Which of the following characteristics is incorrect with respect to cockroach ?

The correct option is: A ring of gastric caeca is present at the junction of midgut and hind gut.

Let's analyze each of the characteristics of a cockroach to understand why this statement is incorrect:
1. Gastric Caeca Position: In cockroaches, a ring of 6 to 8 blind tubules called hepatic or gastric caeca is present at the junction of the foregut and midgut, not at the junction of the midgut and hindgut. At the junction of the midgut and hindgut, another ring of about 100 to 150 yellow-colored thin filamentous Malpighian tubules is present, which helps in excretion. Therefore, statement 1 is incorrect.
2. Hypopharynx: The hypopharynx acts as a tongue and lies within the cavity enclosed by the mouthparts. This is a correct characteristic.
3. Female Genital Pouch: In female cockroaches, the 7^th sternum is boat-shaped and together with the 8^th and 9^th sterna forms a genital pouch (brood or genital chamber). This is a correct characteristic.
4. Anal Cerci: In both males and females, the 10^th abdominal segment bears a pair of jointed filamentous structures called anal cerci. This is a correct characteristic.

Hence, the incorrect characteristic is that the gastric caeca are present at the junction of the midgut and hindgut.