Know your ticks. Ticks are parasitic arthropods, members of the subgroup acari, of the class arachnid. They are not insects. Ticks are more closely related to spiders. There are at least 889 species of ticks in the world. In the US these are usually broken into two categories: dog ticks and deer ticks. Dog ticks are larger, hard ticks (from about 1/8 inch to 1/2 inch when engorged), and are easily identified on the body. Deer ticks are much smaller, and can be hard to spot. They are approximately the size of a sesame seed (until engorged).