My cloud storage is littered with the bones of a thousand unfinished projects. Occasionally some bones get articulated into the skeletons of a papers, which I aim to make available as preprints. Sometimes I finish these manuscripts and submit for peer review, but often—probably too often—they remain a jumble of ideas, questions, and preliminary data. Here’s an example I ran across today – a question that came to us on one of my ENT 432 field trips: What is the function of caddisfly warts?
My colleague and I got to talking at the light sheet one night, while looking at limnephilids. Caddisfly adults have many setiferous, wart-like protuberances on their cuticle, and the pattern of these “warts” is diagnostic. But what function do they serve? He crudely cut one open under the dissecting ‘scope, and the inside looked almost … glandular. We also noted that these caddisflies were chemically defended—to put it bluntly, they stank. I tossed a few in a funnelweb spider‘s web. The spider would rapidly approach the caddisfly but then just as rapidly retreat without biting it. The caddisfly always got away. These observations triggered a sudden obsession, which lasted for several days.
Hypothesis 1: Warts produce a chemical defense against predators.
We rejected this first hypothesis pretty quickly, when a literature review revealed that the source of the stink was a pair of abdominal glands associated with the fifth sternite (homologous with Lepidoptera), and their products are well known. We also found a paper by Oláh and Johanson (2007), experts of Trichoptera, that describes these warts as sensory, rather than glandular. Ok, we were way off, but why do caddisflies need such dense clusters of sensilla? Research by Ivanov (1990) suggested that warts are important for flight function. He put glue on the warts, which induced a highly erratic flight.
Hypothesis 2: Warts are sensory organs that are critical for flight, perhaps by providing feedback regarding air flow and body orientation.
I don’t think of caddisflies as adept fliers. It seems strange that they’d need a sophisticated system of sensory structures to fly effectively. We can’t reject this hypothesis yet, given Ivanov’s experiments, but discussions in my lab group yielded another hypothesis …
Hypothesis 3: Warts are sensory organs that detect bat predators.
Adults seem to be mostly nocturnal, and many moths (close relatives of caddisflies!) have sonar-detecting organs and jamming systems. Maybe Ivanov’s caddisflies were tricked by the glue into flying evasive maneuvers? Also, some day-flying caddisflies (Conoesucidae) lack warts. My colleague here at Penn State, Tom Baker, suggested that we try a crude experiment: jingle your keys in front of these insects and see if they react. He recalled a past publication (Baker and Cardé 1978), in which he described how Noctuoidea respond to the high frequency sounds emanating from jingling keys. I tried it on limnephilids and geometrids at my porch light. First I waved my hand and snapped my fingers in front of these insects. No reaction. Then I jingled my keys (watch around the 0:50-0:59 mark):
I repeated this experiment several times on successful mornings after leaving the porch light on overnight. I definitely need to work on my cinematography skillz.
The geometrid flaps its wings (greatest party trick ever?), while the caddisfly just sits there. Hmmm … Literature searches also revealed that warts evolved something like 100 million years before bats (Gao et al. 2013) and that bats are known to prey on caddisflies (Clare et al. 2011). Hmmmmm … Getting close to rejecting this hypothesis, but it would be great to do some proper experiments.
That’s about as far as we got before a big grant deadline and other distractions interfered. We have some thoughtful correspondence from real trichopterologists and a messy 9-page Google doc where we parked more preliminary results, figures, commentary, and our bibliography. Ultimately, though, we lost momentum and moved on to other projects.
What’s to be done with these bones? It’s a compelling question, I think – one that begs resolution. We spent time and resources exploring the question, but at this point it’s a lower priority than other research. What’s the right outlet for stalled projects or half-baked ideas with preliminary results and partial bibliographies? Maybe sharing these thoughts will inspire others to take the baton or to serve us with some quick resolution.