If you look at the seedheads of a bed of cattails in the fall, some are in pristine condition while others have places where their seeds have started to burst from the stem. Such a “fluffed out” seedhead can indicate that something (s) is living within the 250,000 or so packed seeds. A bit of investigation into these heads often reveals tiny, tan larvae (as well as larval skins and frass) that have burrowed their way into the depths of the seedhead, where they overwinter. These are the larvae of the Shy Cosmet (Limnaecia phragmitella), a slender tan moth. Come spring they will pupate and emerge as adults.
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