An inflatable head

The amazing ptilinum of the common house fly

I recently had the opportunity to photograph one of the odder spectacles among insects: a common house fly emerging from its puparium using a giant inflatable head. What’s deal with this strange behavior?

Many millions of years ago, some flies figured out an ingenious way to protect their delicate developing pupae. Instead of shedding their last larval skin and discarding it, as do most insects, these flies (the Cyclorrhapha) retain it as a sort of armor- the puparium– and metamorphose inside. It’s a great example of evolution re-purposing an existing structure for a novel function.


Emerging as an adult inside an unbroken suit of armor has its own difficulties, however. What makes it hard for predators to get in also makes it hard for flies to get out. So they pump their heads full of hemolymph, inflating a balloon-like structure called the ptilinum, and burst their way to freedom using hydrostatic pressure.

breaking free!

Once the fly has emerged the ptilinum deflates back into the head, leaving a characteristic upside-down U-shaped suture.

photo details:
Canon MP-E 65mm 1-5x macro lens on a Canon EOS 7D camera
f/13, 1/250 sec, ISO 100
(top) background flash; (mid, bottom) diffused foreground flash

12 thoughts on “An inflatable head”

  1. Very cool pics & cool insect order!

    @joshua: Yes, how could I win my fights against e.g. Galactus or Super Villian X without my awesome superpower, the inflatable head…

  2. James.C. Trager

    Josh — Ain’t nobody detaching mine!

    (Or stated more in the laguage of this readership, detachment is overrated.)

  3. I saw these photos in your sideroll the other day and was hoping you’d make a post on them! The top photo has got to be one of the best shots I’ve seen showing the inflated ptilinum and associated chaetotaxy! As always, it’s nice to see some flies invading these myrmecophilous blogs!

  4. Great post! It’s neat how everyday things can amaze you when you look closely.

    @Joshua: What’s the advantage of a detachable penis?

  5. Ecky thumb, that was a revelation! All these flies on which I’ve tried to see the U-shape, and never knew from where it came (OK, so flies are not my specialty. Thery’re wicked, though.)

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