Hematophagy

Fleas are tiny, so they live on furry mammals. They actually suck blood. It would hurt if they were bigger, but since they're so small, it makes a small tickle. This is what causes dogs to itch.

Only female mosquitoes bite, so size doesn't matter. However, there are different types of insects that look like large mosquitoes that don't bite.

Some arthropods are predators, like scorpions and spiders; some are omnivorous and opportunistic like most crabs and lobsters (although some species of crab specialize in catching fish); krill eat plankton (mostly phytoplankton but including some zooplankton); some arthropods specialize in plant and other detritus, many insects are herbivorous and have mouthparts adapted for only eating, scraping, or sucking plants for nutrients; some are parasitic like lice, many ticks and mosquitos suck blood (hematophagy).