Examples Of Prognathous Head Insects

The head is a controversial area for anatomical nomenclature, but it provides some of the best examples of evolutionary trends in anatomy. Most insect morphologists believe that the head of modern insects represents the fusion of several segments that were present in an ancestral condition. However, the number of segments included in the ground plan of the insect head has been a contentious issue among morphologists for more than a century. Any argument that attempts to explain head segmentation must take into account comparative anatomical, embryological, and paleontological evidence, and must examine modern forms of ancestral insects.

Ground Plan of the Pterygote Head

Given the difficulty in homologizing anatomical features of the head, we describe regions associated with landmarks of a ground plan or an idealized hypognathous insect head. In terms of modern insects, the Orthoptera probably come closest to displaying all the important landmark sutures and sclerites that form the head (Fig. 3A).

The vertex (Latin, vertex = top; pl., vertices) is the apex or dorsal region of the head between the compound eyes for insects with a hypognathous or opisthognathous head. This definition does not apply to prognathous heads because the primary axis of the head has rotated 90° to become parallel to the primary axis of the body. The vertex is the area in which ocelli are usually located. In some insects this region has become modified or assumes different names.

The ecdysial suture (coronal suture + frontal suture, epicranial suture, ecdysial line, cleavage line) is variably developed among insects. The suture is longitudinal on the vertex and separates epicranial halves of the head (Fig. 3B). Depending on the insect, the ecdysial suture may be shaped like a Y, a U, or a V. The arms of the ecdysial suture that diverge anteroven-trally, called the frontal sutures (frontogenal sutures), are not present in all insects (Fig. 3B). Some of these complexes of sutures are used by insects to emerge from the old integument during molting.

The frons is that part of the head immediately ventrad of the vertex (Fig. 3A). The frons varies in size, and its borders are sometimes difficult to establish. In most insects the frons is limited ventrally by the frontoclypeal suture (epistomal suture), a transverse suture located below the antennal sockets. As its name implies, the suture separates the dorsal frons from the ventral clypeus (Fig. 3A).

The face is a generalized term used to describe the antero-medial portion of the head bounded dorsally by the insertion of the antennae, laterally by the medial margins of the compound eyes, and ventrally by the frontoclypeal suture. In some insects the area termed the face is coincident with some, most, or all of the frons.

The clypeus (Latin, shield) is a sclerite between the face and labrum (Fig. 3A). Dorsally, the clypeus is separated from the face or frons by the frontoclypeal suture in primitive insects. Laterally, the clypeogenal suture demarcates the clypeus. Ventrally, the clypeus is separated from the labrum by the clypeolabral suture (Fig. 3A). The clypeus is highly variable in size and shape. Among insects with sucking mouthparts the clypeus is large.

The gena (Latin, cheek; pl., genae) forms the cheek or scle-rotized area on each side of the head below the compound eye and extending to the gular suture (Fig. 3). The size of the gena varies considerably, and its boundaries also often are difficult to establish. In Odonata the gena is the area between compound eye, clypeus, and mouthparts. The postgena (Latin, post = after; gena = cheek; pl., postgenae) is the portion of the head immediately posteriad of the gena of pterygote insects and forms the lateral and ventral parts of the occipital arch (sensu Snodgrass) (Fig. 3). The subgenual area is usually narrow, located above the gnathal appendages (mandible and maxillae), and includes the hypostoma (Figs. 3 and 4) and the pleurostoma. The pleu-rostoma is the sclerotized area between the anterior attachment of the mandible and the ventral portion of the compound eye. The hypostoma is posteriad of the pleurostoma between the posterior attachment of the mandible and the occipital foramen. The subgenal suture forms a lateral, submarginal groove or sulcus on the head, just above the bases of the gnathal appendages (Fig. 4). The subgenal suture is continuous anteriorly with the frontoclypeal suture in the generalized pterygote head. Internally, the subgenal suture forms a subgenal

Insect Morphology
FIGURE 4 Generalized view of an insect head. [This and other line drawings after Snodgrass, R. E. (1935). "Principles of Insect Morphology," McGraw-Hill Co.].

ridge that presumably provides structural support for the head above the mandible and maxillae. In some instances, the subgenal suture is descriptively divided in two. The part of the suture that borders the proximal attachment of the mandible to the head (Fig. 4) is called the pleurostomal suture (the ventral border of the pleurostoma). The posterior part of the subgenal suture from the mandible to the occipital foramen is called the hypostomal suture (the ventral border of the hypostoma).

Head Size and Shape

The size and shape of the head and its appendages reflect functional adaptations that can be used to explain biological details of the insect—the realm of morphology as opposed to anatomy.

SIZE Upon casual observation, the size of any given insect's head appears to be in proportion to the size of its body. A head that is disproportionately small or large relative to body size suggests that some adaptation has taken place that serves a functional need. Proportional head size varies considerably in the Insecta. Some fly families have very tiny heads in relation to their body size (e.g., Diptera: Acroceridae). Among Orthoptera, grass-feeding species typically have larger heads than herbaceous-feeding species. The large head is filled with powerful adductor muscles because grasses (monocots) are more difficult to chew than dicotyledonous plants. Furthermore, the postseedling stages of grasses are nutrient poor, meaning that more grass must be bitten, chopped, or ground to provide adequate nutrition.

SHAPE Head shape varies considerably among insects. Many unusual shapes seem to be influenced by behavior and may be used to illustrate examples of structural form and function. The functional importance of head shape may be difficult to determine in preserved specimens. A few hours of observation with living insects can provide considerable insight into the importance of shape. Globular heads are seen in some insects, including the burrowing crickets (e.g., stenopelmatines and gryllids). This form of head is adapted for pushing soil. Hypercephalic heads are seen in the males of some Diptera (Sepsidae, Diopsidae, Drosophilidae, and Tephritoidea) and Hymenoptera (Pteromalidae and Eurytomidae); the broad heads of the males are featured in various aspects of courtship behaviors.

Topographical Features of the Head

Morphologists experience considerable difficulty in defining regions and determining homologies of structure on the insect head. We cannot unambiguously characterize topographical features of the insect head because more than a million species are involved in the definition, and they show incredible diversity in head anatomy. Shape alone is not adequate or suitable because there are many head shapes, and often a head shape can be derived independently in several unrelated lineages. Some head shapes are influenced by behavior.

axial position The posture or orientation of the head in its resting position relative to the long axis of the body can be important in providing definitions of the anatomical features of the head. Axial position in insects typically falls into three basic categories: hypognathous, prognathous, and opisthognathous.

In general zoological usage, the word "hypognathous" (Greek, hypo = under; gnathos = jaw) serves to designate animals whose lower jaw is slightly longer than the upper jaw. In entomological usage, "hypognathous" refers to insects with the head vertically oriented and the mouth directed ventrad. Most insects with a hypognathous condition display an occipital foramen near the center of the posterior surface of the head. The hypognathous condition is considered by most insect morphologists to represent the primitive or generalized condition. The hypognathous position is evident in most major groups of insects and can be seen in the grasshopper, house fly, and honey bee. Other conditions are probably derived from ancestors with a hypognathous head.

In general zoological usage "prognathous" (Greek, pro = forward; gnathos = jaw) refers to animals with prominent or projecting jaws. In entomological usage, the prognathous condition is characterized by an occipital foramen near the vertexal margin with mandibles directed anteriad and positioned at the anterior margin of the head. When viewed in lateral aspect, the primary axis of the head is horizontal. Some predaceous insects, such as carabid beetles and earwigs, display the prognathous condition. In other insects, such as cucujid beetles and bethylid wasps, the prognathous position may reveal a solution to problems associated with living in concealed situations such as between bark and wood or similar confined habitats.

In general zoological usage, "opisthognathous" (Greek, opisthos = behind; gnathos = jaw) refers to animals with retreating jaws. In entomological usage, the opisthognathous condition is characterized by posteroventral position of the mouthparts resulting from a deflection of the facial region.

The opisthognathous condition is displayed in many fluid-feeding Homoptera, including leafhoppers, whiteflies, and aphids.

sutures of the head Head sutures are sometimes used to delimit specific areas of the head, but there are problems. Establishing homology of sutures between families and orders is difficult. From a practical viewpoint, standards have not been developed for naming sutures among insect groups. Some names are based on the areas delimited (e.g., frontoclypeal suture); other sutures are named for the areas in which the suture is found (e.g., coronal suture). Sutures frequently have more than one name (e.g., frontoclypeal suture and epistomal suture are synonymous).

The compound eye is an important landmark on the insect head. An ocular suture surrounds the compound eye and forms an inflection or an internal ridge of the integument (Figs. 3, 4). The ocular suture is not present in all insects and is difficult to see in some insects unless the head is chemically processed for microscopic examination. When present, the ocular suture probably provides strength and prevents deformation of the compound eye.

A subocular suture extends from the lower margin of the compound eye toward the subgenal suture. In some species the subocular suture (Fig. 4). may extend to the subgenal suture; in other species it may terminate before reaching another landmark. This suture is straight and commonly found in the Hymenoptera, where it may provide additional strength for the head.

posterior aspect of the head The entire posterior surface of the head is termed the postcranium (Fig. 3). The surface may be flat, concave, or convex, depending on the group of insects. The occiput (Latin, back of head) of pterygote insects is the posterior portion of the head between the vertex and cervix (Latin, neck). The occiput is rarely present as a distinct sclerite or clearly demarcated by "benchmark" sutures. When present, the occiput signifies a primitive head segment. In some Diptera the occiput forms the entire posterior surface of the head. In other insects it forms a narrow, horseshoe-shaped sclerite.

The occipital suture (hypostomal suture sensu MacGillivray) is well developed in orthopteroids, but it is not present in many other groups of pterygote insects (Fig. 3C). When present, the occipital suture forms an arched, horseshoe-shaped groove on the back of the head that ends ventrally, anterior to the posterior articulation of each mandible. Internally, the occipital suture develops into a ridge, providing strength for the head.

The postoccipital suture is a landmark on the posterior surface of the head and is typically near the occipital foramen (Fig. 3C). The postoccipital suture forms a posterior submarginal groove of the head with posterior tentorial pits marking its lower ends on either side of the head. Some morphologists regard this suture as an intersegmental boundary (labium) between the first and second maxillae.

FIGURE 5 Occipital closures (diagrammatic): (A) hypostomal bridge, (B) postgenal bridge, (C) gula and (D) ventral view of gula of adult ground beetle (Coleoptera: Carabidae).

FIGURE 5 Occipital closures (diagrammatic): (A) hypostomal bridge, (B) postgenal bridge, (C) gula and (D) ventral view of gula of adult ground beetle (Coleoptera: Carabidae).

Internally, the postoccipital suture forms the postoccipital ridge that serves as an attachment for the dorsal prothoracic and cervical muscles of the head. The absence of the postoccipital suture in pterygote insects is a derived condition.

The postocciput of pterygotes forms the extreme posterior, often U-shaped sclerite that forms the rim of the head behind the postoccipital suture. The postocciput is interpreted as a sclerotic remnant of the labial somite in ancestral insects.

In pterygotes such as Orthoptera the occipital foramen and the mouth are not separated. More highly evolved insects have developed sclerotized separations between the mouthparts and the occipital foramen. At least three types of closure have been identified (Fig. 5): the hypostomal bridge, the postgenal bridge, and the gula. An understanding of these structures provides insight into the operation of the head and suggests evolutionary trends in feeding strategies.

The hypostomal bridge is usually developed in adult heads displaying a hypognathous axial orientation. The bridge is formed by medial extension and fusion of hypostomal lobes (hypostoma) (Fig. 5A). The hypostomal bridge is the ground plan condition of closure for the posterior aspect of the head, but it is not restricted to primitive insects. The hypostomal bridge is found in highly developed members of the

Heteroptera, Diptera, and Hymenoptera. In Diptera the hypostomal bridge also has been called the pseudogula.

The postgenal bridge is a derived condition from the hypostomal ground plan and is developed in adults of higher Diptera and aculeate Hymenoptera. The bridge is characterized by medial extension and fusion of the postgenae, following a union of the hypostoma (Fig. 5B). The posterior tentorial pits retain their placement in the postoccipital suture.

The gula (Latin, gullet; pl., gulae) is developed in some Coleoptera, Neuroptera, and Isoptera. Typically, the gula is developed in heads displaying a prognathous axial orientation and in which posterior tentorial pits are located anteriad of the occipital foramen. (Fig. 5C, D). The median sclerite (the gula) on the ventral part of a prognathous head apparently forms de novo in the membranous neck region between the lateral extensions of the postocciput. The gula is a derived condition that is found in some but not all prognathous heads.

Endoskeletal Head Framework

Although the hardened integument of the head forms a structurally rigid capsule, this design is insufficient to solve the problems associated with muscle attachment and

Anterior condyle of the mandible

Mandible

Posterior condyle of the mandible; mandibles open away from the midline of the head

Hypostoma

Acetabulum Condyle

Mandible

FIGURE 6 Mandible articulation. Top: lateral view; bottom: posterior view (Orthoptera: Stenoplematidae).

Anterior condyle of the mandible

Mandible

Posterior condyle of the mandible; mandibles open away from the midline of the head

Hypostoma

Acetabulum Condyle

Mandible maintaining structural integrity during chewing. Thus, insects have evolved a tentorium (Latin, tent; pl., tentoria): a complex network of internal, hardened, cuticular struts that serve to reinforce the head. The tentorium forms as an invagination of four apodemal arms from the integument in most pterygote insects. The tentorium strengthens the head for chewing, provides attachment points for muscles, and also supports and protects the brain and foregut.

Anatomically, the tentorium consists of anterior and posterior arms. In most insects, the anterior arms arise from facial inflections located just above the anterior articulations of the mandibles. Externally, the arms are marked by anterior tentorial pits positioned on the frontoclypeal or subgenal (pleurostomal) suture (Fig. 4). Internally, the anterior region may form a frontal plate. Posterior arms originate at the ventral ends of the postoccipital inflection. They are marked externally by posterior tentorial pits (Fig. 4). The posterior arms usually unite to form a transverse bridge or corpus tentorii (internally) across the back of the head. Dorsal arms (rami), found in many insects, arise from the anterior arms. They attach to the inner wall of the head near antennal sockets. The dorsal arms are apparently not an invagination of cuticle, because pits do not mark them externally.

Mandible Articulation and Musculature

The hypothetical ancestor of insects is thought to possess a mandible with one point of articulation. Later, insects acquired a second point of articulation. The basis of this assumption comes from a survey of the Hexapoda in that the modern Apterygota have a monocondylic mandible and the Pterygota have a dicondylic mandible.

The term condyle (Greek, kondylos = knuckle) refers specifically to a knoblike process. For the mandible, the condyle is the point of articulation with the head. On the head itself is an acetabulum (Latin, acetabulum = vinegar cup), a concave surface or cavity for the reception and articulation of the condyle (Fig. 6).

The dicondylic mandible is the derived condition and is found in the Lepismatidae and Pterygota. The dicondylic mandible has secondarily acquired an articulation point anterior to the first point in the monocondylic mandible. These attachments form a plane of attachment. In the monocondylic mandible there is no plane of attachment, and the mandibles move forward or rearward when the muscles contract. The two points of articulation create a plane of movement that restricts the direction of mandible movement.

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Responses

  • neftalem
    What is the characteristics of an prognathous and a hypognathous head?
    3 months ago
  • angela
    What are the characteristic of an insect with hypognathous type of head?
    3 months ago

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