Monday, January 24, 2011

Micro Caddisflies (family: Hydroptilidae)

The Micro Caddisfly (family: Hydroptilidae) -- also known as the "Purse case-maker" -- is the smallest macroinvertebrate that we find in our samples -- or better yet, try to find!  The size of this larva and its case may best be appreciated by looking at the photo below, in which it is placed next to a grain of rice; the rice grain is about 1/4" long.

Let me admit right away that I myself never succeeded in finding one of these on a net.  The only person I knew that could do it was Rose Brown, the Program Manager of StreamWatch: she is exceptionally talented, probably has more experience in sampling than all the rest of the volunteers put together, and she can always find insects on nets long after the rest of us have given up!

The Micro caddis gets its alternate name of "Purse case maker" from the fact that its case -- normally made of fine grains of sand as in the photos above -- is actually made of two dome-like sides that are then sealed together.  However,  the "Purse case-maker" is only a "case-maker" in a qualified way (we noted that this was true, as well, for the Saddle case-maker: see the entry for 1/15/11.)  Unlike true case makers, the Micro caddis does not build its case until its final instar, as it prepares for pupation: until then, it is "free-living".   In fact, Wiggins and others do not see this as a "portable case maker"at all.  Rather it belongs with the "Free living caddis"(Rhyacophilid) and the Glossosomatid (Saddle case-maker) in the primitive group "Spicipalpia."  ("Spicipalpia," and I quote Glenn Wiggins on this, alludes to the "the pointed apex of the terminal segment of the maxillary palps of the adult insects." [Wiggins, Caddisflies: The Underwater Architects, p.107])

Fortunately, Micro caddisflies are normally found in their cases.  But when they are not, they are still easily identified by two distinct features (if we need anything more than their miniature size!).   Like the "Common Netspinner," the pronotum, mesonotum, and metanotum -- the three sections that follow the head -- are all "sclerotized" (i.e. they're shiny and brown, not fleshy).  And the other feature that gives this larva away is the unusually large abdomen which is much thicker than the thorax and head.

This is a caddis that StreamWatch volunteers do not find very often.  A few have been found in Long Island Creek in Fluvanna, and a few have been found in the Moormans River.  But without any question, the Rivanna River itself provides the habitat they seem to prefer, and even beyond that, most have been found at two sites below Palmyra.

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