The Procuticle

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.

The Epidermis and basal lamina

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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 epidermis and the basal lamina

The innermost element of the cuticle is the basal lamina, which is secreted by the epidermal cells and forms a continuous sheet beneath the epidermis. The primary components of the basal lamina are fibrous protein, collagen, glycoprotein’s and glycosaminoflycans, many of these molecules are charge and probably give the basal membrane the ability to act as a molecular sieve controlling substance exchange between the hemocoel and the epidermis (Locke 1991).

On top of the basal lamina lies the epidermis itself, a single celled layer that lies just under the cuticle and is responsible for the building of the cuticle. The cuticle is secreted in a distinct order during the moults of the insect. The moult is the change of cuticle that allows an insect to change from an exoskeleton that is too small to one that will allow it to grow bigger. For a description of the growth of insects see the section on moulting.

Within the epidermis there are different cells, the basic epidermal cells, oenocytes, gland cells, tormogen and trichogen cells. The epidermal cells are together responsible for secreting different parts of the cuticle and the communication across the insect cuticle.

The epidermal cells comprise the bulk of the epidermis. The epidermal cells are gland cells in the sense that they produce and secrete substances that form the cuticle. Secretion by the epidermal can be through the very folded cell wall that is in contact with the cuticle or through pore canals that can extend all the way through the cuticle opening in the upper layers of the epicuticle. Secretion directly through the wall is usually associated with the building of the endo and exocuticle or with the secretion of agents that can alter the cuticle making it more elastic, harden or causing the cuticle to degrade allowing most of it to be consumed but the epidermal cells during moults. Secretion through the epidermal pore canals is mostly related to the building of the epicuticle by secretion of epicuticular waxes, cements and quinones that all make up the epicuticle and or give the cuticle its mechanical properties.

The oenocytes can be seen as the chemical factories of the epidermis, that produces many of the lipids and polyphenols that are required to produce the outer and inner epicuticle (Wigglesworth 1988). The oenocytes are derived from epidermal cells and are in most insects found in the epidermis where they attach to the basal lamina and lie surrounded by the epidermal cells. In other insects oenocytes can be observed singularly or in clusters outside the epidermis just under the basal lamina. The pruducts synthesized in the oenocytes are transferred to the epidermal cells that then secrete them through pore canals straight to the epicuticle (Chino 1985, Gu et al. 1995).

Like the oenocytes, the epidermal gland cells are also derived from dermal cells. There are a great variety of cuticular gland cells and many of them are specific to specific genera or orders of insects. One of the functions of the epidermal gland cells is the production and secretion of phermones directly to the cuticle surface through pore canals (Chapman 1988).

The tormogen and trichogen cells perform mechanical and chemical sensory functions and are in that sense involved in the communication through the cuticle. Especially tactile sensory function of the environment is achieved by setae that extend from the cuticle forming hair like structures that when touched or disturbed activate nerve cells sending sensory input that in turn effects the behaviour of the insect. Setae can be both tactile or olifactory, meaning they can be sensitive to mechanic or chemical stimulus (see setae). The insect seta is formed by the tormogen and trichogen epidermal cells, the latter forming the exterior extension of the seta and the tormogen cells forming the setal socket that is so characteristic of an insect seta. Setae come in a variety forms and perform a large array of functions some of which are described in the section about setae where the setal sensory cell is also depicted.