The Head

Cephalization has occurred in all arthropods, where one tagma is always present at the anterior end of the body and minimally has the functions of food intake, sensory perception, and neural integration and command. Most of the head is occupied by musculature, and the brain takes up little space. The details of cephalization in the different arthropod lineages, however, differ and can be used to define major arthropod lineages. For any given lineage, we can ask, "How many segments are present in the head?", "How many pairs of appendages on these segments are modified into other structures, such as mouthparts?", and "How distinct is the head from the rest of the body?"

The head of insects has three general positions in relation to the mouthparts. Hypognathous heads are those in which the mouthparts are oriented ventrally (e.g., Orthoptera); this type of head is the primitive condition for insects. Prognathous heads are those in which the mouthparts are oriented anteriorly (e.g., many Coleoptera, Raphidioptera); those structures that typically face anterior in a hypog-nathous insect are directed dorsally in a species with prog-nathy. The last type of gross head morphology is essentially an elaboration of the aforementioned condition of hypog-nathy. Opisthognathous heads are those in which the mouth-parts are in a ventral position but are oriented to the rear of the insect (e.g., Blattodea, Hemiptera).

The lateral surfaces of the head capsule are called the parietal regions. The gena is that portion of the parietal region above the subgenal sulcus. The vertex is essentially the top of the head, above the ocelli and between the compound eyes (Figure 4.2).

The head is etched by a series of lines, most of which are not true sutures (as frequently referred to) but are instead sulci marking internal costae. Because costae are typically employed for functional purposes of strengthening the head capsule, it is perhaps not surprising that as the functional requirements of the insects vary, so do the position and shape of these sulci. The sole exception among extant insects is the

Occipital Sulcus Beetles

mandible

4.2. Basic head and mouthpart morphology of insects, illustrated with a grasshopper.

postoccipital suture. This line marks the boundary between the labial and maxillary segments of the head and is evident from between the labium and maxillae, demarcating behind it a very narrow region called the postocciput. The occipital sulcus is just anterior to the postoccipital suture and defines, between these two lines, the occiput. Frequently, the postoccipital suture is faint or indistinct, and the bulk of the back of the head is composed of the occiput. The frontal line delineates a region on the front of the head called the frons and connects to the coronal line that runs dorsally toward the vertex. The coronal line is most often not present in adults but is easily seen in most larvae. The frontal and coronal lines (together referred to as the epicranial lines) are ecdysial cleavage lines, despite being referred to by many entomologists as sulci or sutures, and are where the integument breaks during molting. Circumscribing the head on its inner surface, near where the mouthparts articulate, is a continuous costa. Externally, this costa is visible by a line that is given different names on different regions of the head. The utility of this complex naming system is that it is immediately clear whether one is referring to the front, side, or back of the head, but it does obscure the fact that the different names refer to a single continuous line and internal ridge. The epistomal (or frontoclypeal) sulcus is that portion of the line that marks the boundary between frons and clypeus on the anterior surface of the head. The clypeus is that region of the head below the epistomal sulcus and anteriorly articulates with the movable labrum (Figure 4.2). The subgenal sulcus is that portion of the line on the side of the head, defining a region called the subgena between the subgenal sulcus and the mouthparts. The subgenal sulcus is often subdivided into two parts: the pleurostomal sulcus above the mandible (the sclerite below this is the pleurostoma) and the hypostomal sulcus behind the mandible (the sclerite below this is the hypostoma). Some insects, such as crickets (Orthoptera: Ensifera) have a sulcus between the bottom of the compound eye and the epistomal-subgenal sulcus. This is the subocular sulcus. There are also sulci around the sockets of the compound eyes and the antennae; these are the circumocular sulci and the circuman-tennal sulci.

The head capsule contains an internal skeleton called the tentorium, which not only provides strength to the head but also provides some sites for the attachment of muscles that insert on mouthparts. On the external surface of the head, the existence of the tentorium is evidenced by anterior and posterior tentorial pits. Within the head the tentorium is typically an H-shaped structure with anterior and posterior arms on each side; it is joined in the middle by a tentorial bridge but generally positioned toward the back of the head. Frequently, the dorsal arms of the tentorium extend toward the upper surfaces of the head capsule. The anterior tentorial pit is located in the epistomal/subgenal sulcus, while the posterior tentorial pit is located in the postoccipital suture. The postgena is that region posterior and ventral to the gena and above the subgenal sulcus. In some prognathous insects the region just proximal to the posterior tentorial pits is expanded anteriorly from the neck region to form a median sclerite called the gula (e.g., Coleoptera).

Mouthparts

The mouthparts principally consist of three pairs of appendages, each pair corresponding to a fused segment. These are (from anterior to posterior): mandibles, maxillae, and labium (Figure 4.2). As already discussed, there are two major divisions of the mouthpart appendages within the Hexapoda: entognathous and ectognathous. In Collembola, Protura, and Diplura the mouthparts are recessed into a pocket within the head capsule, a condition called entog-nathy. These mouthparts can be extruded during feeding and withdrawn at other times. The Insecta (also known as Ectog-natha) consist of all remaining hexapod orders, and their mouthparts are not retracted into the head capsule and are dramatically modified in various lineages. The groundplan morphology for the major structures is outlined next under their respective head segments.

Head Segmentation

Most insects have the following head appendages: antennae, mandibles, maxillae, and a labium. The last three are widely accepted as serially homologous with the thoracic legs and thereby attest to at least four ancestral segments, or metameres, comprising the insect head capsule. The principal criteria for identifying a metamere (proposed by Rempel, 1975) include a pair of appendages, a pair of apodemes corresponding to the appendages, a neuromere (i.e., a ganglion associated with the ancestral segment), and a pair of mesodermal somites (coelomic sacs during development, which also correspond to the ancestral body cavity of the original segment). Using these criteria, insects appear to have six or perhaps seven fused segments in the head, although the debates continue to rage. For now there are six purported head segments.

The Preantennal Segment. This segment is a bit counterintuitive because there is no obvious pair of appendages anterior to the antennae. In fact, some authors dismiss the idea of a preantennal segment entirely (e.g., Snodgrass, 1935: despite the fact that he himself cites some embryological studies that identify "a pair of evanescent appendage-like lobes in the embryo ... lying anterior to the antennae"). The appendages of the preantennal segment (argued by Rempel, 1975) are the fused components that form the labrum. The labrum in insects is an articulated sclerite anterior to the clypeus. The labrum is clearly associated with coelomic sacs in development and forms embryologically from the fusion of two appendage-like outgrowths of the first segment (e.g., in Pieris, Tenebrio, Lytta, some Phasmatodea, and almost any other insect that has been studied). Thus, it appears that the labrum is the remnant of an ancestral appendage.

The Antennal Segment. The origin of antennae has been of some debate and has been interpreted in two ways: (1) as a pair of modified appendages serially homologous with mouthparts and thoracic legs (favored by Rempel and most other workers) or (2) as sensory structures associated with the presegmental part of the body, at least analogous, if not homologous, with tentacles on the heads of certain worms (an interpretation favored by Snodgrass). Modern developmental and phylogenetic evidence favors the first hypothesis and antennae meet all four of the criteria for a true segment discussed earlier. Antennae arise as lateral outgrowths of the segment, the antennal buds are provided with large coelomic cavities, the antennae have their own neuromere (which becomes the deutocerebrum of the brain), and apodemal invaginations are also present. The antennae are composed of three principal units (from base to apex): scape, pedicel, and flagellum (itself subdivided into flagellomeres). In primitive hexapods (i.e., Collembola and Diplura; the Protura have lost their antennae), there are intrinsic muscles to each fla-gellomere; in all other groups the scape and pedicel only have musculature. The scape is inserted within a membranous antennal socket and articulates via a single point called the antennifer. The pedicel is typically a small joint in insects. In adult insects the pedicel contains the Johnston's organ (a chordotonal organ, or specialized group of cells designed to detect deformations of the cuticle). The flagellum is variable in length and shape and frequently divided into annuli or fla-gellomeres, which are joined to each other by membranes so that it is overall quite flexible. Minute sensory structures (sensilla) are most abundant on the flagellum, and nerves traversing the flagellum are entirely sensory, not motor neurons.

The Intercalary Segment. There is now conclusive evidence for a segment between the antennae and the mandible of insects (believed to be the homologue of the second antennal segment of Crustacea). This segment is evident as a coelomic cavity in development and is associated with the tritocere-brum, although in the later stages of development the trito-cerebrum becomes secondarily associated with the labrum.

The Mandibles. The mandibles of hexapods principally vary in their articulation with the head capsule. The point at which a mandible articulates with the head is called a condyle. The condyle is a ball-like structure that fits into a socket, called the acetabulum, to form the articulation. Two types of mandibles are known. Monocondylic mandibles articulate via a single, dorsal condyle and therefore make a rotary motion around this point (Figure 5.9). The condyle is on the mandible and the acetabulum on the head capsule. Dicondylic mandibles articulate via two condyles (one anterior condyle and one posterior condyle) (Figures 4.2, 5.9). Thus, the mandible can only move in a single plane of motion. The posterior condyle is homologous with the dorsal condyle of monocondylic mandibles; at the posterior point of articulation, the ancestral condyle is on the mandible and the acetabulum on the head capsule; the anterior articulation point is the novel feature of the dicondylic hexapods, and the morphology is reversed - condyle is on the head capsule and acetabulum is on the mandible.

The Maxillae. The maxilla has five basic components: cardo, stipes, galea, lacinia, and maxillary palpus (Figure 4.2). The homology of mouthparts with leg structures is most obvious in the maxillae and is identical in insects with generalized chewing mouthparts. The cardo has a monocondylic articulation with the head, identical to that of a mandible.

The Labium. The labium is essentially a fused set of maxillae and homologous with the "second maxillae" of the Myri-apoda and Crustacea (Box 4.2). Interestingly, in embryonic development the labium starts off as separate limb buds (like the mandibles and maxillae), which then fuse. The main elements are postmentum, prementum, paraglossa, glossa, and labial palpus (Figure 4.2). The postmentum is typically subdivided into two sclerites: mentum (anterior) and submen-tum (posterior). The ligula is a general term referring to the glossa and paraglossae; these are often fused into a single structure in holometabolous insects. Anterior to the labium is a lobe that forms the posterior wall of the mouth (or gnathal) region called the hypopharynx. A small cavity is formed between the posterior wall of the hypopharynx and the anterior surface of the labium called the salivarium. The labial salivary glands open into the salivarium. Posterior to the labrum is a lobe called the epipharynx. Between the hypopharynx and epipharynx is a cavity called the cibarium.

Some authors hypothesized that there is an additional segment in the head associated with the compound eyes, and that the eyes are modified appendages on a hypothetical metamere called the ocular segment (see discussion

BOX 4.2. Serial Homology Between Maxillary and

Labial Structures

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