Why do arthropods need a hard exoskeleton?

Answer 1

Firstly, any sizeable organism be it Arthropod or otherwise would need some kind of infrastructure if it's important to that organism to maintain a consistent general body plan or morphology. Not all organisms require this; for example there are amorphous organisms like the amoeba or certain fungi; some slime molds can grow to a significant size and survive quite well with a highly generalized and nonspecific shape. In the case of those with a high degree of motility, some consistency in shape is useful. Even jellyfish with no significant skeleton, have sufficient rigidity for mobility. The important thing is, the degree of infrastructure is consistent with the habitat and behaviors to which it's adapted. In the case of arthropods, an exoskeleton is very convenient and an endoskeleton would be redundant; for vertebrates, an endoskeleton serves this role and an exoskeleton would prove cumbersome. To be sure, some vertebrates evidence similar adaptations for which an arthropod uses an exoskeleton, for example, the plating on an armadillo, the shell on a tortoise, the thick skin on large mammals, etc; yet for their mass, the thickness of an exoskeleton to provide the necessary rigidity for the purpose of the body plan in vertebrates would be impractical. The reverse can be said to be true of some arthropods; since some have muscles which anchor not to the exoskeleton but to internal cartilaginous structures which are arguably somewhat endoskeletal.

One might say, the laws of physics, particularly mass and gravity, dictate the usefulness of an exoskeleton to Arthropods given their size, role, and adaptations; the hardness of it reflecting the requirements placed upon the organism for its functionality and thus, its survival.

Answer 2

First, it provides the gross structural integrity for the organism. Without the exoskeleton an arthropod would be an amophous mass of soft tissue and liquid. Secondly, it provides a barrier between the organism and its environment and performs the same role as skin; a barrier against organisms and pathogens. For most arthropods it also functions as a kind of armor against predators; some exploit this as their main defense strategy like the pill bugs/woodlice (Armadillidiidae). Unfortunately, exoskeletons also present a strategic challenge; since they are inflexible and can't grow, they need to be removed entirely (moulted) in order to allow the organism to increase in size.

Answer 3

Protection (it typically forms a hard carapace), maintaining structural integrity, camo (for some), stopping fluid loss, etc.