This is the first of it's family I've seen, collected from a tiny, fishless stream in the Cascades. The three species of this genus all live in the Northwest and are predators that primarily eat stonefly nymphs Merritt R.W., Cummins, K.W., and Berg, M.B. (2019).
Both anglers and entomologists know these diminutive flies as "Microcaddisflies." The term refers to this family specifically, not to all tiny caddisflies. Many are large enough for imitation on small hooks, but some are impossibly small, as tiny as hook size 36.
Like the Tricorythodes mayflies, Microcaddisflies can be important to trout because of their extreme abundance in certian waters.
Where & when
In 2375 records from GBIF, adults of this family have mostly been collected during June (29%), July (27%), August (18%), May (12%), and September (7%).
In 755 records from GBIF, this family has been collected at elevations ranging from 3 to 10177 ft, with an average (median) of 1713 ft.
Pupae emerge on the surface, where they struggle for a long time to break through.
Egg-Laying behavior
Females dive to the bottom to lay their eggs.
Larva & pupa biology
Diet: Plankton or algae (including filamentous)
Shelter type: Plant matter, rock, sand, or silk
Hydroptilidae larvae don't build cases until their fifth and last instar, although they do speed through the first four instars in the first two weeks of their lives. In the fifth instar, their abdomens grow proportionally huge and they build flat cases from a variety of materials. Leucotrichia pictipes, for example, builds oval silk cases which at first are not recognizable as caddis cases at all to anyone not familiar with the species.
I'm tentatively classifying this one as Dibusa angata because I cannot see any ocelli in my dorsal view of the head (and because it has a single spur on each front tibia). However, sometimes they can be difficult to spot when obscured behind setae in general, so I'm not 100 % sure on this one. If there are ocelli I missed, then based on ranges and colors the genus ought to be Stactobiella.
There's a stonefly nymph in the bottom right corner of this picture, but what's really interesting is those white blotches. They're pretty common in my Wisconsin home river river, stuck flat onto the rocks--lots of rocks have a speckled look as a result. They are microcaddis cases, made by larvae of the caddisfly family Hydroptilidae. These are made by larvae of the subfamily Leucotrichiinae, most likely the genus Leucotrichia. They spin little flat oval cases of silk tight and immobile against the rocks.
I had not fished Bennett Spring for many years and ended up there on Nov 26. I was not very prepared for the stream's entemology, spoke to someone at the flyshop and went with fishing a midge or two and this was very successful. But it was quite obvious that the trout were feeding on the surface or just under and it wasn't a midge. As the day progressed and late afternoon brought sunshine, a hatch of Tiny Caddis(Black) appeared. I had a size 18 but it wasn't dark enough or small enough. A gent who fished the stream regularly gave me a #24 black Elk Hair(or other hair) caddis. The fish took this fly as a dry off the surface. I am hard pressed to ID this fly aside from Protoptila. This, however, is not a marginally temp spring, the water is consistent flow a cold. The description of the Protoptila indicates that this insect is more a warm water insect. The hatch was occurring right at the spring itself. Any ideas?
Again, this was a size 24 fly, and maybe,just maybe, a 26 or 28 might have been the appropriate size for the insect as there were obvious refusals at the surface.