Glossosomatidae (pronounced: glos-so-so-ma-ti-dee) is a family in the aquatic insect order of Trichoptera (caddisflies). You will usually find the larvae living inside their tiny pebble huts where they scrape plankton and algae off of the rock. Glossosoma nigrior is common in northern Wisconsin’s trout streams. The small, size #18 pupae and adults emerge about the time as Ephemerella subvaria (Hendrickson mayfly hatch). The trout do feed on both the emerging pupae and egg laying female caddis.

As the larvaegrow, they must abandon their case and build a new one. It has been recorded in Minnesota (Waters 1962) hundreds of caseless glossosomatid larvae in the drift. That makes them extremely easy prey for trout and other fish in the streams.

Below are some characteristics of the Glossosomatidae caddisfly family.

• Saddle case makers (sort of like a tortoise shell case made from tiny pebbles)
• The larvae live inside the stone case and are pinkish or creamy colored, and chubby
• Larva lives underneath their pebble case scraping algae and plankton off the rock
• When it passes from one instar to the next the larva must abandon its case
• Often at dusk and early dawn many G. larvae drift downstream (in large numbers) to find a suitable new place on another rock to erect their new pebble shell house (¹)
• Pupae are good swimmers
• Most female G. species dive under to lay their eggs
• Glossosoma nigrior
  • common in Midwest, larva about 9.5mm
• • the pupae swims to surface to emerge
• • emergence around the same time as Ephemerella subvaria
  • adults have dark brownish splotchy wings and gray to brownish bodies
• • adults 7-10 mm long
• • females lay their eggs on the bottom of the stream
• • Glossosoma nigrior adults were found to be abundant in Hilsenhoff’s Pine-Popple study (late April to mid-September)
  • DuBois also found evidence of the genus Glossosoma in his Brule River study

Some additional information about Glossosomatidae from Kurt Schmude, Senior Scientist/Professor at University of Wisconsin – Superior

• The genus Glossosoma is represented by two species in WI: G. nigrior and G. lividum. I am not sure of the total range of G. lividum in WI, but it likely occur in our area.
• The family also has two more genera, with a total of five more species: Protoptila erotica, P. maculata, and P. tenebrosa; Agapetus hessi and A. tomus. These species (and genera) are usually smaller than Glossosoma, especially Protoptila, which is tiny. Agapetus is rare in northern WI and it is found in slightly warmer streams than Glossosoma. Protoptila occurs throughout the state in even warmer and larger streams.
• The larvae of Glossosoma will readily abandon their cases, as you mentioned, so they do not invest a lot of material (silk) and energy into making these cases. Consequently, these cases fall apart readily when you collect them, leaving you with a bunch of small pebbles. However, they will build a sturdier case when it is time to pupate. Compare these larval cases to those created by the larvae of Helicopsychidae (Helicopsyche – the snail-case maker), Thremmatidae (Neophylax – little caddisflies), and Odontoceridae (Psilotreta – the mortarjoint caddisflies, or strong case makers). The cases of these caddisflies are very difficult to break or crush. In fact, it is incredible how strong Helicopsyche and Psilotreta build their cases. Consequently, these larvae rarely leave their cases – they have invested too much material and energy into building them.

(1) pg 90 from Caddisflies A Guide To Eastern Species For Anglers and Naturalist by Thomas Ames Jr.

Note: The above is based from my years of observations while on northern Wisconsin trout streams along with several aquatic insect related books.