Final answer:
Multicellular organisms that live on the outside of their hosts, such as fleas, ticks, lice, and mosquitoes, are called ectoparasites. They consume resources from their hosts and often act as vectors for transmitting diseases, although they may not cause significant direct harm themselves.
Step-by-step explanation:
Multicellular organisms such as fleas, ticks, lice, and mosquitoes are referred to as ectoparasites because they live outside their host. Ectoparasites, such as ticks (Parasitiformes: Acari), fleas (Siphonaptera), lice (Phthiraptera), and many louse flies (superfamily Hippoboscoidea: Diptera), inhabit the host surface for at least part of their life, consuming host-derived resources like blood, secretions, keratin, and skin cells. These ectoparasites are vectors for harmful pathogens but are often not the cause of substantial fitness costs or health issues for their hosts on their own. Instead, their role as vectors in transmitting invasive pathogens like the parasites causing malaria or bacteria causing Lyme disease is crucial in public health and epidemiology.