Plants are far from helpless in their efforts to repel herbivores. They can cover themselves with thorns, make themselves taste bad, or produce leaves that are difficult to digest, just to name a few examples. However, my personal favorite strategy is the purchasing of protection that occurs when plants produce extra-floral nectar to attract ants. Ants are major and effective predators, but are also attracted to sweets. Plants can induce ants to swarm about on their leaves and stems by producing droplets of a sweet liquid.
I notice this phenomenon most frequently on sunflowers, especially plains sunflower (Helianthus petiolaris), an annual plant common on sandy soils around Nebraska. Over the years, I’ve photographed a number of ants that have been drawn to sunflowers by the extra-floral nectar those plants have produced. Below is a selection of those images. The ants aren’t a foolproof strategy, and sunflowers still get eaten by lots of animals – large and small. Regardless, it’s an admirable and fascinating tactic in sunflowers’ fight to survive and reproduce.
The Prairie Ecologist 
Best blog ever, Chris! (But you knew I’d say that.)
Maxiimillian sunflower does have tiny (one or few-celled?) nectaries on the abaxial side of leaf petioles near the base.
They also may tend the common, composite-feeding Membracidae, Entylia carinata and Publilia concava on this and a wide variety of other plants.
Ant IDs in order of appearance (by picture numbers)
1 – Myrmica “AF-evani” -This is a code name for a common, but as yet undescribed grassland species, which has long been called M. americana, but is considered distinct.
2 – Formica subsericea
3 – Winged ones are queens of the slave-making parasitic ant F. pergandei, and the lone worker is the host species F. incerta.
4 – Minor workers of Pheidole pilifera
5 – F. subsericea
6 – M. “AF-evani”
7 – F. dolosa
8 – F. subericea
Hope that helps for image labeling.
Many thanks, James. You’re the best (but you knew I’d say that…)
I wonder whether the weevil on picture #6 also has a means to buy protection or escape notice from the ants. The one ant just seems to walk right over it.
Ants have been an annoyance in my kitchen this week. I’m surprised at how good you make them look. Thanks.