The Hox gene Ubx is fully expressed in crustaceans (region shaded in green) including the expression of the legs. The Hox gene Ubx is partially expressed in modern-day insect (region shaded in pink) resulting in formation of the homologous body segment similar with the crustacean but with the absence of legs.
Urry, Lisa A.. Campbell Biology (p. 543). Pearson Education. Kindle Edition.