"Shell" - what is it, definition of the term
A shell is a program that offers a textual interface through which users communicate with an operating system; it parses command strings, initiates system utilities, controls input and output streams, and typically provides a scripting language that enables automation of routine tasks.
Detailed information
The protective outer covering of arthropods consists of a multilayered cuticle composed primarily of chitin reinforced with proteins and lipids. This structure provides mechanical resistance, prevents desiccation, and serves as a barrier against pathogens. Molting replaces the covering periodically, allowing growth.
Ticks possess a dorsal shield called the scutum, a sclerotized plate that protects the anterior portion of the body. The scutum varies in size among species; in females it covers only part of the dorsum, permitting expansion during blood feeding. Beneath the scutum, a flexible cuticle accommodates engorgement. Sensory setae are embedded in the cuticle, enabling detection of host cues.
True bugs (order Hemiptera) feature a forewing divided into a hardened basal region and a membranous distal region. The hardened section, often termed the corium, reinforces the dorsal surface and shields the abdomen during locomotion. The cuticle is thin yet resilient, allowing wing flexion while maintaining protection.
Lice exhibit a series of sclerotized plates (tergites) that interlock along the dorsal midline. These plates create a rigid framework that resists abrasion from host hair and skin. The intersegmental membranes retain flexibility, permitting movement across the host’s body.
Fleas display a compact, dome‑shaped dorsal plate (pronotum) that covers the thorax and contributes to an overall streamlined exoskeleton. The cuticle is densely covered with microspines that increase friction, aiding attachment to the host’s fur. The ventral side remains softer to accommodate leg articulation.
Key characteristics of the protective outer covering across these groups:
- Composition: chitin‑protein matrix, often reinforced with sclerotized plates.
- Function: mechanical protection, moisture retention, barrier to microbes.
- Adaptation: species‑specific modifications (e.g., scutum in ticks, corium in bugs, tergites in lice, pronotum in fleas).
- Molting: periodic shedding allows growth while preserving protection.