"Phylum" - what is it, definition of the term
The term denotes a principal taxonomic rank that assembles related classes sharing a fundamental body plan; it sits between kingdom and class, indicating common evolutionary ancestry and major morphological characteristics, such as the segmented, jointed‑limb organization observed in arthropods including ticks, bugs, lice, and fleas.
Detailed information
Arthropoda represents a principal taxonomic division within the animal kingdom, encompassing organisms characterized by segmented bodies, jointed appendages, and an external chitinous exoskeleton. The group exhibits bilateral symmetry, a ventral nerve cord, and open circulatory system. Development typically proceeds through successive molts, allowing growth despite the rigid cuticle.
Members such as ticks, true bugs, lice, and fleas belong to this division. All share the following structural features:
- Body divided into distinct regions (head, thorax, abdomen) or fused segments.
- Paired, jointed limbs adapted for locomotion, attachment, or feeding.
- Sensory structures including compound eyes, antennae, or specialized mouthparts.
- Respiratory organs varying from tracheae to book lungs, depending on lineage.
Ecologically, these insects and arachnids occupy diverse niches. Ticks function as ectoparasites on vertebrates, transmitting pathogens. True bugs (order Hemiptera) display a range of feeding strategies, from plant sap extraction to predation. Lice (order Phthiraptera) specialize in permanent ectoparasitism of mammals and birds. Fleas (order Siphonaptera) are wingless, laterally compressed parasites capable of rapid jumping.
Reproductive strategies differ across the division. Many undergo internal fertilization, with females laying eggs in protected environments. Some groups, such as certain true bugs, exhibit ovoviviparity, retaining eggs until hatching.
Molecular analyses confirm the monophyly of this division, supported by conserved gene sequences (e.g., Hox clusters) and shared developmental pathways. Fossil records trace its origin to the Cambrian explosion, with early representatives displaying primitive exoskeletal features.
Overall, the division integrates a vast array of morphological adaptations that enable successful colonization of terrestrial, freshwater, and marine habitats, exemplified by the diverse lifestyles of ticks, bugs, lice, and fleas.