Figure caption
1. Epicuticle, 2. Chitin fibril orientation, 3. Exocuticle, 4. Endocuticle, 5. Epidermal pore, 6.Basal lamina, 8. Cuticular gland cell pore, 9. Microtrichia, 10. Seta, 11. Epidermal cell pore canal, 12. Tormogen cell, 13. Epidermal cell, 14. Trichogen cell, 15. Gland cell, 16. Oenocyte
The procuticle (endo- and exocuticle)
The procuticle is a usually divided into the endo- and exocuticle that do not differ so much in their basic composition but more so in the additional agents that are present in the two types of procuticle giving them different mechanical properties.
The procuticle is made up of polysaccharide fibers (Chitin) scaffolding, embedded in an extensive stabilized protein matrix (Vincent 2002). Even though chitin is usually the component associated with the insect cuticle, it rarely comprises more than 50% of the cuticle dry weight. Furthermore chitin is not what gives the cuticle its hardness or rigidness (Wigglesworth 1953) in fact chitin is present in larger amount in soft extensible cuticle (usually the endocuticle) than in the harder tanned (sclerotised) cuticle (usually the exocuticle). Chitin molecules resemble those of cellulose, the chitin chains are very straight and are arranged anti-parallel (a-chitin) combining into highly crystalline structures. The stability and hardness of the chitin is due to the sugar residues of the chitin chains being heavily H-bonded. In the cuticle the chitin molecules are arranged in nanofibrils of 19 molecules about 3 nm in diameter and 0.3 mm long (Vincent 2002).
When the chitin and the protein matrix are secreted by the epidermis it is done so in sheets or laminae. Within a lamina the chitin nanofibrils are all oriented in one preferred direction (Wigglesworth 1953, Chapman 1998). Subsequent lamina secreted by the epidermis however, have the nanofibrils oriented at an angle to the previous layer, usually at an angle of 60°. This rotation of the nanofibrils creates a latticework within the protein matrix that affords the cuticle maximum shear strength from multiple directions (Scherge and Gorb 1965).
The other basic component of the cuticle the proteins, makes up a matrix that stabilizes the chitin. Furthermore it is the proteins that give the procuticle the different mechanical properties. The amount of proteins present in the cuticle can to some degree be compared to the hardness of the cuticle as flexible and stretchy cuticle of the abdomen of locusts have about 20 proteins (Vincent and Shawky 1978) and the harder and stiffer cuticle has about 200 proteins (Andersen et al. 1995). There is also a varying amount of water present in the procuticle. Usually there is a larger amount of water in the softer more flexible cuticle than in the harder and stiffer cuticle.
The differences in the endocuticle and the exocuticle can be roughly summed based on the known mechanical properties and the above mentioned characteristics of the procuticle in general. The endocuticle forms the bulk of the procuticle, but may in some species or parts of some species, as the elytra (the hardened forewings) of certain beetles, be completely missing. The endocuticle is soft pliable, suggesting that it contains mostly chitin and water and less proteins.
The procuticle usually comprises the bulk of the cuticle itself. Some organism that require extremely hard cuticle can have no endocuticle at all. A lack of endocuticle is usually seen in the elytra (the modified forewings of beetles) of beetles. Insects that require very soft and pliable cuticle can have almost completely reduced exocuticle. The most important role of the procuticle is to provide the mechanical properties of the cuticle. The epidermal cells can regulate the composition of the procuticle when this is secreted during moult, but the epidermal cells can also modify the properties of old cuticle by secreting additional proteins, lipids and water or by removing said components. The only Procuticle that is not alterable is tanned or sclerotised procuticle. Sclerotised procuticle is usually tanned brown and becomes extremely hard, but cannot be broken down and ingested by the epidermal cells during the moult.
