TOEFL

The exoskeleton of the arthropod.
There are more arthropods alive on Earth than there are members of any other phylum of animals. Given that not only insects and spiders but also shrimp, crabs, centipedes, and their

numerous relatives are arthropods, this fact should not occasion surprise. For all their diversity, arthropods of any type share two defining characteristics: jointed legs (from which the phylum takes its name) and

exoskeleton (the recognizable hard outer shell). Though the shell itself is made of dead tissue like that of human hair and fingernails, it is dotted with sensory cells. These give the arthropod information about its

surroundings, much as the nerve endings in human skin do. Also like human skin, the shell protects fragile internal organs from potentially hazardous contact with the enviroment. It seals in precious moisture that would

otherwise evaporate but permits the exchange of gases. Its primary component is chitin, a natural polymer that contains calcium and is very similar in structure to the cellulose in wood. Chitin and

proteins are secreted in the epiderms, the living tissue just below the shell, after which they bond to form a thin sheet. Each new sheet is produced so that its chitin fibers are not parallel with those directly above,

which increases their combined strength. The result is the endocuticle, a mesh of molecules that forms the lowest layer of the shell. The endocuticle is not tough enough for daily wear and tear. Over time, however, its

molecules continue to lock together. As the endocuticle is pushed upward by the formation of new sheets by the epidermis, it becomes the middle shell layer called the exocuticle. With its molecules bonded so tightly, the

exocuticle is very durable. There are points on the body where it does not form, since flexibility is needed around joints. This arregement allows supple movement but provides armor-like protection. Though strong, the

chitin provides armor-like protection. Though strong, the chitin and protein exocuticle itself would provide a poor barrier against moisture loss. Therefore, it must be coated with lipids, which are also secreted by the

epiderms. These lipids, mostly fatty acids and waxes, form the third, outermost layer of the shell. They spread over the cuticles to form a waterproof seal even in dry weather. This lipid layer gives many arthropods

their distinctive luster. Combined, the endocuticle, exocuticle, and lipid coating form a shell that provides formidable protection. The external shell has other advantages. One is that, because it has far more surface

area than the internal skeleton found in vertebrates, it provides more points at which muscles can be attached. This increased number of muscles permits many arthopods to be stronger and more agile for their body size

than birds or mammals. The coloration and markings of the exoskeleton can be beneficial as well. Many species of scorpion, for instance, have cuticles that contain hylaine. The hyaline is excited by ultraviolet radiation

, so these scorpions glow blue-green when a black light is flashed on them. Scientists are not sure why scorpions have evolved to fluoresce this way, but the reason may be that their glow attracts insects that they can

capture and eat. Adaptive as their shell is, it leaves arthropods with at least one distinct disadvantage: the cuticle cannot expand to accommodate growth. As the animal increases in size, therefore, it must occasionally

molt. The existing cuticle separates from newer, more flexible layers being secreted beneath it, gradually splits open, and can be shaken or slipped off. The new chitin and protein will harden and be provided with a

fresh lipid coating, but this process can take hours or days after molting occurs. The arthropod must first take in extra air or water to swell its body to greater than its normal size. After the shell has hardened in

its expanded form, the arthropod expels the air or water. It then has room for growth. But until it hardens, the new coat is tender and easily penetrated. Accordingly, the arthropod must remain in hiding. Otherwise, it

risks being snapped up by a predator clever enough to take advantage of its lowered defenses.