Basic entomology

T.Y.B.Sc. Zoology (Pune Univeisty) Basic entomology Course.


ZY-335: Paper V
Total Lectures: 45
1. Introduction:                    
1.1 Definition, distinguishing features of Insects.
1.2 Importance & Scope for Entomology.
1.3 Branches of Entomology : Agricultural, Medical, Forest, Forensic & Industrial.
2. Body covering:                         (2)
2.1 Integument structure & function.
2.2 Cuticular processes & appendages.
3. Body organization:             (15)
3.1 Head: General morphology & it’s articulation patterns.
3.1.1 Antenna : Basic structure & types.
3.1.2 Eyes : Location,structure & functions of ocelli & compound eyes.
3.1.3 Mouth parts: Basic types.
3.2 Thorax: Segmentation & various sclerites.
3.2.1 Leg: Structure of typical leg & it’s modifications
(fossorial, cursorial, saltatorial, natatorial, clasporial, raptorial & suctorial).
3.2.2 Wing: Basic structure & wing venation in a generalized insect, wing coupling structures, flight mechanism & wing modifications (Tegmina, Elytra, Hemelytra, Halters, Brachypterous & Hairy fringed wings)
3.3 Abdomen: Segmentation & sexual dimorphism
3.3.1 Abdominal appendages: Pregenital & genital appendages
4. Sensory receptors:                   (3)
General account of tactile, auditory, olfactory, gustatory,
hygro & thermal receptors.
5. Insect metamorphosis:               (6)
5.1 Definition & types of metamorphosis (ametabola,
hemimetabola,paurometabola & hypermetabola)
5.2 Hormonal control of metamorphosis.
5.3 Structure of insect egg & types of eggs,larvae & pupae.
6. Insect pheromones: (3)
6.1 Definition & importance of pheromones.
6.2 Types of pheromones : Alarm, aggregation, trail making, releaser, primer
& sex pheromones
7. Bioluminescence in insects: (4)
7.1 Definition with examples of insects.
7.2 Structure of light producing organ & it’s mechanism.
7.3 Significance of bioluminescence in insects.
8. Sound production in insects: (3)
8.1 Structure & mechanism of sound producing organs in cicada & crickets.
8.2 Significance of sound production in insects.
9. Insect biotechnology:
General introduction
9.1 Use of insects in tissue culture & genetic studies as model animals.
9.2 Use of insects as biological weapons.
9.3 Importance of insects in medicines & cosmetics.
Reference Book:
1. Imm’s General Text book of Entomology Vol. I & 11(1993), Richards O.W. & Davis R.F.., B.I. Pul (Indian edition) New Delhi.
2. Principals of insect morphology, Snodgrass R.E. (1994) Indian Reprint, SBS Pub. New Delhi.
3. Structure & functions of Insects. Chapman R.F. (1983), 3 edition, ELBS, London.
4. Entomology, Gillott Cedric (1980), Plenum Press, New York.
5. The Science of Entomology, Romoser W.S. (1981) 2’ edition, Mac millon Co., New York.
6. General Entomology, Mani M.S. (1998) Reprint Oxford- IBH, India.
7. An Introduction to Entomology, Srivastav R.D. & Singh R.P. (1997), Concept Pub. New Delhi.
8. General & Applied Entomology, Nayar K.K., T.N. anantkrishanan & B.V. David, (1983), tata McGrow Hill, Pub. New Delhi.
9. Insects, Mani M.S. (2006) Reprint NBT Pub. New Delhi. 

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  Beaments  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 insects 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,  O,  N, and  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 avones 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).

A compound eye consists of many quite distinct elements, the ommatidia, each represented externally by one of the many facets of which the cuticular layer of the eye is composed. As the ommatidia of a given eye are similar, a description of the structure of one will serve to illustrate the structure of the eye as a whole.
The structure of an ommatidium.— The compound eyes of different insects vary in the details of their structure; but these variations are merely modifications of a common plan; this plan is well-illustrated by the compound eyes of Machills, Figure (155) represents a longitudinal section and a series of transverse sections of an ommatidium in an eye of this insect, which consists of the following parts.
The cornea.—The cornea is a hexagonal portion of the cuticular layer of the eye and is biconvex in form (Pig. 155, c).
The corneal hypodermis.—Beneath each facet of the cuticular layer of the eye are two hypodermal cells which constitute the corneal hypodermis of the ommatidium. These cells are quite distinct in Machills and their nuclei are prominent (Fig. 155, h y); but in many insects they are greatly reduced, and consequently are not represented in many of the published figures o compound eyes.
The crystalline-cone-cells.—Next to the corneal hypodermis there are four cells, which in one type of compound eyes, the eucone eyes, form a body known as the crystalline-cone, for this reason these cells are termed the crystalline-cone-cells (Fig. 155, cc). Two of these cells are represented in the figure of a longitudinal section and all four, in that of a transverse section. In each cell there is a prominent nucleus at its distal end.
The iris-pigment-cells.—Surrounding the crystalline-cone-cells and the corneal hypodermis, there is a curtain of densely pigmented cells, which serves to exclude from the cone light entering other ommatidia; for this reason these, cells are termed the iris-pigment (Fig.155, i). They are also known as the distal retinula cells; but as they are not a part of the retina this term is misleading.
There are six iris-pigment-cells surrounding each crystalline-cone; but as each of these cells forms a part of the iris of three adjacent ommatidia, there are only twice as many of these cells as there are ommatidia. This is indicated in the diagram of a transverse section (Fig. 155, i).
The retinula.—At the base of each ommatidium, there is a group of visual cells forming a retinula (Fig. 155, r); of these there are seven in Machills; but they vary in number in the eyes of different insects. The visual cells are so grouped that their united rhabdomeres form a rhabdom, which extends along the longitudinal axis of the ommatidium (Fig. 155, r h). The distal end of the rhabdom abuts against the proximal end of the crystalline-cone;, and-the nerve-fibers of which the visual cells are the endings pass through the basement membrane (Fig. 155, b) to the optic nerve.
The visual cells are pigmented and thus aid in the isolation of the ommatidium.
The accessory pigment -cells.—In addition to the two kinds of pigment-cells described above there is a variable number of accessory pigments-cells (Fig. 155 a p), which lie outside of and overlap them from the above, it will be seen that each ommatidium of a eucone eye is composed of five kinds of cells, three of which, the corneal hypodermis, the crystalline-cone-cells, and the retinular cells produce solid structures; and three of them are pigmented.
Three types of compound eyes are recognized: first, the eucone eyes, in tse each ommatidium contain a true crystalline-cone, as described above, and the nuclei of the cone-cells are in front of the cone; second, the pseudocone eyes, in these the four cone-cells are filled with a transparent fluid medium, and the nuclei of these cells are behind the refracting body; and third, the acone eyes, in which although the four cone -cells are present they do not form a cone, either solid or liquid.

Fig: An ommatidium. The lettering is explained in the text

Abdominal segmentation and abdominal appendages
The abdomen is composed of series of segments which are more equally developed that in the other regions of the body.
Through out the abdomen, they retain their simple annular forms, the terga and sterna are generally undivided shield, while, the pleura are membranous and usually with out differentiated sclerites.
Each inter-segmental sclerite is fused with the segmental plate behind it.
Reduction or modification of certain segments is observed at the anterior and posterior ends at the abdomen, more especially in the later regions, and this specialization increases for the lower to the higher orders.
The primitive abdominal segments are 11, with a non segmental region or telson. They are all visible in embryonic conditions. The telson is present in embryos of certain insects, but it rarely persists in adults.
The 11th segment is present in the adults of the lower orders where its tergum is represented by the epiproct above the anus (Often fused with 10th tergum), while vestiges of its sternum are seen in the paraprocts, which lies on either side of the anus.
The 10th segment is distinct and forms the terminal segments in the higher orders.
In Protura, the number of abdominal segments increases during post embryonic development, the youngest instars having only 8th segments and a telson.
In Collembola, the number of abdominal segments is never more than six, either in embryo or the adult.
In most insect the 1st abdominal segment and more especially its sternum, is reduced or vestigial.
In hymenopterans, a 1st segment become fused with the metathorax during the change from the larva to the pupa and is known as propodeura or epinotum or median segment.

Appendages and process of abdomen
Primitive appendages
Anal Cerci
The appendages of the segment 11 form a pair of structure called cerci. This arise form the membrane between the epiproct and Paraproct even where the segments 11 is absent, the cerci may be present, appearing to arise form segment 10.
Cerci are present and well developed in the apterygota and the hemimetabolous orders other than the Hemiptera. They may be simple unsegmented structure as in orthopteran or annulated as in dictyoptera (Cockroach). They may be very short or slightly visible or form long filaments as long as or longer than the body as in Thysanura.
The cerci usually function as sense organs; they are sensitive to tactile stimuli and to air movement and sometime act as sound receptors.
Dr. CSJ Gen Entom. Page 1
Sometime the cerci differ in two sexes of as species, they play role in copulation and used for sexual dimorphism.
In earwig they are highly sclerotised and form powerful forceps with incurved, toothed end, in male. They are called as pincers.
In larval zygoptera, the cerci are modified to form the two lateral gills. In Ephemeroptera, the long featherlike cerci, together with the medial caudal filament, can be used for swimming in water.
The insect genital opening lies just below the anus
It is surrounded by specialised Sclerites that form external genitalia
In females.
Paired appendage of the eight and ninth abdominal segments fit together to form an egg laying mechanism called a the ovipositor.
The appendage consists of four valvifers (basal sclerites with muscle attachments) and six valvulae (apical sclerites which guide the egg as it emerges form the female’s body).
In males the genital opening is usually enclosed in a tube like Aedeagus which enters the female’s body during copulation.
The external genitals may also include other sclerites (e.g. Sub genital plate, claspers, styli etc). That facility’s mating or eggs laying. The structure of these genital sclerites differs from species to species to the extent that it usually prevents interspecies hybridization and also serves as tool for identification for insect taxonomist.

Nongenital appendages
Prolegs--Legs like outgrowth of body wall, known as prolegs and common feature of he abdomen of holometabolous larva. These appendages are expanded by blood pressure and moved mainly by the normal muscles. Frequently the prolegs are armed distally with spines or crochets which forms grips on he surfaces.
Cornicals --Paired secretory structure located dorsally on the abdomen of aphids. The cornicals produce substances that repel predators or elicit care giving behavior by the symbiotic ants.
Sting - Modified ovipositors, found only in the females of hymenoptera.
Abdominal gills—Respiratory organs found in the nymph (Naids) of certain aquatic insect. In Ephemeroptera (mayflies), paired gills are located along the sides of each abdominal segment, in Odonata (damselflies) the gills are attached to the end of the abdomen.
Furcula – the springtail jumping organ found in Collembola on the ventral side of the 5th abdominal segment.
Tenaculum – a clasp on the 3rd abdominal segment holds the springtail in its cocked position.
Collophore--- a fleshy, peg like structure found in Collembola on the ventral side of the first abdominal segment. It appears to maintain homeostasis by regulating absorption of water from the environment.

Aggregation pheromones
Pheromones are chemical signals from one organism that stimulate a response in another individual of the same species. Generally, this behavior is either attraction to the opposite sex or part of courtship interaction and are referred as sex attractants pheromones. Male-produced sex attractants often are referred to as aggregation pheromones because they typically result in the arrival of both sexes at a calling site leading to an increase in density of conspecifics in the vicinity of the pheromone source. Aggregation pheromones have been reported for members of the Coleoptera, Dictyoptera, Hemiptera, Homoptera and Orthoptera and have been identified for hundred of species. In pest management survey and monitoring with pheromones and other attractants are practiced worldwide against a broad array of insect pest, and these techniques are integral parts of a growing number of control programs
e.g in control of Cotton boll weevil the use of male-produced pheromone, grandlure, in conjunction with traps and/or trap crops.
Pea and bean weevil (Sitonia lineatus (L.). This weevil is a pest of leguminous crops. pea and bean weevil produce aggregation pheromones and presumably does assist the aggregation of the weevils in legume fields for the purpose of both feeding and mating. The aggregation pheromone, 4-methyl-3,5-heptanoide, has been utilized in mass trap of these insects and to time insecticide application.
Stored product weevils (Sitophilus zeamais (L.), Sitophilus granarius (L.) and Sitophilus oryzae (L.)). These three weevil species have long been recognized as a serious pests of stored grains in all the world. In these weevils a male-produced aggregation pheromones and cause interspecific attraction between weevils. This strong cross (interspecific) attraction facilitates the control of the weevil using trap baited with aggregated pheromones.
There is a wide range of methods of pest management using aggregation pheromones among the most ecologically selective pest suppression agents. Unlike the conventional insecticides, they are no toxic and they are effective at very low concentrations. The most promising results are obtained with the combination with other techniques such as color attractants and the application of lower doses of insecticides.
Use for pest control
When used in combination with traps, sex pheromones can be used to determine what insect pests are present in a crop and what plant protection measures or further investigations might be in order to assure that there will be no excessive damage to the crop. If the synthetic attractant is exceptionally seducing and the population level is very low, some control can be achieved with pheromone traps or with a technique called "attract and kill".
Generally, however, a technique called mating disruption is more effective: Synthetic pheromone is released from numerous sources placed throughout the crop to be protected; the males are then unable to locate the females and the number of matings and offsprings is reduced.
Mating disruption has been successful in controlling a number of insect pests. More than 20% of the grape growers employ this technique and produce their wine without using insecticides.
The  cuticle  is  secreted  by  the  epidermis  and  covers  the whole of the outside of the body as well as lining the foregut and hindgut and the tracheal system, which are formed as in- vaginations of the epidermis. Most of the cuticle is composed of a mixture of proteins and the polysaccharide chitin. Out- side this chitinous cuticle is a chemically complex epicuticle that does not contain chitin. It is only a few microns thick.

Chitinous cuticle
Chitin occurs as long molecules that are bound together to  form  microfibrils.  These  microfibrils  lie  parallel  to  the plane of the surface and, at any depth below the surface, to each other. In successive layers the orientation changes, usu- ally giving rise to a helicoid (spiral) arrangement through the thickness of the cuticle. This gives strength to the cuticle in all directions. Sometimes layers of helicoidally arranged mi- crofibrils alternate with layers in which the microfibrils have a consistent orientation. These layers differ in their refractive indexes, and the metallic colors of insects typically are the re- sult of differences in the optical properties of the successive layers, so that only specific wavelengths of light are reflected.

The helicoid arrangement of microfibrils provides strength to the cuticle, but it does not impart hardness or rigidity. Hard- ness in insect cuticle derives from the linking together of pro- teins. The process of linking the proteins is called sclerotization, and the hardened cuticle that results is said to be sclerotized or tanned. Hardening is restricted to the outer parts of chitinous cuticle, so that the cuticle becomes differentiated into the outer sclerotized exocuticle and an inner endocuticle that remains un- sclerotized. Sclerotization does not take place until the cuticle is expanded fully after a molt and depends on the transport of chemicals from the epidermis. This is achieved via a series of slender processes of the epidermal cells that extend through the chitinous cuticle, creating canals in the cuticle that run at right angles to the surface. These are called pore canals.

Sclerotization  affords  some  rigidity  in  addition  to  hard- ness, but in many areas of the cuticle this rigidity is enhanced by shallow folds in the cuticle. Their effect is comparable to that of a T-girder. The folds are seen as grooves, called sulci (singular:  “sulcus”),  on  the  outside  of  the  cuticle.  Sulci  are most common on the head and thorax, where they define ar- eas of cuticle that are given specific names. Additional rigid- ity  is  achieved  where  fingerlike  inpushings  of  the  cuticle, called apodemes, meet internally, forming an endophragmal skeleton.  This  occurs  in  the  head  of  all  insects,  where  two pairs of apodemes, originating anteriorly and posteriorly on the head, join beneath the brain to form the tentorium, which provides the head with great rigidity in the horizontal plane. In winged insects lateral and ventral apodemes in the thorax may join or be held together by muscles forming a strut that holds the sides (pleura) of the thorax rigid with respect to the ventral surface (sternum). This is essential for the movement of the wings in flight. The tubular form of the legs and other appendages makes them rigid.

Flexibility in the cuticle, which allows different parts of the body to move with respect to each other, depends on regions of  movable  cuticle  between  the  hardened  plates  (sclerites). Sclerotization does not occur in this flexible cuticle, which is referred to as “membranous.” It is most extensive in the re- gion of the neck, between the abdominal segments, and between segments of the appendages. Membranous cuticle also is found where the wings join the thorax and at the bases of the antennae, mouthparts, and other appendages, giving them freedom  to  move.  Precision  of  movement  is  achieved  by points of articulation at which there is only a very small re- gion of membrane between adjacent sclerites.

A rubberlike protein, called resilin, also is known to be pre- sent in some insects and may occur more widely. When it is distorted, it retains the energy imparted to it and, like a rubber ball, returns to its original shape when the tension is re- leased. There is a pad of resilin in the hind wing hinge of the locust and also in the side of the thorax of the flea, where the release of stored energy gives rise to the jump. Small amounts also are present in the hinge of the labrum in the locust and in the abdomen of some beetles.
The strength, rigidity and articulations of the cuticle pro- vide  the  insect  with  support,  protection,  and  precision  of movement. In larval forms, such as caterpillars and fly larvae, most of the cuticle remains unsclerotized. In these cases, the hemolymph  (insects’  blood)  functions  as  a  hydrostatic  (held by  water  pressure)  skeleton,  and  movements  are  much  less precise.
Three or, in some species, four chemically distinct layers are present in the epicuticle. The innermost layer (inner epi- cuticle)  contains  lipoproteins  but  is  chemically  complex.  Its functions are unknown. The next layer, the outer epicuticle, is made of polymerized lipid, though it probably also contains some protein. It is believed to be inextensible, such that it can unfold but not stretch. It defines the details of patterns on the surface of the cuticle. Outside the outer epicuticle is a layer of  wax.  This  comprises  a  mixture  of  chemical  compounds whose  composition  varies  considerably  between  insect  taxa. The wax limits water loss through the cuticle and so is a major feature contributing to the success of insects as terrestrial organisms, for whom water is at a premium. Because this layer becomes abraded (worn away) during normal activities, it has to be renewed continually. New compounds are synthesized in the epidermis and are thought to be transported to the sur- face via wax canal filaments that run through the pore canals and the inner and outer epicuticles. A fourth layer sometimes occurs outside the wax, but its functions are unknown.

The epidermis is a single layer of cells. In addition to pro- ducing the cuticle, it contains many glands that secrete chem- icals  to  the  outside  of  the  insect.  These  chemicals  include many  pheromones,  involved  in  communication  with  other members of the same species, and defensive compounds that often are repellent to potential enemies. In the latter case, the glands  frequently  include  a  reservoir  in  which  the  noxious substances are accumulated until they are needed.