1. The problem states that the scale drawing is 12 : 1, meaning the drawing is 12 times larger than the real insect. 2. Given the length of the insect in the drawing is 8.4 cm, we need to find the real length. 3. Let the real length be $x$ cm. According to the scale, $12 \times x = 8.4$. 4. Solve for $x$: $$x = \frac{8.4}{12} = 0.7 \text{ cm}.$$ 5. Convert the real length from cm to mm: $$0.7 \text{ cm} = 0.7 \times 10 = 7 \text{ mm}.$$ 6. Therefore, the real length of the insect is 7 mm.