This is where the morphological approach falls down. This is an "analogous trait", i.e. something that's arrived at through two independent evolutionary pathways.

These palps are due to a reproduction process called "stamping". Sounds violent, but that's nature for you. This is an asexual process. The tabs give an advantage to the overall clutch of young, not individuals.

There's a morphologically similar trait in other Plasticae that's the result of "injection moulding". This involves the mating of two (or sometimes more) parents. This method allows for the evolution of more complex features.

Overmoulding is also possible, which produces symbiotic organisms.