Somebody on reddit posted a ridiculous picture today of a fly pierced onto a needle of a cactus: http://www.reddit.com/r/pics/comments/xarue/what_are_the_odds_of_this_accident/

Whilst the OP claims the picture to be real, several people doubted the fly could ever reach enough momentum to actually pierce itself onto the cactus.

Using estimates of the appropriate variables needed, could somebody calculate the plausibility of the OP's claim? How would one go about such a calculation? My hunch would be that it can be estimated using the fly's momentum but how can the required piercing force be calculated?

  • $\begingroup$ Good puzzle. No solution but I would bet it's possible, its body is a rather soft material full of pores and the speed it may achieve is rather high. Moreover, it's plausible that the immediate speed of the body is somewhat higher but fluctuating back and forth as the insect flaps its wings about 200 periods per second vias.org/physics/example_3_1_2.html. That's ideal for piercing. $\endgroup$ Jul 28, 2012 at 19:15
  • 5
    $\begingroup$ Surely this calls for experiment. $\endgroup$ Jul 28, 2012 at 19:27
  • 1
    $\begingroup$ @EmilioPisanty: Your Ig Nobel Prize is waiting. $\endgroup$ Dec 9, 2014 at 12:46

1 Answer 1


According to http://www.speedofanimals.com/animals/housefly a house fly weighs $1.2 \times 10^{-5}$kg and flies at 2m/sec. This gives it a kinetic energy of $2.4 \times 10^{-5}$J.

A house fly is about 10mm long, so looking at the picture it looks as if the spine has gone in about 1mm. Assuming a constant deceleration the work is $10^{-3}$F so the force is 0.024N or 2.4g.

The question is whether a force of 2.4g is enough to pierce the fly, and at this point I have to admit defeat as I have no idea whether it is or not. I imagine it will depend a lot on exactly how sharp the cactus spine is. Sadly I have no houseflies to hand so I cannot do the experiment.

  • 1
    $\begingroup$ Another variable to consider is where on the fly the piercing occurs. locations where the exoskeleton are segmented would be decidedly softer than a solid plate of exoskeleton. The fly in the OP appears to have been impaled on the back of its thorax, which can be segmented or solid depending on species. $\endgroup$ Dec 9, 2014 at 15:19

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.