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