Acari: Definition & Classification

Instructor: Elizabeth Friedl

Elizabeth, a Licensed Massage Therapist, has a Master's in Zoology from North Carolina State, one in GIS from Florida State University, and a Bachelor's in Biology from Eastern Michigan University. She has taught college level Physical Science and Biology.

The Acari are the ticks and mites of the world. In this lesson, you'll learn about this broad and diverse group of tiny animals, as well as how they are classified in the animal kingdom.

Acari are Arachnids

You may already know that while they sometimes look similar, spiders are not insects. Spiders are in the class Arachnida, of which they are the largest order. But Arachnida also consists of other animals such as scorpions and harvestmen. Arachnida also contains the subclass Acari, which are the ticks and mites, and the focus of this lesson.

Before we get into acarids, let's take a step back to see where they fit into the larger scheme of things. We now know that acarids are arachnids. Arachnids are in the subphylum Chelicerata, which also consists of horseshoe crabs and sea spiders. Chelicerates get their name from their chelicerae, which are fang- or claw-like appendages that help the animal grasp and pinch.

Some acarids are so small they can only be seen with a microscope
rust mite

From Chelicerata we move up to the phylum Arthropoda, which is enormous. In fact, it is the largest phylum in the kingdom of Animalia. Insects, lobsters, crabs, spiders, centipedes, and of course, ticks and mites, make up this group.

Heading back down to Acari, we find that they are divided into two superorders: Parasitiformes, which has more than 15,000 described species, and Acariformes, with more than 40,000 species.

Ticks and Mites

With about 55,000 species of acarids, there is an entire field of study devoted to them called Acarology. And while ticks and mites are often confused for one another, they are different. Additionally, there are different kinds of ticks and there are different kinds of mites.

Acarids come in a wide range of sizes, shapes, and colors
peacock mite

Acarids range in size from only a few hundred micrometers to a few centimeters (when fully engorged). They live on land and also in aquatic environments. Some are parasitic, meaning they feed on other animals like us! Others are harmless, like the dust mites that live in your house and eat the dead skin cells you leave behind.

When you look at a spider you'll see two body segments - the cephalothorax, which is a fused head and thorax, and the abdomen. Ticks and mites generally do not have this type of segmentation, though their mouth parts sometimes look like a separate segment. Like other arachnids, acarids have four pairs of legs. Some have eyes while others are completely blind and depend on other sensory structures to guide them.

Parasitic ticks can grow quite large after having a big meal
engorged tick

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