The main mystery is the fact that adults of the same insect species don’t have any gearing—as the juveniles grow up and their skin molts away, they fail to regrow these gear teeth, and the adult legs are synchronized by an alternate mechanism (a series of protrusions extend from both hind legs, and push the other leg into action).

Burrows and Sutton hypothesize that this could be explained by the fragility of the gearing: if one tooth breaks, it limits the effectiveness of the design. This isn’t such a big problem for the juveniles, who repeatedly molt and grow new gears before adulthood, but for the mature Issus, replacing the teeth would be impossible—hence the alternate arrangement.

Well that's exceptionally neat