Tuesday, September 7, 2010

antenna

The antenna

The antenna are a pair of jointed appendages articulated with the head in front of the eyes or between them. The antenna vary greatly in form; in some insects they are thread-like, consisting of a series of similar segments; in others certain segments are greatly modified. The thread-like form is the more generalized.

In descriptive works names have been given to particular parts of the antennae, as follows (Fig. 5 i):
The Scape.—The first or proximal segment pf an antenna is called the scape (a). The proximal end of this segment is often subglobose, appearing like a distinct segment; in such cases it is called the bulb (a1).

Pedicel—The pedicel is the second segment of an antenna (b). In it differs greatly in form from the other segments.

The Clavola.—The term clavola is applied to that part of the antenna distad of the pedicel (c) ; in other words, to all of the antenna except the 1st and second segments. In some insects certain parts of the clavola are specialised antenna and have received particular names. These are the ring-joints, the funicle, and the club.

Ring-joints.—In certain insects (e.g., Chalcididae) the proximal segment or segments of the clavola are much shorter than the proceeding segments; in such cases they have received the name of ring-joins (c1).

The Club.—In many insects the distal segments of the antennae are more or less enlarged.

in such cases they are termed the club (c2).

The Funicle.—The funicle (e’) is that part of the clavola between the dub and the ring joints; or, when the latter are not specialized, between the club and the pedicel.

The various forms of antennae are designated by special terms. The more common of these forms are represented in Fig. 52. They are as follows: -

1. Setaceous : or bristle-like, in which the segments are successively smaller and smaller,

the whole organ tapering to a -point.

2. Filiform : or thread-like, in which the segments are of nearly uniform thickness.

3.Moniliform: or necklace-form, in which ‘the segments are more or less globosa, suggesting a string of beads.

4. Serrate: or saw-like, in which the segments are triangular and project like the teeth of a saw.

5. Pectinate : or comb-like, in which the segments have long processes on one side, like the teeth of a comb).

6. Clavate :club-shaped, in which the segments become gradually broader, so that the whole organ assumes the form of a club.

7. Copitate : or with a head, in which the terminal segment or segments form a large knob.

8. Lamellate: in which the segments that compose the knob are extended on one side into broad plates.

9. Geniculate: When an antenna is bent abruptly at an angle like a bent knee (Fig. 5) it is

said to be geniculate.

10. Aristate: (in house fly) pouch-like with lateral bristle on last clubbed segment.

11. Plumose: (brush-like) in mosquito.

Simple eye



THE ORGANS OF PHOTORECEPTION
A. THE GENERAL FEATURES
The two types of eyes of insects.—insects possess two types of eyes, the ocelli or simple eyes and the compound or facetted eyes. Typically both types of eyes are present in the same insect, but either may be absent. Thus many adult insects lack ocelli, while the larva of insects with a complete metamorphosis lack compound eyes. When all are present there are two compound eyes and, typically two pairs of ocelli; but almost invariably the members of one pair of ocelli are united and form a single median ocellus The median ocellus is absent in many insects that possess the other two ocelli. The distinction between ocelli and compound eyes.—The most obvious distinction between ocelli and compound eyes is the fact that in an ocellus there is a single cornea while in a compound eye there are many. Each ommatidium of a compound eye has been considered as a separate eye because its nerve-endings constituting the retinula are isolated from the retinube of other ommatidia by surrounding accessory pigment cells; but a similar isolation of retinui exist in some ocelli. It has also been held that in compound eyes there is a layer of cells between the corneal hypodermis and the retinas. the crystalline-cone- cells, which is absent in ocelli; but in the ocelli of adult Ephemerida there is a layer of cells between the lens and the retina, which, at least, is in a position analogous to that of the crystallinecone-cells; the two may have had a different origin, but regarding this, we have, as yet, no conclusive data.

The absence of compound eyes in most of the Apterygota.—
Typically insects possess both ocelli and compound eyes; when either kind of eyes is wanting it is evidently due to a; loss of these organs and
not to a generalized condition. Although compound eyes are almost universally absent in the Apterygota-in the few cases where they are present in this group they are of a highly developed type and not rudimentary; the compound eyes of Machills, for example, are as perfect as those of winged insects.

The absence of compound eyes in 1arve.—
The absence. of compound eyes in larva is evidently a secondary adaptation to their particular mode of life, like the internal development of wings in the same forms. In the case of the compound eyes of larva, the development of the organs is retarded, taking place in the pupal stage instead of in an embryonic stage, as is the case with nymphs and naiads. While, the development of the compound eyes as a whole is retarded in larva, a few ommatidia may be developed and function as ocelli during larval life.

B. THE OCELLI: There are two classes of ocelli found in insects: first, the ocelli of adult insects and of nymphs and naiads, which may be termed the primary ocelli; and second, the ocelli of most larva possessing ocelli, which may be termed adaptive ocelli.

The primary ocelli.—The ocelli of adult insects and of nymphs and naiads having been originally developed as ocelli are termed the primary ocelli. Of these there are typically two pairs; but usually when they are present there are only three of them, and in many cases only a single pair. When there are three ocelli, the double nature of the median ocellus is shown by the fact that the root of the nerve is double, while that of each of the other two is single. In certain generalized insects, as some Plecoptera, (Fig. 150) all of the ocelli are situated in the front; but in most insects, the paired ocelli have either migrated into the suture between the front and the vertex (Fig. xi), or have proceeded farther and are situated in the vertex.

Monday, September 6, 2010

Cuticular appendages

Cuticular appendages:
Cuticular structural components, waxes, cements, pheromones (Chapter 4), and defensive and other compounds are products of the epidermis, which is a near continuous, single-celled layer beneath the cuticle. Many of these compounds are secreted to the outside of the insect epicuticle. Numerous fine pore canals traverse the procuticle and then branch into numerous finer wax canals (containing wax filaments) within the epicuticle (enlargement in Fig. 1); this system transports lipids (waxes) from the epidermis to the epicuticular surface. The wax canals may also have a structural role within the epicuticle. Dermal glands, part of the epidermis, produce cement and/or wax, which is transported via larger ducts to the cuticular surface. Wax-secreting glands are particularly well developed in mealybugs and other scale insects. The epidermis is closely associated with molting – the events and processes leading up to and including ecdysis (eclosion), i.e. the shedding of the old cuticle. Insects are well endowed with cuticular extensions, varying from fine and hair-like to robust and spine-like. Four basic types of protuberance (Fig. 2), all with sclerotized cuticle, can be recognized on morphological, functional, and developmental grounds:
1. Spines are multicellular with undifferentiated epidermal cells;
2. Setae, also called hairs, macrotrichia, or trichoid sensilla, are multicellular with specialized cells;
3. acanthae are unicellular in origin;
4. microtrichia are subcellular, with several to many extensions per cell.
Setae sense much of the insect’s tactile environment. Large setae may be called bristles or chaetae, with the most modified being scales, the flattened setae found on butterflies and moths (Lepidoptera) and sporadically elsewhere. Three separate cells form each seta, one for hair formation (trichogen cell), one for socket formation (tormogen cell), and one sensory cell (Fig. 4.1). There is no such cellular differentiation in multicellular spines, unicellular acanthae, and subcellular microtrichia. The functions of these types of protuberances are diverse and sometimes debatable, but their sensory function appears limited. The production of pattern, including color, may be significant for some of the microscopic projections. Spines are immovable, but if they are articulated, then they are called spurs. Both spines and spurs may bear unicellular or subcellular processes.


Fig. 1 The general structure of insect cuticle; the enlargement above shows details of the epicuticle.

Fig. 2 The four basic types of cuticular protuberances: (a) a multicellular spine; (b) a seta, or trichoid sensillum; (c) acanthae; and (d) microtrichia.

Saturday, September 4, 2010

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).

Cuticle structure

Cuticle
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.
Epicuticle
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.

Epidermis
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.

Thursday, September 2, 2010

Insect Head

The Head

The head, being the anterior tagma, bears the major sense organs and the mouth parts. Considerable controversy still surrounds the problem of segmentation of the insect head, especially concerning the number and nature of segments anterior to the mouth. At various times it has been argued that there are from three to seven segments in the insect head, though it is now widely agreed that there are six.

The embryological studies have demonstrated convincingly that an acron is present. However, it is never seen in fossil insects or other arthropods because it moved dorsally to merge imperceptibly into the region between the compound eyes. Both embryology and paleontology have confirmed that there are three preoral and three postoral segments The first preoral segment is preantennal; it is called the protocerebral clypeolabral segment. The segment itself has disappeared but its appendages remain as the clypeolabrum. The second preoral (antennal/deutocerebral) segment bears the antennae which are therefore true segmental appendages. The third preoral (intercalary/tritocerebral) segment appears briefly during embryogenesis, then is lost. Its appendages, however, remain as part of the hypopharynx (Kukalova´-Peck, 1992). Head segments 4–6 are post-oral and named the mandibular maxillary, and labial, respectively. Their appendages form the mouthparts from which their names are derived. In addition, the sternum of the mandibular segment becomes part of the hypopharynx.

General Structure

Primitively the head is oriented so that the mouthparts lie ventrally (the hypognathous condition) (Figure 3.3B). In some insects, especially those that pursue their prey or use their mouthparts in burrowing, the head is prognathous in which the mouthparts are directed anteriorly (Figure 3.4A). In many Hemiptera the suctorial mouthparts are posteroventral in position (Figure 3.4B), a condition described as opisthognathous (opisthorhynchous).
The head takes the form of a heavily sclerotized capsule, and only the presence of the antennae and mouthparts provides any external indication of its segmental construction. In most adult insects and juvenile exopterygotes a pair of compound eyes is situated dorsolater- ally on the cranium, with three ocelli between them on the anterior face (Figure 3.3A). The two posterior ocelli are somewhat lateral in position; the third ocellus is anterior and median. The antennae vary in location from a point close to the mandibles to a more median position between the compound eyes. On the posterior surface of the head capsule is an aperture, the occipital foramen, which leads into the neck. Of the mouthparts, the labrum hangs down from the ventral edge of the clypeus, the labium lies below the occipital foramen, and the paired mandibles and maxillae occupy ventrolateral positions (Figure 3.3B). The mouth is situated behind the base of the labrum. The true ventral surface of the head capsule is the hypopharynx (Figure 3.3D), a membranous lobe that lies in the preoral cavity formed by the ventrally projecting mouthparts.
There are several grooves and pits on the head (Figure 3.3A–C), some of which, by virtue of their constancy of position within a particular insect group, constitute important taxonomic features. The grooves are almost all sulci. The postoccipital sulcus separates the maxillary and labial segments and internally forms a strong ridge to which are attached the muscles used in moving the head and from which the posterior arms of the tentorium arise (see following paragraph). The points of formation of these arms are seen externally as deep pits in the postoccipital groove, the posterior tentorial pits The epicranial suture is a line of weakness occupying a median dorsal position on the head. It is also known as the ecdysial line, for it is along this groove that the cuticle splits during ecdysis. In many insects the epicranial suture is in the shape of an inverted Y whose arms diverge above the median ocellus and pass ventrally over the anterior part of the head. The occipital sulcus which is commonly found in orthopteroid insects, runs transversely across the posterior part of the cranium. Internally it forms a ridge that strengthens this region of the head. The subgenal sulcus isa lateral groove in the cranial wall running slightly above the mouthpart articulations. That part of the subgenal sulcus lying directly above the mandible is known as the pleurostomal sulcus; that part lying behind is the hypostomal sulcus, which is usually continuous with the postoccipital suture. In many insects the pleurostomal sulcus is contin- ued across the front of the cranium (above the labrum), where it is known as the Epistomal ( frontoclypeal) sulcus. thin this sulcus lie the anterior tentorial pits, which indicate the internal origin of the anterior tentorial arms. The antennal and ocular sulci indicate internal cuticular ridges bracing the antennae and compound eyes, respectively. A subocular sulcus running dorsolaterally beneath the compound eye is often present.
The tentorium (Figure 3.5) is an internal, cranial-supporting structure whose morphology varies considerably among different insect groups. Like the furca of the thoracic segments (Section 4.2), with which it is homologous, it is produced by invagination of the exoskeleton. Generally, it is composed of the anterior and posterior tentorial arms that may meet and fuse within the head. Frequently, additional supports in the form of dorsal arms are found. The latter are secondary outgrowths of the anterior arms and not apodemes. The junction of the anterior and posterior arms is often enlarged and known as the tentorial bridge corporotentorium In addition to bracing the cranium, the tentorium is also a site for the insertion of muscles controlling movement of the mandibles, maxillae, labium, and hypopharynx.
The grooves described above delimit particular areas of the cranium that are useful in descriptive or taxonomic work. The major areas are as follows. The frontoclypeal area is the facial area of the head, between the antennae and the labrum. When the epistomal sulcus is present, the area becomes divided into the dorsal frons and the ventral clypeus. The latter is often divided into a postclypeus and an anteclypeus The vertex is the dorsal surface of the head. It is usually delimited anteriorly by the arms of the epicranial suture and posteriorly by the occipital sulcus. The vertex extends laterally to merge with the gena, whose anterior, posterior, and ventral limits are the subocular, occipital, and subgenal sulci, respectively. The horseshoe-shaped area lying between the occipital sulcus and postoccipital sulcus is generally divided into the dorsal occiput, which merges laterally with the postgenae. The postocciput is the narrow posterior rim of the cranium surrounding the occipital foramen.
It bears a pair of occipital condyles to which the anterior cervical sclerites are articulated. Below the gena is a narrow area, the subgena, on which the mandible and maxilla are articulated. The labium is usually articulated directly with the neck membrane (Figure 3.3C), but in some insects a sclerotized region separates the two. This sclerotized area develops in one of three ways: as extensions of the subgenae which fuse in the midline to form a subgenal bridge, as extensions of the hypostomal areas to form a hypostomal bridge, or(in most prognathous heads) through the extension ventrally and anteriorly of a ventral cervical sclerite to form the gula. At the same time the basal segment of the labium may also become elongated (Figure 3.4A).




FIGURE 3.3. Structure of the typical pterygotan head. (A) Anterior; (B) lateral; (C) posterior; and (D) ventral
(appendages removed). [From R. E. Snodgrass. Principles of Insect Morphology. Copyright 1935 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company.]





FIGURE 3.4. (A) Prognathous; and (B) opisthognathous types of head structure. [A, from R. E. Snodgrass, Principles of Insect Morphology Copyright 1935 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company. B, after R. F. Chapman, 1971, The Insects: Structure and Function By permission of Elsevier North-Holland, Inc., and the author.]




FIGURE 3.5. Relationship of the tentorium to grooves and pits on the head. Most of the head capsule has been cut away. [From R. E. Snodgrass. Principles of Insect Morphology Copyright 1935 by McGraw-Hill, Inc. Used with permission of McGraw-Hill Book Company.]