What is the most critical physical feature that places ticks in the Class Arachnida rather than the Class Insecta?

Answer

Adult ticks possess eight legs, whereas insects have six

Ticks are classified as arachnids because adult ticks possess eight legs. Insects, by definition, possess only six legs. Other distinguishing arachnid features mentioned include the lack of antennae and the body structure.

What is the most critical physical feature that places ticks in the Class Arachnida rather than the Class Insecta?
classificationarachnidparasitetick