Caddis Pupa Husk

When mature, caddis larvae seal themselves in a pupal chamber and metamorphose into the pupa. The mature pupa is rightly called the “Pharate Adult.” It is the adult inside the pupal husk. There has been much discussion of the appearance of the Pharate Adult as it rises to the surface to hatch out. Some say it is a shining ball of movement that rockets to the surface and emerges in a heartbeat. And that is what I thought for years–it’s what many of the books say. But after much onstream observations, reading much scientific literature, and going back and reading the accounts in angling texts, I am convinced otherwise.

The insects seen with a shining bubble appearance that pop to the surface and “hatch” in an instant are caddis females returning from egg laying. They dive or crawl to the bottom and lay eggs directly on the bottom (see Caddisflies in the archives). They carry an air bubble with them so they don’t drown, and when done, they swim to the top (assisted by the buoyancy of the air bubble; the air bubble bursts, and the adult flies off.

The Pharate Adult is dressed in a loose cuticle (the pupal skin) which is separated from the adult inside by molting fluid. This gives the Pharate Adult a bit of a glistening appearance; the cuticle hangs on the adult like a filmy negligee. My friend Will Koukkari and his colleague Tracy Anderson have provided some excellent photos of the Pharate Adult showing the filmy cuticle and its pale, almost pearlescent glow. He has graciously allowed me to post these for all to see. Thanks Will.

Abdomen of Glossoma intermedium abdomen showing cuticle

Abdomen of Glossoma intermedium showing cuticle, copyright WLK

Head of Glossoma intermedium showing cuticle--note specifically legs and eyes

Head of Glossoma intermedium showing cuticle--note specifically legs and eyes, copyright WLK


  1. Paul Roberts says:

    Hi Gary,

    Gary Lafontaine described caddis pupae as generating gasses within the cuticle to buoy them to the surface, and shining like quicksilver in the process. Do you think that what Gary LaFontaine observed as caddis pupae glistening silver were actually diving egg layers?


  2. Gary Borger says:

    Hi Paul, No one that I know of in the scientific literature has seen caddis pupa pumped full of gasses. They have seen and clearly showed that there is molting fluid between the pupal husk and the enclosed adult and that it makes the pupa glisten. The diving adults certainly have a encasing air bubble, which may in fact be the reason that some anglers believe that the caddis emergence is instantaneous. When an egg laying female is finished ovipositing, she lets go of the bottom and rides swiftly to the surface in an air bubble. At the surface the air bubble bursts and the female flies away–looking for all the world like a caddis that has emerged in an instant. I’ve watched the actual molting any number of times, and it takes them several seconds to get out of the husk and get the wings expanded and ready to fly.