The argument never cites a binary for behaviors (i.e. instinctual/flexible), so why is it necessary that insect brain size < required size for flexible behavior ----> insect behavior = pure instinct?
I felt the more qualified statement in (E) was appropriate. Please advise.
Instinctual and noninstinctual are logical opposites and represent the only possibilities for behavior in this question. A behavior either is instinctual or it is not. The argument states:
capable of flexible behavior -> large number of neurons
The contrapositive is:
insufficient number of neurons -> not capable of flexible behavior
Since we're told that no insect has a brain size large enough to hold a sufficient number of neurons, we can infer that insects are not capable of flexible behavior, and must, therefore, engage exclusively in instinctual behavior.
In choice E, you're mistaking a necessary condition for a sufficient one. Without the word only, E would be correct. Can we infer that if a species engages solely in instinctual behavior, its brain is as small as or smaller than an insect's? No. We don't know the threshold past which brain size and neuron numbers are sufficient for noninstinctual behavior. We only know that insect brain sizes and neuron numbers are insufficient for flexible behavior. Further, would having a large enough brain size and a sufficient number of neurons guarantee that a species engaged in noninstinctual behavior? No. These two are necessary, not sufficient.