Flies in Winter

This winter has been unusually mild, with snowfall not accumulating until late December and temperatures abnormally warm except for a few freezes in my area. There was one single huge snowstorm just a week ago that buried the northeast and the south, but that bypassed my area and was over quickly, even in places like NYC and Washington, DC that were hardest hit. And these past few days, we have been in the midst of a heat wave, with daytime temperatures reaching the 60s. Not so bad for mid-winter.

Because it was so sunny and warm yesterday, I decided to take some of the houseplants out in the backyard for some fresh air n’ photosynthesis. There, I made my first interesting natural history discovery of 2016.


While moving my pelargoniums from the lawn onto the back porch, I noticed a pale insect huddling in the litter in the pot. This was a newly emerged fly, seen in the minutes after it emerged from its puparium, with its wings recently expanded but its cuticle still soft and unpigmented. How it ended up in the pot isn’t clear; I’m thinking it emerged from the lawn and climbed up and into the pot, rather than remaining dormant in the pot in my house throughout the winter. And these insects are relatively difficult to find in this state: many flies emerge, expand their wings, harden, and are ready for liftoff in as little as an hour.

You’ll notice this fly has a weird snout on its face. That snout is the remains of a structure called a ptilinum, a soft, bladderlike structure between the eyes which the fly repeatedly inflates with blood during its emergence. The puparium, the cramped outer shell that surrounds a fly pupa during its development, can be extremely tough to break from the inside, especially for an insect that has no biting mouthparts. But by pumping blood into its head and expanding the ptilinum, the fly uses a hydraulically powered battering ram to literally push the top off of the puparium and escape its time capsule. In some parasitic flies, the ptilinum is even used to break out of the earthen nests made by their bee hosts:

Given its importance in a brief but crucial time in the fly life cycle, the ptilinum is a pretty sophisticated structure, with associated musculature and sensory hairs. But once the fly has escaped, the ptilinum, now useless, begins to retract into the head. Over the next few days, the cuticle of the ptilinum dissolves, its muscles disappear, and all that is left externally is a thin line (the ptilinal suture) that marks the site where the ptilinum has shrunken.

From this angle you can see the shrinking ptilinum from the side. The rounded tip of the abdomen indicates this fly is male. I wondered what the orange spot was–I’m guessing it’s a testis, since some male caterpillars can be sexed based on the presence of testes showing through the thin cuticle as faint yellow spots.

Over the next thirty minutes, the fly colored up considerably. The stripes on the thorax indicate this fly is a flesh fly, in the large genus Sarcophaga. From these photos alone it is virtually impossible to pin the ID down to species.


That afternoon I saw two other flies buzzing around outside; one came to settle on a plant dish for some water. Indeed, flesh flies, along with blow flies like Pollenia and Calliphora, are some of the earliest ‘large’ insects to appear outdoors, and many flies that overwinter as adults will come out briefly during warm sunny days and retreat when the cold returns. But I don’t know if it’s normal for flies like this one to break out of diapause in the middle of winter, or whether these callow individuals survive until spring without a chance to feed. But who knows whether this behavior is normal at all, what with the eerily uneventful winter we have this year?