Functions of the Integument

Functions of the Integument

Most functions of the integument relate to the physical structure of the cuticle though the latter may serve as a source of metabolites during periods of starvation. The primary functions may be discussed under three headings: strength and hardness, permeability, and production of color.


1. Strength and Hardness

The few studies that have been carried out on the mechanical properties of insect cuticle indicate that is of medium rigidity and low tensile strength (Locke, 1974). There is, however, wide variation from this general statement; for example, the cuticles of most endopterygote larvae are extremely plastic, whereas the mandibular cuticle of many biting insects may be extremely hard, enabling them to bite through metal. Further, there is an obvious difference
in properties between sclerites and intersegmental membranes, and between typical non- elastic cuticle and that which contains a high proportion of resilin.
Though the above properties indicate that the cuticle is satisfactory as a “skin” pre- enting physical damage to internal organs, discussion of the suitability of the cuticle as a skeletal component must include an appreciation of overall body structure (Locke, 1974). Most components of insect (and other arthropod) bodies may be considered as cuticular cylinders or spheres. Such a tubular shell (used here in the engineering sense to mean a surface-supporting structure that is thin in relation to total size) is about three times as strong as a solid rod of the same material having the same cross-sectional area as the shell
(i.e., they both contain the same amount of skeletal material). The force required to dis- tort the shell is proportional to the thickness of the shell and inversely proportional to the cross-sectional area of the whole body. Thus, in small organisms where the thickness of the shell is great relative to the cross-sectional area of the body, the use of a shell as an xoskeletal structure is quite feasible. In larger organisms the advantage of the extra strength relating to a shell type of skeleton is greatly outweighed (literally as well as metaphori-
cally) by the massive increase in thickness of the shell that would be required and, perhaps,
by the physiological problems of producing the large amounts of material required for its construction.


4.2. Permeability

For different insects there exists a wide range of materials that are potential permeants
of the integument, and of factors that affect their rate of permeation. Sometimes specific regions of the integument are constructed to facilitate entry or exit of certain materials; more often the integument is structured to prevent entry or loss. At this time we shall consider only the permeability of the cuticle to water and insecticides, of which the latter may now be considered a normal hazard for most insects. The passage of gases through the integument
is considered in Chapter 15.
Water. Water may be either lost or gained through the integument. In terrestrial insects, which exist in humidities that are almost always less than saturation, the problem is to prevent loss through evaporation. In freshwater forms the problem is to prevent entry related to osmosis.
In many terrestrial insects the rate of evaporative water loss is probably less than 1% per hour of the total water content of the body (i.e., of the order of 1–3 mg/cm2 per hr for most species). Most of this loss occurs via the respiratory system, despite the evolution of mechanical and physiological features to reduce such loss (Chapter 15). Water loss through the integument (sensu stricto) is extremely slight, mainly because of the highly impermeable epicuticle and in particular the wax components. Early experiments demonstrated that permeability of the integument is relatively independent of temperature up to a certain point (the transition temperature), above which it increases markedly. As a result of his studies on both artificial and natural systems, Beament (1961) concluded that the initial impermeability is related to the highly ordered wax monolayer whose molecules sit on the tanned cuticulin envelope at an angle of about 25 to the perpendicular axis, with their polar ends facing inward and nonpolar ends outward. In this arrangement the molecules are closely packed and held tightly together by van der Waals forces. As temperature increases, the molecules gain kinetic energy, and eventually the bonds between them rupture. Spaces appear and water loss increases significantly. The nature of the wax and its transition temperature can be correlated with the normal niche of the insect. Insects from humid environments or that have access to moisture in their diet, for example, aphids, caterpillars, and bloodsucking insects, have “soft” waxes, with low transition temperatures. Forms from dry environments or stages with water-conservation problems, for example, eggs and pupae, are covered with “hard” wax, whose transition temperature is high (in most species above the thermal death point of the insect).
More recent studies have questioned the validity of Beament’s ordered monolayer model. Evidence against it includes the observation that hydrocarbons (non-polar molecules) are the dominant component of wax, physicochemical analyses that indicate that the lipids have no preferred orientation, and mathematical calculations that show the abrupt permeability changes at the so-called transition point to be artifactual (Blomquist and Dillwith, 1985).
Some insects that are normally found in extremely dry habitats and may go for long periods without access to free water, for example, Tenebrio molitor and prepupae of fleas, are able to take up water from an atmosphere in which the humidity is relatively high.
Originally it was believed that uptake occurred across the body surface perhaps via the pore canals. However, it has now been demonstrated that uptake occurs across the wall of the rectum (Chapter 18, Section 4.1).
In many freshwater insects, for example, adult Heteroptera and Coleoptera, the cuticle is highly impermeable because of its wax monolayer and water gain is probably 4% or less of the body weight per day. In most aquatic insects, however, the wax layer is absent. Thus, gains of up to 30% of the body weight per day are experienced, the excess water being removed via the excretory system (Chapter 18, Section 4.2).
Insecticides. Economic motives have stimulated an enormous interest in the perme- ability of the integument to chemicals, especially insecticides and their solvents (Ebeling,1974). Though, for the most part, the cuticle acts as a physical barrier to decrease the rate of entry of such materials, there is evidence that in some insects it may also bring about metabolic degradation of certain compounds, and consequently reduction of their potency.
It follows that increased resistance to a particular compound may result from changes in ei- ther the structure or the metabolic properties of the integument (see also Chapter 16, Section 5.5). For most insects, the primary barrier to the entrance of insecticides is the epicuticular wax, which dissolves and retains these largely lipid-soluble materials. For the same reason, the cement layer also probably provides some protection against penetration. The procuti- cle offers both lipid and aqueous pathways along which an insecticide may travel, but the precise rate at which a compound moves depends on many variables, especially thickness of the cuticle, presence or absence of pore canals, and whether the latter are filled with cytoplasmic extensions or other material. It follows that the rate of penetration will vary according to the 1ocation of an insecticide on the integument. However, it has also been noted that dissolution in the wax will facilitate lateral movement of the insecticides, perhaps allowing them to reach the tracheal system and thus gain access. Thin, membranous cuticle such as occurs in intersegmental regions or covers tactile or chemosensory hairs generally provides little resistance to penetration. The tracheal system is another site of entry. The extent to which tanning of the procuticle occurs is also related to penetration rate. As the chitin-protein micelles become more tightly packed and the cuticle partially dehydrated, permeability decreases.
In addition, but obviously related to the physical features of the cuticle, the physico- chemical nature of an insecticide is an important factor in determining the rate of entry. Especially significant is the partition coefficient (the relative solubility in oil and in water) of an insecticide or its solvent. In order to penetrate the epicuticular wax the material must be relatively lipid-soluble. However, in order to pass through the relatively polar procuti- cle and, eventually, to leave the integument to move toward its site of action, the material must be partially water-soluble. Thus, correct formulation of an insecticidal solution is an important consideration.
It should be apparent from the above discussion that few generalizations can be made.
At the present time, therefore, the suitability of an insecticide must be considered separately for each species. Because of the factors that affect the entry of insecticides, a great difference usually exists between “real toxicity,” that is, toxicity at the site of action, and “apparent toxicity,” the amount of material that must be applied topically to bring about death of the insect. The chief feature that relates the two is obviously the “penetration velocity,” that is, the rate at which material passes through the cuticle. When the rate is high, the real and apparent toxicity values will be nearly identical.
4.3. Color
As in other animals, the color of insects serves to conceal them from predators (some- times through mimicry), frighten or “warn” predators that potential prey is distasteful, or facilitate intraspecific and/or sexual recognition. It may be used also in thermoregulation. The color of an insect generally depends on the integument. Rarely, an insect’s color may be the result of pigments in tissues or hemolymph below the integument. For example, the red color of Chironomus larvae is caused by hemoglobin in solution in the hemolymph. Integumental colors may be produced in two ways. Pigmentary colors are produced when pigments in the integument (usually the cuticle) absorb certain wavelengths of light and reflect others (Fuzeau-Bresch, 1972). Physical (structural) colors result when light waves of a certain length are reflected as a result of the physical features of the surface of the integument.
Pigmentary colors result from the presence in molecules of particular bonds between atoms. Especially important are double bonds such as C C, C O, C N, and N N which absorb particular wavelengths of light (Hackman, 1974; Kayser, 1985). The integument may contain a variety of pigment molecules that produce characteristic colors. Usually the molecule, known as a chromophore, is conjugated with a protein to form a chromo- protein. The brown or black color of many insects results usually from melanin pigment. Melanin is a molecule composed of polymerized indole or quinone rings. Typically, it is located in the cuticle, but in Carausius it occurs in the epidermis, where it is capa- ble of movement and may be concerned with thermoregulation as well as concealment. Carotenoids are common pigments of phytophagous insects. They are acquired through feeding as insects are unable to synthesize them. Carotenoids generally produce yellow, orange, and red colors, and, in combination with a blue pigment, mesobiliverdin, produce green. Examples of the use of carotenoids include the yellow color of mature Schistocerca and the red color of Pyrrhocoris and Coccinella. Pteridines, which are purine derivatives, are common pigments of Lepidoptera, Hymenoptera, and the hemipteran Dysdercus, and produce yellow, white, and red colors. Ommochromes, which are derivatives of trypto- phan, an amino acid, are an important group of pigments that produce yellow, red, and brown colors. Examples of colors resulting from ommochromes are the pink of immature adult Schistocerca, the red of Odonata, and the reds and browns of nymphalid butter- flies. In some insects the characteristic red or yellow body color is the result of flavones originally present in the foodplant. Uric acid, the major nitrogenous excretory product of insects (Chapter 18, Section 3.1), is deposited in specific regions of the epidermis in some insects. For example, in Dysdercus it is responsible for the white areas of the integument.
Physical colors are produced by scattering, interference, or diffraction of light though the latter is extremely rare. Most white, blue, and iridescent colors are produced using the first two methods. White results from the scattering of light by an uneven surface or by granules that occur below the surface. When the irregularities are large relative to the elength of light, all colors are reflected equally, and white light results. An interference color is produced by laminated structures when the distance between successive laminae is similar to the wavelength of light that produces that particular color. As light strikes the laminae light waves of the “correct” length will be reflected by successive surfaces, and the color they produce will therefore be reinforced. Light waves of different lengths will be out of phase. Changing the angle at which light strikes the surface (or equally the angle at which the surface is viewed) is equivalent to altering the distance between laminae. In turn, this will alter the wavelength that is reinforced and color that is produced. This change of color in relation to the angle of viewing is termed iridescence. Iridescent colors are common in many Coleoptera and Lepidoptera.


4.4. Other Functions

The cuticular waxes may have important roles in preventing the entry of microorganisms and in chemical communication (i.e., they serve as semiochemicals). It has been suggested that the waxes may prevent adhesion of microorganisms or may be toxic to them. Cuticular hydrocarbons are also known to serve as contact sex pheromones, for example, in female Diptera and Blattella, attracting or inducing copulatory behavior in males, or serving as
an aphrodisiac to keep the male in position until insemination has occurred (Schal et al 1998). In termites, the cuticular hydrocarbon blend is highly specific and serves as a species- and/or caste-recognition pheromone. (See also Chapter 13, Section 4.1.2.) Interestingly, some beetles that live in termite colonies produce the same hydrocarbon profile as the host, enabling them to remain unmolested in the nest. The species-specific nature of the lipids has been turned to advantage by some parasitic Hymenoptera who use these chemical cues (known as kairomones [Chapter 13, Section 4.2.) to locate their host (Blomquist and Dillwith, 1985).