Final answer:
Daddy longlegs, or harvestmen, differ from all other arachnids by having a complete fusion of the cephalothorax and abdomen.
Step-by-step explanation:
The class Arachnida includes a variety of joint-legged invertebrates, all of which have two main body segments: the cephalothorax and the abdomen. Arachnids are known for having four pairs of legs and additional appendages like chelicerae and pedipalps.
Among arachnids, daddy longlegs, also known as harvestmen, are distinct in that they have a complete fusion of the cephalothorax and abdomen, unlike other arachnids such as scorpions, ticks, and spiders.