Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Exuviae is a term used in biology to describe the remains of an exoskeleton and related structures that are left after ecdysozoans[verification needed] (including insect, crustacean or arachnid) have moulted. The exuviae of an animal can be important to biologists as it can often be used to identify the species of the animal and even its sex. As it is not always practical to study insects, crustaceans or arachnids directly and because exuviae can be collected fairly easily, exuviae can play an important part in helping to determine some general aspects of a species overall life cycle such as distribution, sex ratio, production and proof of breeding in a habitat. "Exuvia" is a derived singular usage that is becoming more common; "exuvium" is an incorrect singular formation.An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton (endoskeleton) of, for example a human.