I'm in Central Texas in Limestone County and photographed some ants that were behaving like leafcutter ants. But I have never seen ants with such huge spikes on their backs! Will you please tell me exactly what kind of ant these are?
You're right! The ants in your photographs are indeed leafcutter ants. This particular species is Atta texana. All species of leafcutter (or fungus-farming) ants display a spiny exterior, as you point out, and in fact the number of these thoracic spines tells you the difference between the two "true" genera of leafcutters, Atta and Acromyrmex. Please see this previous post to learn more about the geographic distribution of leafcutter ants, and the second point of this post for an explanation of their spiny morphology.
Thanks for your interest!
Alexandra Westrich & the AntAsk Team