Are animals that harm their hosts and help themselves arachnids?

No, the term "arachnid" doesn't describe animals based on their behavior towards their hosts.

Arachnids are a specific class of arthropods that includes:

* Spiders

* Scorpions

* Ticks

* Mites

* Harvestmen

While some arachnids like ticks and mites are parasitic and harm their hosts, many other arachnids are not. For example, spiders and scorpions are predators, while harvestmen are scavengers.

Therefore, it's incorrect to define animals as arachnids based solely on their parasitic nature.

To accurately classify an animal as an arachnid, it must have the following characteristics:

* Eight legs

* Two body segments (cephalothorax and abdomen)

* No antennae

* Simple eyes (ocelli)

* Specialized mouthparts called chelicerae

So, while some arachnids are parasitic, not all parasitic animals are arachnids. There are many other types of animals that are parasitic, including insects, nematodes, and flatworms.