"The most wonderful plants in the world"

In the words of Darwin, insectivorous plants are "the most wonderful plants in the world", as he described them on his 1875 book. 

Pitcher Plant at Bishop's Bog, Portage MI

In my Ecology class at Kellogg Biological Station, I saw my first wild carnivorous plants. I started wondering why would a plant that could photosynthesize invest so much energy in developing  carnivory? Would they still be considered primary producers?

Illustration of Sun Dew from Darwin's  "Insectivorous Plants"

They are photosynthetic organisms that have motile organs or other traps like pits or chemical attractants for insects, which they later digest with enzymes they secrete. How could this evolve? And why?

One of the reasons is that they often live at very low nutrient or even anoxic environments, so they make up for the lack of nutrients by trapping and breaking down insects. 

Sun Dew at Bishop's Bog, Portage MI

Because of this intriguing adaptations, Ellison and Adamec have described them as great model organisms to study the evolution of trait-based cost benefit models.  

References, cool readings, links, etc.

Ellison, A.M. & Adamec, L. (2011). Ecophysiological traits of terrestrial and aquatic carnivorous plants: are the costs and benefits the same?. Oikos, 120(11).

A link to Darwin's book online!

http://darwin-online.org.uk/EditorialIntroductions/Freeman_InsectivorousPlants.html