Step 1: Understand the question.
In a fruit fly, the sex index is $ 0.66 $. We must find the sexual phenotype that goes with this value.
Step 2: Recall the balance theory.
In Drosophila, sex is decided by the balance between X chromosomes and sets of autosomes, not by the Y. The sex index is found as $ \dfrac{\text{number of X chromosomes}}{\text{number of autosome sets}} $.
Step 3: Note the key values.
A sex index of $ 1.0 $ gives a normal female. A sex index of $ 0.5 $ gives a normal male.
Step 4: Look at the in-between zone.
When the index falls between $ 0.5 $ and $ 1.0 $, the fly is neither fully male nor fully female. This middle form is called an intersex.
Step 5: Place the given value.
Here the index is $ 0.66 $. Since $ 0.5 < 0.66 < 1.0 $, it lies in the intersex range.
Step 6: State the answer.
So a sex index of $ 0.66 $ gives an intersex fly.
\[ \boxed{\text{Intersex}} \]