LBird,
First of all, critiquing the very existence of individuals is not an answer to a criticism of the concept of society. If the individual doesn't exist, society certainly can't exist either (just as the concept of water cannot exist without hydrogen and oxygen atoms).
Second, it is somewhat useful to note that I exist as an individual - there is no other way for me to exist. If I'm not an individual I'm not 'me' and therefore I do not exist... which seems like a bit of a paradox since I happen to be writing this text.
As for the mysterious 'essence' of 'the individual' which you seem to be seeking, I'm sad to say you are once again just asking for an abstraction. There is no Platonic world, where the definite, perfect 'individual' himself/herself exists. The best attempt I could make at a quick definition would be to say that an individual is the one who feels himself/herself separate from the group - an autonomous entity which recognizes its own selfhood and separation from others. And the individualist is the one who values that separation!
If you do not value it, all power to you - live as part of the mob. I prefer, in the spirit of Max Stirner, to be the individual nothing rather than a subservient part of something. The only relations I will yield to are voluntary relations which I consent to fully and which I feel are beneficial to me. Against all other impositions and involuntary relations I will struggle.
CA, as do all 'individualists', will just fall back on 'biological' existence as 'proof' that 'individuals' exist, whereas 'society', not being a biological structure, is thus 'prooved' not to exist.
Just like 'water', not being an 'atomic' concept but a higher level structure, doesn't exist for our reductionist individualist friends.
There are only atoms and individuals, such as hydrogen, oxygen and ComradeAppleton.