jumping cocoon and put it on a very sensitive digital scale and video shoot the whole jump in slow motion I should be able to see the down force on the scale...
 
  • #6
A.T.
Science Advisor
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btw are you experienced in the jumping cocoon field or just general physics?
First time I heard of those jumping cocoons.

Anyway if I have very strong jumping legs and jump inside an 8ft ball I could jump 800 meters in this case , right? or half a mile.
SO if I build small strong plastic ball and Install a very powerful hydraulic mechanism inside it, it should jump 100 times its length, right?
Yes, theoretically. But biomechanical systems are not scale-invariant. The force of muscles depends on the the crossectional area, while the mass on the volume. That's why small animals are stronger compared to their weight than big ones.

Furthermore If I take a jumping cocoon and put it on a very sensitive digital scale and video shoot the whole jump in slow motion I should be able to see the down force on the scale...
Yes.
 
  • #7
sophiecentaur
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4- ALRIGHT here is where I think it gets tricky to understand: as the cocoon lifts of the ground the upper wall is bounced off the larva´s head (there is some space between larvas head and upper wall?) and the lower part is now hitting the lower larva´s body which should cancel any big jumps. Yes indeed it will jump but only 1 times its length only. Please remember by no means I am certain but I am trying to imagine how this works...
This doesn't make sense. the 'floor' of the cocoon is travelling upwards by that time, in the same direction as the insect inside. There won't be any cancellation.
None of this is relevant once the larva and case have left the ground. They have whatever momentum the initial jump gave them and share the momentum according to their relative masses.
Imagine putting a flea inside a very light balloon (ignore the difference in air resistance). The flea would be able to jump as high as without the balloon, it would merely be carrying the balloon along with it. The larva and its container have evolved to make the most of whatever energy the larva has available and, as with many small creatures, their feats of strength are only impressive when you try to judge them by the standards of human scale animals.
 
  • #8
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Ok, I am thankful and I really appreciate your help guys.

I may make a little experiment once I find the 6mm cocoon. I will loosen a little piece of cotton wool by pulling it and put the cocoon simply on it. What will happen now when the cocoon is attempting to jump? will the very loosen cotton absorb the motion or at least dramatically decrease the jumping distance?
 
  • #9
sophiecentaur
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You will probably find that the suspension thread that the insect produces is part of 'the system' and its elasticity will be matched to the mass of the cocoon etc to maximise the height of the jump. Hanging it from a different thread will probably reduce the performance. It's a bit like the way a trampoline is designed for the weight range of the gymnast. But you could try, of course and you could see the effect.
Endless hours of fun. :smile:
 
  • #10
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You will probably find that the suspension thread that the insect produces is part of 'the system' and its elasticity will be matched to the mass of the cocoon etc to maximise the height of the jump. Hanging it from a different thread will probably reduce the performance. It's a bit like the way a trampoline is designed for the weight range of the gymnast. But you could try, of course and you could see the effect.
Endless hours of fun. :smile:
It sure will be fun :) :rolleyes: but the cotton bed should be as soft and light as possible. furthermore if the cotton is extremely elastic(is it the right word?) the poor cocoon will sink down instead haha.. am I thinking right?

the ultimate test would be to send the cocoon into outer space but actually we can drop a cocoon from a 50 story building and see how good it can jump:wink:, when it is in free weightless fall...
 
  • #11
sophiecentaur
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Like I said, the suspension needs to be matched to the insect's mass and the way it jumps. Too soft or too hard won't do - as with a trampoline. The quantity of interest here is the 'spring constant' of the suspension. The insect needs some resonance (like a pole vaulter, for instance) so it can actually channel all its energy into vertical velocity. It's a universal design problem, to be found everywhere in nature. For instance, we find that our achilles tendon works as an energy return system (matched to our weight and muscles) which gives us efficient running and a fishing rod springiness is matched to the value of weight used and the fisherman's arms to maximise the length of cast.
Resonance rocks!
 

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