An insect is considered an animal, belonging to the vast biological kingdom Animalia. This classification places them within the same fundamental group as all other animals on Earth. Understanding why insects are categorized as animals requires delving into the defining characteristics of the animal kingdom.
Defining the Animal Kingdom
The animal kingdom, Animalia, encompasses organisms that share several fundamental biological features. All animals are multicellular, meaning their bodies are composed of many cells. Their cells are eukaryotic, possessing a true nucleus and other membrane-bound organelles. Unlike plants, animals are heterotrophic, obtaining nutrition by consuming other organisms or organic matter.
Animals possess specialized tissues, which are groups of similar cells working together to perform specific functions. These tissues can organize into organs and organ systems. Most animals also exhibit motility at some stage of their life cycle, meaning they can move independently. While some animals become sessile as adults, their larval stages often demonstrate movement.
Insects as Animals: Key Characteristics
Insects exhibit the core characteristics that define the animal kingdom. They are multicellular organisms with eukaryotic cells. Insects are also heterotrophic, consuming various forms of organic matter for energy.
Their bodies demonstrate distinct specialization, with cells organized into tissues that perform specific roles. Insects possess an external skeleton, called an exoskeleton, made of chitin, which provides structural support and protection. This is a key feature of arthropods, the larger group to which insects belong. Insects are motile, with most species capable of movement through walking, jumping, or flying. They are distinguished by a three-part body plan—head, thorax, and abdomen—and typically have six legs attached to the thorax.
Placing Insects in the Animal Family Tree
The animal kingdom is remarkably diverse, containing millions of species. Insects are a significant part of this diversity, belonging to the phylum Arthropoda. Arthropoda is the largest phylum within the Animalia kingdom, encompassing creatures with jointed legs and exoskeletons, such as spiders, crustaceans, and millipedes.
Within the phylum Arthropoda, insects are further classified into the class Insecta. This class is distinguished by their three-part body and six legs. The number of insect species, estimated to be over a million, makes them the most diverse group of animals on the planet. This classification highlights insects as an integral branch of the animal family tree.