I spent a great deal of my childhood hiking through the rainforest on the side of Grouse Mountain. In particular, my friends and I explored the areas around Mackay Creek, a small stream that burbled over a rocky bed. The creek was filled with small trout, crayfish, and caddis fly larvae (apparently a sure sign of pollution-free water: we often drank the cold, clear water in cupped hands with no noticeable deleterious effects). About a decade ago I visited my old stomping grounds and I was so distressed with the loss of habitat to the burgeoning suburbia that I haven’t been back since; fortunately, my memories are, for the most part, intact.
Remarkably, the caddis fly larvae left one of the greatest impressions on my memory. I didn’t know what the creatures were at the time (I had no idea they were a larval form of a flying creature: to me they were aquatic bugs), but they fascinated me, and it was the caddis fly larvae, I think, that helped spark my life-long interest in biology (and, now that I think about it, the time I spent peering into the crystal water — long after my friends had gone home — was possibly my initiation into meditation).
There are over a thousand species of caddis fly in North America, and the Pacific Northwest is home to nearly two-hundred, but there is a particular species that attracted my attention.
Caddis fly adults are nocturnal and look like small moths with long antennae and silken hair on their wings; but they are not moths, they are in the Order Trichoptera (from the Greek for hair and wings). The adults are a favored snack of trout and are commonly used as models for fly-fishers. The caddis fly’s adult life is brief, just long enough for reproduction. Some adult female species deposit their eggs on the surface of the water and others dive underwater to lay their eggs.
As I mentioned, it was the larvae stage that fascinated me as a boy. A caddis fly larva is a long, segmented creature: its head and six legs are packed into its anterior end and two grasping hooks are situated at the posterior end.
Some larva species live like underwater spiders, spinning webs as a home and as a means to gather food, but most species build homes around their bodies. The habitats are constructed from various materials; some use slivers of wood, tree needles, or grains of sand, but there is a small group of species that uses miniature stones to create a cylindrical structure (the larvae secret a cementing chemical and laboriously build a house of pebbles around themselves), and it is this variety — two or three centimeters long (about an inch) and about a half-centimeter in diameter — that caught my attention while wading in and around Mackay Creek.
Using the grasping hooks at their anterior end, they held fast to rocks in the stream and maneuvered about using their six legs. I studied the creatures and, though they were all obviously quite similar in structure, each home was composed of tiny pebbles that were unique in color, size and shape. I tried to imagine each tiny creature choosing pebble fragments as they flowed downstream within reach: the creature would presumably grasp a stone, turn and spin it this way and that, and then mysteriously fasten the stone to the outside of its body (perhaps they forage for construction material and assemble their homes in an area of the creek that has no current, but I preferred the image of creatures plucking boulder-sized (to them) rocks from the stream’s current). I suppose the construction is instinctual, like a spider’s web, but it baffled my young mind.
At a glance they appeared to be sessile creatures, but I watched long enough to observe them as they crawled across small boulders in the middle of the current. I marveled at their tenacity: their petite black heads could be observed bobbing out the top of their homes, and their dark legs moved slowly and carefully across the rock surface: it appeared as though the legs helped anchor the larva for motility in the current. The cylindrical structures waggled in the current, but I never saw one lose its grip (I tested their resolve by plucking some of the creatures off of their boulder: they held on tenaciously, but I was able to tear them off. I tried to place them back on the boulder, but they had retracted their legs, probably fearing the worst. I have no idea whether I had injured them; I hope not).
Apparently, the larvae writhe inside their home, which enables oxygenated water to flow through fissures in the pebble-structure and along the gills that are located on the creature’s abdomens.
Before pupating in late summer, the larva attaches itself to a rock and seals its casing; within two weeks an adult emerges from the pupal enclosure, makes its way to the water-surface and searches for a mate in order to continue its genetic heritage by contributing to the succeeding generation.
I’m almost curious enough to go back to Mackay Creek and check for larvae, but I think my memory will suffice.