I have been doing some reading of Classical conditioning which is considered to be an indication of animal's ability to learn...
Before this I considered insects to be machine like - devoid of any sort of 'thinking' just doing things that evolution 'made' them to do to survive...
However I have found that cockroaches and even fruit flies could be conditionally trained...
Which made me wonder What would be the least complex species that respond to classical conditioning?
I have also read this question about single-celled organisms capable of learning and I don't think that this 'cuts it'... this is nothing more than chemical response... however classical conditioning is different and in my opinion certainly does indicate ability to learn...