The phylum Arthropoda comprises of a vast assemblage of animals, with over 800,000 species classified at present. Found in virtually all habitat types, if numbers of species and diversity is used to measure success, then this group is the most successful of all the terrestrial invaders. Their success in exploiting the riches of terrestrial life is primarily due to the chitin and protein exoskeleton common to all arthropods. It is the specialised histology of the arthropod integument that gives protection from predators, radiation, desiccation and friction, thus concurring great advantages to the phylum. The most negative factor involved in being encased in a non-living material is the problem of growth. The structure of the exoskeleton has allowed arthropods to develop a faster and more efficient means of locomotion than the annelids. The strength of the cuticle has even allowed the power of flight in pterygota. This report will endeavour to explain the enormous impact the development of the exoskeleton had on this phylum. Terrestrial examples of the three living lines of arthropod evolution will be given to elucidate how the different subphyla have manipulated the general theme of an exoskeleton into their own lifestyles.

Before examples of the subphyla and their particular exoskeletal designs are discussed, a description of the arthropod integument is necessary. This will serve to enlighten both the advantages and the constraints that this solid outer covering provides.Diagram 1 A Cross Section of Insect Cuticle.
The arthropods integument consists of four main layers. The outer epicuticle, composed of several layers provides a waterproof coating in most arthropods. Its shape also determines the form of the underlying cuticle and its surface appearance (Chapman 1982). The cuticle is a bilaminar structure formed from an outer exocuticle which overlays an endocuticle. The cuticle is secreted from the epidermis which is attached to a basement membrane. When first secreted the cuticle is soft and pliable. The formation of hydrogen cross bonds between protein chains forms the hard cuticle that is common to the group. This process called tanning or sclerotisation is not universal over the entire area as moveable joints must be present for locomotion. In these joint regions two specialised cuticles are present called resilin and arthrodial membrane (Mc Neill Alexander 1983). These have greater flexibility and allow flexion of a joint in various planes.

The cuticle is secreted from the epidermis and forms a series of plates called slcerites which cover the body. The tough exocuticle is formed from proteins in successive layers producing a laminated structure. The successive layers run at oblique angles which reduces planes of weakness running through the cuticle. The exocuticle containing chitin is up to 20 micrometers thick, whilst the chitin free epicuticle is 1-4 micrometers thick. The inner epicuticle consists mostly of tanned lipoproteins. If damaged it has an extracellular enzyme, phenoloxidase which produces extra tanning. The outer epicuticle is approximately 18nanomicrometers thick comprising of a trilaminar membrane formed from polymerised lipids which are at right angles to the surface. Outside this epicuticle is a wax layer formed from long chain hydrocarbons and esters of fatty acids and alcohols. They form a hydrophilic oriented monolayer directly on the surface of the cuticulin. This layer is constantly being moved and replaced. The molecules of the monolayer are closely packed and angled making intermolecular spaces too small for water molecules to pass between. The orientation of molecules also presents a row of aliphatic groups on their outside which is one reason for the hydrofuge properties of the cuticle. (Richards 1951). This general pattern of cuticle histology differs somewhat throughout various groups. The basement membrane is not as discrete in Crustacea as in the Insecta. Crustacea often have large deposits of calcium carbonate, whilst the terrestrial Isopoda lack the waterproof lipid layer.

The evolution of the exoskeleton has been used to form a system of levers. Inside these tubular levers are projections, apodemes, which allow for muscle attachment. Joints between the lever systems have resulted in jointed appendages. Each segment is attached to its neighbour by means of a ring of arthrodial membrane and inner strips of resilin. This allows each segment which acts as a lever to be moved independently of its adjacent component. For this lever system the physical properties of the cuticle allow firm anchorage points for muscle origins and insertions (Curry 1980). The muscles work as antagonistic pairs and unlike the annelids are of the striated type.

These factors have all been instrumental in the excellent locomotion that has evolved throughout the phylum. It is evident then that this tubular form of exoskeleton is efficient for locomotion and supporting both body on legs, and the internal organs. It can support a much greater weight without collapse than can a solid cylindrical strut of the same mass (Neville 1970). Throughout the group, portions of the exoskeleton have been modified to form a variety of structures serving many different functions. Examples can be seen in the poison claws of Lithobius, the piecing stylet mouth parts of Culex pipens, the pincers on Eupagurus and the pedipalps on Androctonus.

Examples from the three living subphyla will now be discussed with respect to exoskeleton modifications.

The subphylum Chelicerata includes horseshoe crabs, scorpions, and the spiders. They have no antennae and four pairs of legs. The body is divided into two parts, the cephalothorax, and the abdomen. The name of this subphylum is derived from the first pair of appendages behind the mouth, the chelicerae, meaning claw horn. A single carapace may be present and behind the chelicerae are a pair of appendages called pedipalps. The majority of chelicerates are members of the class Arachnida. Scorpionida are large arachnids, they have a thick well sclerised cuticle, and very well developed pedipalps. The abdomen is long and segmented with a terminal sting. The cuticle forms this piecing hypodermic sting which can be brought over the body due to the abdominal segments being jointed. They are moved using extensor and flexor muscles attached to internal apodemes (Buchsbaum 1976). The exoskeleton in scorpions affords them great protection from predators and the harsh temperature fluctuations that their habitat in general subjects them to. The pedipalps are very well developed, and the cuticle is heavily scleroised with ridges along the inside edges of the claws. The exoskeleton in this particular order can be seen to provide protection from both prey that they feed on, and predators. They are ancient arachnids and may have been among the first terrestrial arthropods.

The subphylum Crustacea, although a highly successful group are predominantly aquatic. For this reason the woodlice will be briefly discussed, with mention as to why the exoskeleton is not as well suited to terrestrial life as that of other land arthropods. Woodlice belong to the order Isopoda. They still retain much physiology that is common to the aquatic members of their group. The dorsal surface forms a segmented carapace which is more heavily sclerotized than the ventral surface. The cuticle does not offer the same waterproofing properties as that found in other terrestrial arthropods. This is due to the absence of the lipid layer in the exocuticle (Darlington 1981). This makes them extremely susceptible to desiccation. They also have slightly modified pleopods for respiration which also results in large amounts of moisture being lost. The terrestrial isopods show how important the waterproof cuticle and tracheal system, or book lungs are. The restriction of their physiology has confined these land isopods to a nocturnal existence in damp habitats.

The most successful of all the arthropod subphylums is the Uniramia, containing the centipedes, millipedes and insects. The myriapods are all terrestrial with heads that usually exhibit no compound eyes and a long segmented trunk with many uniramious legs. In general arthropods have reduced the number of legs and compacted the body to increase mobility. This group however is extremely mobile even with its long body and numerous legs.

The insects are the largest class of arthropods and exhibit extraordinary diversity. The exoskeleton itself varies greatly in amount of scleroization and flexibilty across the group. The combination of the insects cuticle combined with their ability to fly is probably fundamental in their success. The exoskeleton forms a series of sclerites, dorsal terites, the lateral pleurites and the ventral sternites. Running between these plates like sutures is flexible arthrodial membrane (Sharov 1966). The power of flight is inextricably linked with the exoskeleton. The thorax forms a box section providing anchorage and housing for the flight muscles. The cuticle itself also provides elastic properties which assist in creating the power stroke, and recovery stroke (Pringle 1957). The mechanism of flight in Locusta for example is similar to a small diaphragm pump. The wings themselves are attached between the tergum and the pleurite. The tergum is moved rapidly up and down, similar to a diaphragm pump, causing rapid movement of the wings. The wings themselves are excellent modifications of the cuticle. They vary among the group, some like those of butterflies are covered in coloured scales. Those of Locusta are formed from a double layer of cuticle strengthened by veins.

Another adaptation which all except the isopoda have evolved is a gaseous exchange system. This is due to the cuticle forming an effective barrier to the environment, and so preventing the type of gaseous exchange seen in their annelid ancestors. The system is either a tracheal one, seen in Periplaneta, or book lungs, as seen in Androctonus and sometimes both, Araneus. The tracheal system is comprised of tubes formed by invaginations of the cuticle. These are supported by cuticular spiral thickenings, called taenida. These respiratory tubes invade deep into the body, branching into smaller tubes the tracheoles. This mechanism results in each individual cell being in close proximity to a tracheole to allow diffusion to take place. This passive method of gaseous exchange can be partially active in large insects by compression and expansion of the tracheae (Buck 1962)

Book lungs are formed from invaginations of the exoskeleton. They occur on the ventral surface of the abdomen forming a chitinised pocket. One wall is composed of thin lamellae which are bathed in a blood sinus. The very thin cuticle of these lamellae allow for passive gaseous exchange across the surface. The lung is connected by a spiracle to the outside. Movement of air into and out of the lungs is mainly by diffusion.

These are just some of the modifications of the exoskeleton seen among arthropods. The lens of the compound eye are also derived from the cuticle. All these structures combined with the properties of the cuticle itself far outweigh the disadvantages of periodic ecdysis. It can clearly be seen that the development of this integument was the most significant factor in the successful colonisation of land.