it is quite certain that the plant is well adapted for catching and drowning insects.
The connate leaves form cups holding from 12 to 100 c.c. of fluid ; the leaves are smooth (although those of the seedlings are rough, with large prickly hairs) and are inclined so as to form a large angle with the horizon and a small one with the vertical; they form, therefore, two steep and slippery slides, leading to a pool of water. The stalk of the plant is covered with sharp prickles, but these cease where the stalk dips into the water in the cup. If it were not for the loss of the prickles at this point, a ladder of escape would be provided for the drowning victims. I have seen a beetle struggling to get out, and observed his tarsi slipping over and over again on the smooth stalk. The cups undoubtedly form a most efficient trap. In
some wild teasels the following insects were found:—In one cup six large malacoderm beetles, from half to three quarters of an inch in length, one fair-sized caterpillar, and two flies; in another, seven of the same beetles, one earwig, a bluebottle fly, besides many smaller flies and much debris.
Francis Darwin cited his great-grandfather Erasmus, who, the previous century, hypothesized that the teasel cups actually protected the nectar from insects, being unaware of the relationships between flowers and insects. Frances, who thought the teasel was a carnivorous plant, went on to investigate how teasels could absorb the nutrients afforded by the insects and made numerous, if somewhat puzzling, experiments on some glandular cells and filaments produced by the leaves. He also explained how beetles died faster in teasel water than in pure water, as if the teasel produced some chemical to "narcotize" them. Francis Darwin believed that the teasel benefited from these catched insects, and even designed the experiment to prove it:
I hope to decide by a comparative experiment, in which a number of teasels raised from seed under similar conditions will be divided into two lots, one half being starved and the other fed with insects or pieces of meat.
ReferencesShaw PJ, & Shackleton K (2011). Carnivory in the teasel Dipsacus fullonum--the effect of experimental feeding on growth and seed set. PloS one, 6 (3) PMID: 21445274
Darwin, F. (1877). On the Protrusion of Protoplasmic Filaments from the Glandular Hairs of the Common Teasel (Dipsacus sylvestris). Proceedings of the Royal Society of London, 26 (179-184), 4-8 DOI: 10.1098/rspl.1877.0003