I read through the other thread titled "Lafontaine Sparkle Pupa" by dryfly and decided to add to this one, due to the title being more direct. Maybe it should go in the Insect Behavior section. Anyway, here goes...
A while back I tried to answer the question for myself. From enough experiences under my belt now, the underwater look of an insect matters. One of the most amazingly effective flies I’ve created was a caddis adult diver. I was sampling on a day when lots of female Hydropsychids were splatting onto the water. I picked up a cobble and turned it over while still submerged. A little brilliant silver bubble was clinging to the rock. It let go, buoyed to the surface, and out popped one of the Hydropsychids. Nearly every cobble I turned had one or more quicksilver egg layers beneath it. I was not catching particularly well that day I remember, fishing dry’s and simple soft hackle pupae, considering the number of caddis on the water.
To make a long story short I found a material that mimiced that quicksilver look, and the difference in catch rate was astonishing. Instead of begging, filching a fish out of each cut I was getting everyone's attention! I’d just throw it in beneath an indicator and the trout were on it. It was truly astounding and more deeply solidified my belief in accurate mimicry –Lafontaine’s “search image” or whatever he called it.
But with pupae I could not verify such a look. All the pupae I’d ever seen taken in bottom samples and in trout stomachs never had that. I also observed them in water in my hand, in coolers, or dishes. I never saw a gas bubble. You’d think I’d see it -or someone would. Nada!
I also own a stream tank and for a couple of years kept, hatched out, and photo’d a lot of stream critters. At one point I was determined to capture an emerging Hydropsychid and sat with my lens over Petri dishes with pupae. One afternoon I caught one with several rapid frames. In the dish, the pupal skin was visible as a loose bag, esp at the very tip of the abdomen leaving an off-white bag hanging there. This told me the pupa was close. When it popped under my lens it simply split the shuck and burst out with those side "paddle" legs kicking –an adult Hydropsychid. But I never saw a bubble.
The image quality is poor in part bc they are digitized via a point-n-shoot from 35mm transparencies. And the adult rises above the focal plane.
See the loose bag of pupal skin at the end of the abdomen.
Splits and kicks out. See the swimming “oars”.
Flies off around the house.
Now…in a half inch of water that was far from a natural emergence. If the pupa had enough water to swim in, and COULD swim a distance to the surface, that would be interesting to know. If they do not swim well, then maybe they do need a bubble.
In Ralph Cutter’s "Bugs of the Underworld" trailer, he appears to shed some light on the question of whether pupae can swim well. His camera follows a Hydropsychid pupa swimming up from the bottom –sans bubble. And it doesn't look very buoyant. (0:46sec in the clip: http://www.youtube.com/watch?v=isNLPP91NXA&feature=related). The loose skin is visible. His next shot in the clip is of one shedding the shuck on the surface. No sign of bubbles, esp encasing the insect as Lafontaine's Sparkle Pupa requires.
Here’s the image from Ralph Cutter’s site (http://www.flyline.com/tips_trivia/all_that_glitters/) GONZO posted in the other thread. It shows a bright shine on an emerging pupa, but it is not clear whether is a gas bubble or just a reflection from sun angle. What it does NOT show is a pupa encased in bubbles.