Caddisfly Gladiators

I don’t remember why, but I was skimming through the Hydropsychidae caddisfliy Wikipedia page a few days ago. I was surprised to find it describe how the larvae are known to stridulate (like what a cricket does, make sounds by rubbing legs/wings/body parts together). And I know, you’re probably thinking what I’m thinking, “whoa! caddisflies make noise?”

The Wikipedia references an article published in the 1979 Behavior journal by researchers at a university in Finland:

Jansson, A; Vuoristo, T (1979). “Significance of stridulation in larval Hydropsychidae (Trichoptera)”. Behaviour 71: 197–186.

Here’s my synopsis:

Caddisfly larvae live in flowing rivers and streams. Most live in cases they create out of rocks, sticks, or leaves for protection against predators. And they’re all different, one type of caddisfly will only build rock cases and another type will only build with leaves. Now, given the nature of fast moving aquatic environments, these areas where caddisflies reside are easily disturbed. Moving rocks and logs will undoubtedly damage or dislodge them. When an unlucky caddis loses it’s case, it then has two choices: he can either start from scratch and build again or he can barge into a neighbors home.

Let me repeat that: A homeless caddisfly will force it’s way into an inhabitated case.

Imagine, you’re a caddisfly, and thanks to your prime location on the rock, that huge boulder rolled by you, and you’re relieved it didn’t careen into you. Unfortunately, your fellow caddisfly neighbor wasn’t so lucky. He’s now homeless and trying to steal your rock case home! Quite the uninvited guest! How dare he!

Of course, no caddisfly gives up his home without a good fight. The caddisflies fight viciously over the case (I’m not yet sure how to visualize dueling caddisflies).

Excerpt: “…larvae which have left their retreats often try to enter retreats of other larvae nearby, and the situation usually leads to a fight about the ownership of the retreat. Such fights may be fierce enough to cause death of a participant, but usually one or other of the larvae gives up before any serious damage has occurred.”

SO the Finnish researchers decide to explore this fighting behavior by provoking the animals to fight, observing the outcomes, and focusing on any sounds that are produced. They remove innocent caddisflies from their river home, place them in aquarium tanks (under horrible lighting I’m sure), then torture them by forcing them to repeatedly fight. A total of 529 fights are recorded.

Side Note: It continues to get better, they fed the caddisflies individually to keep them equal and happy, lest fighting break out among the prisoners over meager rations (and of course keep food controlled in the experiment). They did this by placing frozen chironomids at the netted openings. At least they’re pampered a bit with a few last meals before they’re forced to fight. Now is this starting to sound like Gladiator?

The Experiment and General Results: After allowing the caddisflies to acclimate to their new lab environment. The scientist begin their experiments. To force a fight, they remove a caddisfly (the intruder) from his home and place him at another occupied home (the defender). Cases have one real entrance, but intruders were sometimes observed breaking through the back door. When the defender moves to face the intruder, the battle begins.. When they meet, they bite and struggle and shake, and even lock mandibles! Sometimes, the defender retreats back inside and begins to stridulate. And then it kind of goes back and forth until someone gives up or dies. Usually, the defenders emerged victorious.

The Finnish scientists experimented with 4 different species of Hydropsychids, testing inter and intra species. All species were equally successfully at defending, while some species were more successful at intruding than others. Defending caddis’s were usually the ones to stridulate (not the intruders). And if they did stridulate, they were more likely to be successful at defending their case. The Finnish scientists also postulate about the differences between species. They guess that certain species were better fighters due to their habitat. Species that favored smaller rapids were more likely to become dislodged, and therefore more exposed to and better adapted to fighting.

On Producing Sound: Basically, the stridulations are made by rubbing ventrolateral (bottom side) grooves of the head against specialized protuberance on the anterodorsal (forward upper diagonal) sides of the fore leg femora (it’s like your forearm).


Grooves on the head:

Spike on arm to rub against ridges:

If I understand correctly, I imagine it works a lot like those frog noise makers:


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If you’ve made it this far, congrats, please enjoy enjoy the only caddisfly dueling video I was able to find, featuring two Phryganeids.

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