To compare the sizes of $-\frac{1}{2}$ and $-\frac{1}{3}$, we start by comparing their positive counterparts:

$$\frac{1}{2} \gt \frac{1}{3}.$$

Since $\frac{1}{2}$ is greater than $\frac{1}{3}$, when we take the negative of both, the inequality sign flips. This is because on the number line, a more negative number is considered "less than" a less negative number. Therefore,

$$-\frac{1}{2} \lt -\frac{1}{3}.$$

Thus, the correct comparison operator to fill in the blank is $\boxed{\lt}$.