Saturday, October 8, 2016

A project for a rainy day: identifying that pupa

On September the 6th I posted an entry on the caddis pupa I'd found at the Rapidan River.  This one.

At the time, I speculated on the species ID suggesting three possible choices: Ceratopsyche/Hydropsyche sparna, Ceratopsyche/Hydropsyche slossonae, or Ceratopsyche/Hydropsyche alhedra, the three larvae that I've found in that stream.  But how to determine which one?

Well, I've been able to do it thanks to Jessica Fong, an Aquatic Biologist with the Toronto Conservation Authority who has helped me before.  She kindly sent me a key to Hydropsyche pupae:  Jane E. Rutherford, "An Illustrated Key to the Pupae of Six Species of Hydropsyche (Trichoptera: Hydropsychidae) Common in Southern Ontario Streams," The Great Lakes Entomologist, 1985.

The pupa was Ceratopsyche slossonae, and this is the larva.

(The pale spot in the middle of the head is a key feature of the species.)  How did I determine the species ID.

p. 128 of Rutherford's study: Pupa large, abdominal length 6.9-9.0 mm... with a dense patch of fine hairs present on dorsum of segment IV but the rest of the dorsal surface relatively hairless; left and right plates of each pair on segment III well-separated by at least 1 hook-plate width... H. slossonae.

The abdominal length of our pupa was a little over 7 mm.  Remember, our pupa looked like this.

But let's zoom in on segments 3, 4, and 5.

The dense setae on segment 4 -- vs. the paucity of setae on segments 3 and 5 -- is very clear, and as you can see, the space between the posterior hook plates on segment 3 is close to 3 times the width of the plates.

That will do it.  I love learning new things.  Thanks Jessica.

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