I don't understand the distinction.
Insects cannot rationalize on human level - they do not have the capacity to do so. What's the difference between "Insects do not have the capacity to evidence that they have a conception of death" and "Insects cannot have a conception of death". They of course cannot evidence a conception, because they cannot rationalize in this way (as far as we know). It seems you've added this *before* factor, which I'm not quite clear on.
I personally felt somewhat unsure of the clarity of my post short after providing it. I apologize let me try to rephrase, even if my rephrasing casts my prior post to a trash bin. I should like to clarify my argument, not so much that particular post. I should be brief since you might very well agree with me (but I cannot promise miracles; sometimes even God could not keep me from being long-winded).
What I'm trying to say is this: It is not the case that insects (animals generally) do not give us evidence that they have concept possession (which is presupposed by having a conception of). It's not like "that child is not giving me evidence that he is hurt (by crying)."
Crying would be a kind of evidence for "is in pain." That is because we consider it as such. It's criterial evidence. Any one child is or is not making this evidence available. Any one child could, in principle, provide it.
However, an insect could not give such evidence for concept possess. It's not that an insect, or some insects, somewhere, is or is not giving out such evidence. It's not that it's concealing its mastery of concept possession from us, as if we were supposed to look harder and harder for it, invest more empirical research into its study. An insect, as it conventionally stands, fails to be the kind of thing which could give us such evidence. It would be like asking for the evidence that a piece of pie is an atheist. It's not that the pie is not now, rather than some time later, giving the confirming or confuting evidence. The pie just isn't the kind of thing that could.
Thus, could not possibly
(modal) and does not
(empirical) is my distinction. It would not be an empirical discovery that insects do not have concept possession and further certain conceptions; it would have to be a decision about the meaning of our terms; i.e., "concepts," "conceptions," "possession of a concept," "to have a conception of," etc (along with the backing reasons for why we should now allow insects to engage in such an activity).