You wouldn't think there would be much to attract ants to a cactus. They have waxy stems covered with spines.
There is one structure in this photograph, however, that is highly attractive to ants.
Do you see it yet? No, it isn't the clusters of extrafloral nectaries.
It's a bud.
What are the ants doing?
In case you aren't familiar with the relationship between ants and cacti, the ants are visiting to obtain a sugary liquid produced by the plant. It would appear that the softer, quickly growing tissue of a bud is more susceptible to being eaten by plant-feeding caterpillars or beetles. It is likely the ants will defend their free "soda fountain" and chase these sorts of pests away.
Those of you who have seen extrafloral nectaries on cacti, do you agree that those yellow bulges between the spines above the bud are extrafloral nectaries?
Here's an ant visiting an extrafloral nectary on a barrel cactus.
Any ideas why the two separate structures?