In shallow streams and ponds those long, swordlike leaves emerge from the water. When back-lit the conductive tissues are visible, the separations between the superposed long vascular cells show as darker spots.
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They are the leaves of the Yellow Iris (
Iris pseudacorus). The spider has put its long legs into a prolongation of its long body to make itself less conspicuous.
The flowers look very much like the irises found in gardens.