Which characteristic is unique to German cockroaches?

The characteristic that is unique to German cockroaches is the presence of two distinctive dark stripes on their thorax. These stripes run parallel from the head to the back of the thorax and are a key identification feature in distinguishing German cockroaches from other species. This marking is typically not found in the same manner on other common cockroach species.

While dark color, long antennae, and large size may describe physical aspects of cockroaches in general, they are not uniquely associated with German cockroaches. Many cockroach species can display dark coloring, have long antennae, and can vary greatly in size. Therefore, the specific presence of the two stripes on the thorax is the most identifying characteristic of German cockroaches.