Chirping Caddis
Ever wonder what really goes on down on the bottom of a trout stream? When you look closely it can be rather amazing what one finds. In this case I want to expose some unusual behavior of a very common caddisfly that nearly all fly fishers are familiar with, the net-spinning caddis of the family Hydropsychidae.
Hydropsychidae is a large family with six genera and over 100 species. They’re called net-spinning caddis because the larvae feed by constructing spider-like webs or nets made of silk thread on the sides of rocks. The nets are positioned so water flows through them and thus strain suspended particles - including potential food - from the water column. A good idea, at least appears to be quite successful since net-spinning caddis are often one of the most numerous species of insects in streams.
As we all know however, success comes with its own set of problems. Just like over population can be a serious problem for people, net-spinning caddis larvae sometimes become so numerous that their nets get in the way of each other reducing their ability to collect food. Plus larvae are known to raid a neighbors net for food. Not good for the larvae on the downstream end of the pecking order. To help reduce this over crowding problem they have come up with an interesting solution.
Imagine you are in a city park on a nice summer day. People are sitting around on the grass enjoying the sun and many are also enjoying music with portable boom boxes. We all know that one person’s music is another person’s noise, so wherever these boom boxes play there’s a certain cushion of space around them varying in size by the type and volume of music (and the age, sex and amount of clothes worn by the players, but that’s a different topic). Well, net-spinning caddis larvae have come up with a similar approach, but without boom boxes they have to make their own music. In this case it’s chirps rather than music, but the effect is the same. The noise produced helps define their territory and prevent other larvae from invading their net.
These chirps are made by rubbing part of the front legs against a series of fine grooves on the underside of their head. The resulting chirps are loud enough to be picked up by microphones placed underwater (sorry, I don’t have any recordings to play for you, so for now you’ll just have to take my word for it). As you might guess different species have different sized grooves and different front legs, so they produce different sounding chirps. Just so you know I’m not totally making this up, here’s a quote from the book, Caddisflies - The underwater architects by Glenn B. Wiggins, a respected authority on caddisflies: Larvae of the genus Hydropsyche are known to produce sound by stridulating.... Transverse, sculptured striations on the ventral surface of the head are drawn across a ridge on each femur of the fore legs to generate sound detectable with a microphone.
So, the next time you are fishing in a trout stream standing in a riffle remember that all around your feet are hundreds, if not thousands, of little net-spinning caddis larvae chirping away, telling all their neighbors, this is my space stay away.
