The Head

The rigid cranial capsule has two openings, one posteriorly through the occipital foramen to the prothorax, the other to the mouthparts. Typically the mouthparts are directed ventrally (hypognathous), although sometimes anteriorly (prognathous) as in many beetles, or posteriorly (opisthognathous) as in, for example, aphids, cicadas, and leafhoppers. Several regions can be recognized on the head (Fig. 2.9): the posterior horseshoe-shaped posterior cranium (dorsally the occiput) contacts the vertex dorsally and the genae (singular: gena) laterally; the vertex abuts the frons anteriorly and more anteriorly lies the clypeus, both of which may be fused into a frontoclypeus. In adult and nymphal insects, paired compound eyes lie more or less dor-solaterally between the vertex and genae, with a pair of sensory antennae placed more medially. In many insects, three light-sensitive "simple" eyes, or ocelli, are situated on the anterior vertex, typically arranged in a triangle, and many larvae have stemmatal eyes.

The head regions are often somewhat weakly delimited, with some indications of their extent coming from sutures (external grooves or lines on the head). Three sorts may be recognized:

1 remnants of original segmentation, generally restricted to the postoccipital suture;

2 ecdysial lines of weakness where the head capsule of the immature insect splits at molting (section 6.3), including an often prominent inverted "Y", or epi-

cranial suture, on the vertex (Fig. 2.10); the frons is delimited by the arms (also called frontal sutures) of this "Y";

3 grooves that reflect the underlying internal skeletal ridges, such as the frontoclypeal or epistomal suture, which often delimits the frons from the more anterior clypeus.

The head endoskeleton consists of several invaginated ridges and arms (apophyses, or elongate apodemes), the most important of which are the two pairs of tento-rial arms, one pair being posterior, the other anterior, sometimes with an additional dorsal component. Some of these arms may be absent or, in pterygotes, fused to form the tentorium, an endoskeletal strut. Pits are discernible on the surface of the cranium at the points where the tentorial arms invaginate. These pits and the sutures may provide prominent landmarks on the head but usually they bear little or no association with the segments.

The segmental origin of the head is most clearly demonstrated by the mouthparts (section 2.3.1). From anterior to posterior, there are six fused head segments:

1 labral;

2 antennal, with each antenna equivalent to an entire leg;

3 postantennal, fused with the antennal segment;

4 mandibular;

5 maxillary;

6 labial.

The neck is mainly derived from the first part of the thorax and is not a segment.

2.3.1 Mouthparts

The mouthparts are formed from appendages of all head segments except the second. In omnivorous insects, such as cockroaches, crickets, and earwigs, the mouthparts are of a biting and chewing type (mandibulate) and resemble the probable basic design of ancestral pterygote insects more closely than the mouthparts of the majority of modern insects. Extreme modifications of basic mouthpart structure, correlated with feeding specializations, occur in most Lepidoptera, Diptera, Hymenoptera, Hemiptera, and a number of the smaller orders. Here we first discuss basic mandibulate mouthparts, as exemplified by the European earwig, Forficula auricularia (Dermaptera: Forficulidae) (Fig. 2.10), and then describe some of the more common modifications associated with more specialized diets.

There are five basic components of the mouthparts:

1 labrum, or "upper lip", with a ventral surface called the epipharynx;

2 hypopharynx, a tongue-like structure;

3 mandibles, or jaws;

4 maxillae (singular: maxilla);

The labrum forms the roof of the preoral cavity and mouth (Fig. 3.14) and covers the base of the mandibles; it may be formed from fusion of parts of a pair of ancestral appendages. Projecting forwards from the back of the preoral cavity is the hypopharynx, a lobe of probable composite origin; in apterygotes, earwigs, and nymphal mayflies the hypopharynx bears a pair of lateral lobes, the superlinguae (singular: superlingua) (Fig. 2.10). It divides the cavity into a dorsal food pouch, or cibarium, and a ventral salivar-ium into which the salivary duct opens (Fig. 3.14). The mandibles, maxillae, and labium are the paired appendages of segments 4 -6 and are highly variable in structure among insect orders; their serial homology with walking legs is more apparent than for the labrum and hypopharynx.

The mandibles cut and crush food and may be used for defense; generally they have an apical cutting edge and the more basal molar area grinds the food. They can be extremely hard (approximately 3 on Moh's scale of mineral hardness, or an indentation hardness of about 30 kg mm-2) and thus many termites and beetles have no physical difficulty in boring through foils made from such common metals as copper, lead, tin, and zinc. Behind the mandibles lie the maxillae, each consisting of a basal part composed of the proximal cardo and the more distal stipes and, attached to the stipes, two lobes - the mesal lacinia and the lateral galea - and a lateral, segmented maxillary palp, or palpus (plural: palps or palpi). Functionally, the maxillae assist the mandibles in processing food; the pointed and sclerotized lacinae hold and macerate the food, whereas the galeae and palps bear sensory setae (mechanoreceptors) and chemoreceptors which sample items before ingestion. The appendages of the sixth segment of the head are fused with the sternum to form the labium, which is believed to be homologous to the second maxillae of Crustacea. In prognathous insects, such as the earwig, the labium attaches to the ventral surface of the head via a ventromedial sclerot-ized plate called the gula (Fig. 2.10). There are two main parts to the labium: the proximal postmentum, closely connected to the posteroventral surface of the

coronal suture epicranial suture (an ecdysial line)

frontal suture

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