Mystery Cocoons

A small update: It’s summer 2021 and I’m still hoping to identify the critters that built these cocoons. Below are a couple of new stacks taken from the same specimens. If you’ve ever seen something like this before, please drop me a line!

This first set is from a monarch butterfly.

Points of interest:

  • Collected in summer 2019 in Frontenac county, Ontario.
  • These butterflies were all roadkills, some with some weathering.
  • The cocoons/capsules are approximately 300-400 microns in length, not counting the membrane tab, with some being slightly smaller.
  • Species I’ve examined that are affected by these cocoons include Monarch, White Admiral, Question Mark and Painted Lady.
  • The cocoons are most richly deposited around the thorax, with some nestled in along wing veins, and others appearing in surprising locations such as being attached to a leg, suggesting they may be made by a detritovore well after the butterfly died.
  • The types and colours of scales used are not always to be found close to the location of the cocoon adorned with them, suggesting the critters that built these may be quite mobile.
  • The colours of both the capsules and the mosaic surface layer vary greatly, the latter influenced by the types and colours of scales selected. I don’t know what causes the varied iridescence of some of the capsule walls. Most have a blue cast, while a few are green or purple. Perhaps the critter produces some kind of paste from butterflies scales it consumes in order to construct them.
  • In summer 2019 I collected a number of road killed butterflies along a highway in the general vicinity of Ardoch, Ontario. Sometime later when I started photographing them under my microscope, I found these brilliantly coloured cocoons. Whatever made them used chewed up bits of the butterfly’s scales, and the variety in their appearance is due to the types and colours of scales selected. Extending from each capsule is what looks to be a dried membrane with a distinctive elongated diamond shaped tip. What I can see of the inside of them looks quite clean. I’ve been captivated by these tiny capsules ever since. They’re just so cool, and I’ve never encountered anything like them.

    I collected my specimens that year in containers where they were exposed to each other for some time, so the critter responsible may have spread among my specimens. I can’t be sure which species natively host them, but I’ve found them on a Question Mark, a Painted Lady, White Admirals and Monarchs, all collected dead along the highway on the same days in the same location.

    In 2020 I gained the ability to start photographing them on this scale. I haven’t found any signs of what eclosed from the cocoons. They may have escaped storage, but I didn’t notice anything. I still have the specimens, and I’d love to know more about them. If you recognize them, please contact me.