Lice are arthropods, placing them within the largest and most diverse group in the animal kingdom. The classification of lice, like all living organisms, follows a system called taxonomy, which organizes life based on shared characteristics. Understanding this system is necessary to confirm their place in the Phylum Arthropoda and their more specific designation as insects.
Key Characteristics of Arthropods
Arthropods are characterized by three unifying physical traits. All arthropods possess an external skeleton, known as an exoskeleton, which is a rigid, protective shell primarily made of chitin and protein. This hard outer covering provides support and protection, requiring the animal to periodically shed it through a process called molting to allow for growth.
A second defining feature is a segmented body plan, where the body is divided into distinct regions, such as the head, thorax, and abdomen. Finally, the third defining trait is paired, jointed appendages. These jointed limbs are specialized for functions like walking, feeding, or sensing the environment.
Placing Lice within the Insect Class
Lice exhibit the fundamental characteristics of the Phylum Arthropoda, leading to their placement into the Class Insecta. Their classification is further refined by their placement into the Order Phthiraptera, which includes all species of sucking and chewing lice. The specific human-infesting lice belong to the suborder Anoplura, or sucking lice, which are obligate parasites that feed exclusively on mammals.
Adult lice are small, wingless insects with bodies flattened from top to bottom, spending their entire life cycle on a single warm-blooded host. Their legs are short, stumpy, and equipped with specialized claws adapted for tightly grasping hair shafts, a modification necessary for their parasitic existence.
The Specific Biology of Human Lice
There are three main types of lice that infest humans: head lice (Pediculus humanus capitis), body lice (Pediculus humanus humanus), and pubic lice (Pthirus pubis). These species are highly host-specific, meaning they live, feed, and reproduce only on humans, and they are named for their preferred habitat on the body. All human lice are blood-feeders, using specialized piercing-sucking mouthparts to pierce the skin and ingest blood several times a day.
The life cycle of human lice progresses through three stages: egg (nit), nymph, and adult. Female lice cement their eggs, or nits, to the base of hair shafts or, in the case of body lice, to clothing fibers near the body. Nymphs hatch from the nits and must take a blood meal quickly to survive, molting three times before reaching the adult stage. Adults live for approximately 20 to 30 days, but they require the host’s body temperature and humidity, dying within a day or two if they fall off the host.