The phylum Arthropoda, which includes insects, spiders, and crustaceans, accounts for over 80 percent of all known animal species. It is now understood that crustaceans and insects are indeed related, sharing a common evolutionary history.
Shared Arthropod Heritage
All arthropods share fundamental characteristics: segmented bodies, jointed appendages, and a hard external skeleton known as an exoskeleton. This exoskeleton, primarily composed of chitin, provides support, protection, and muscle attachment for movement. To grow, arthropods must periodically shed their exoskeletons in a process called molting, leaving them temporarily vulnerable.
Arthropod bodies are divided into segments, often grouped into specialized regions called tagmata, such as a head, thorax, and abdomen. This segmentation allows for specialized functions in different body parts. Their versatile jointed appendages, from which the name “arthropod” (meaning “jointed feet”) is derived, are adapted for a wide range of activities, including eating, sensing the environment, and locomotion.
The Pancrustacea Connection
Modern scientific understanding reveals that insects are a specialized lineage that evolved from within the crustacean group. This relationship is formalized by the clade Pancrustacea, which includes all crustaceans and hexapods (insects and their close relatives). The concept of Pancrustacea has gained strong support from various lines of evidence.
Molecular studies consistently show that hexapods are nested within the crustacean family tree. This genetic evidence indicates that the traditional classification of Crustacea as a single, distinct group is paraphyletic, meaning it does not include all descendants of a common ancestor. Morphological characteristics also support this connection, including similarities in compound eye structure and the presence of a nauplius larval stage in some crustaceans.
Distinguishing Features
Despite their deep evolutionary connection, insects and crustaceans exhibit clear differences, largely due to their adaptation to different environments. Insects primarily dominate terrestrial habitats, while most crustaceans are aquatic, living in fresh or saltwater. Exceptions like terrestrial woodlice exist, but water remains their primary domain.
A key difference lies in their body segmentation and number of appendages. Insects typically have three distinct segments (head, thorax, abdomen) and three pairs of legs attached to the thorax. In contrast, crustaceans often have two primary regions (a cephalothorax and abdomen), generally more than six legs (often five pairs or more), and their limbs can be biramous (two-branched).
Insects are distinguished by wings, a feature absent in crustaceans. Insects usually have one pair of antennae, while crustaceans often have two pairs. Their respiratory systems also differ significantly; insects use a tracheal system with spiracles, whereas crustaceans typically breathe using gills.