Tarantulas, despite their often hairy appearance, are fundamentally different from mammals. The short answer is no; they belong to a separate group of animals known as arachnids. Understanding the strict biological classification of both groups clarifies why these creatures occupy distant branches on the tree of life.
What Makes an Animal a Mammal?
The class Mammalia is defined by a specific set of biological characteristics. Mammals are vertebrates, meaning they possess an internal skeleton with a backbone made of vertebrae. They are also endothermic, commonly called warm-blooded, which allows them to internally regulate a consistent body temperature, independent of the external environment.
Two defining traits are unique to the group: all female mammals possess mammary glands to produce milk for their young, and all mammals have true hair or fur at some point in their development. Furthermore, mammals breathe using lungs and a muscular diaphragm, possess a four-chambered heart, and have a unique brain region called the neocortex.
Tarantulas Are Arachnids
Tarantulas are classified within the family Theraphosidae, placing them in the class Arachnida, alongside scorpions, ticks, and mites. This classification makes them invertebrates, meaning they lack the internal vertebral column. Their body structure consists of two main segments: a fused head and thorax (cephalothorax), and the abdomen (opisthosoma).
A tarantula possesses eight jointed legs, a defining feature of the Arachnida class, plus two additional pairs of appendages near the mouth used for feeling and manipulating food. Instead of an internal skeleton, they rely on a rigid external shell, or exoskeleton, made of chitin for support and protection. Although many tarantulas appear hairy, these are specialized sensory bristles called setae, which are extensions of the exoskeleton used to detect vibrations and air movement.
How Their Internal Systems Differ
Mammals possess a complex endoskeleton, providing internal support and attachment points for muscles. Tarantulas rely on their exoskeleton for muscle attachment and hydraulic pressure to extend their legs, though they have internal structures like the entosternum for support.
Respiration operates on a different principle, as tarantulas do not have lungs and a diaphragm. Instead, they breathe using specialized organs called book lungs, which are stacks of thin tissue arranged like the pages of a book, maximizing the surface area for gas exchange. The circulatory systems are also distinct; mammals have a closed, high-pressure system where blood remains contained within vessels. Tarantulas have an open circulatory system, where a fluid called hemolymph is pumped by a tubular heart and then flows freely throughout the body cavity, bathing the internal organs directly.