"Die Wilde Karde ist eine zweijährige Halbrosettenpflanze. Man nennt sie Zisternenpflanze, weil die gegenständigen, unten verwachsenen Blätter ein Wassersammelbecken (Phytotelm) bilden. Deren Funktion wird als Aufkriechschutz gegen Ameisen interpretiert. Möglicherweise stellt Insektenfang und Ansiedlung von Kleinlebewesen eine zusätzliche Stickstoffversorgung dar." (http://de.wikipedia.org/wiki/Wilde_Karde)
"... cup-like formation made where sessile leaves merge at the stem. Rain water can collect in this receptacle; this may perform the function of preventing sap-sucking insects such as aphids from climbing the stem. A recent experiment has shown that adding dead insects to these cups increases the seedset of teasels (but not their height), implying partial carnivory." (http://en.wikipedia.org/wiki/Dipsacus...)
The teasel, Dipsacus fullonum is known to catch invertebrates in its water filled leaf bases, but experimental testing of reproductive benefits of this have been lacking. We report the effects of insect supplementation/removal and water removal during spring/summer on Dipsacus in two field populations. There were no significant treatment effects on biomass, but addition of dead dipteran larvae to leaf bases caused a 30% increase in seed set and the seed mass:biomass ratio. This study provides the first empirical evidence for reproductive benefit from carnivory in Dipsacus fullonum."
***** My thoughts: Even from looking at just those few individuals of Dipsacus fullonum, it seems highly unlikely to me that there isn't any kind of mechanical and/or chemical trap mechanism involved in catching the invertebrates (wirbellose Tiere). Otherwise I don't see why an insect as large as the green shield bug (Palomena prasina, Grüne Stinkwanze), which I found in several mini pools at the leave bases, shouldn't get out of such a tiny puddle of water with ease. As the base of the leaves didn't feel exceptionally slippery or waxy to me (which of course is too crude a way to determine it with certainty), I suppose there is some kind of chemical mechanism involved. The plant contains saponins, which - like soap - can reduce the surface tension of water (Oberflächenspannung des Wassers herabsetzen): Voila, this might do (part of) the trick! They also produce chemical substances that are known to be harmful to invertebrates, e.g. terpenes and caffeic acid; perhaps Dipsacus fullonum releases chemicals like that into the mini pools and poisons the critters that way.