Box

Dorsal

Dorsum: the entire upper portion of a segment or of the whole insect.

Tergum (pl. terga): the dorsal exoskeletal plate or plates of a segment (also called notum; pl. nota).

Tergite: a subdivision of the tergum.

Lateral

Pleural Area: the lateral portions of a segment or of the whole insect.

Pleuron (pl. pleura): the lateral exoskeletal plate or plates of a segment.

Pleurite: a sclerotized subdivision of the pleuron.

Ventral

Venter: the entire undersurface of a segment or of the whole insect.

Sternum (pl. sterna): the ventral exoskeletal plate or plates of a segment.

Sternite: a subdivision of the sternum.

The thorax consists of three metameres, the abdomen consists primitively of 11 metameres (the eleventh metamere is lost in almost all holometabolous insects), while the head consists of an uncertain number. Even though these tagmata are dramatically modified in some groups, typically the function of the head is for sensory input and feeding, the thorax is for locomotion, and the abdomen is for visceral functions, mating, and various modes of sensory input.

Insects are, naturally, arthropods; therefore, they are encased by a chitinized cuticle. The cuticle provides protection, support, and locomotion; prevents water loss via a wax layer; provides a site for waste product deposition; protects from ultraviolet radiation; serves communication functions (inter- and intraspecifically) via hydrocarbons on the surface or through coloration, etc. The cuticle is divided into a series of distinct plates or sclerites. The boundaries of sclerites may or may not correspond to boundaries between body segments (frequently they do not). Differing terms are given to sclerites located on different parts of the body (Box 4.1).

Another important feature of the integument is the externally visible lines used to subdivide the sclerites. Sutures are intersegmental lines, demarcating two primitive metameres, now fused (e.g., postoccipital suture). This term is almost universally misapplied by insect systematists, biologists, and others for any line on the integument of the insect. It is important to be precise, however, because this will greatly affect our interpretations as to where particular structures originally derived. A sulcus (pl. sulci) is a line marking where an internal ridge is formed for muscle attachment (e.g., epis-tomal sulcus). The internal ridge associated with the sulcus is called a costa (sometimes the costa forms a large, internal plate called a phragma). An external ridge (i.e., an evagination, rather than invagination) is called a carina or crest. Costae and carinae are used either to strengthen the cuticle or to serve as a site for muscle attachment. An ecdysial cleavage line is a point of weakness in the cuticle where it splits at molting (e.g., epicranial "suture," or, more correctly, epicranial line). Such lines are never indicated internally by a costa or phragma. Apodemes (synonymous with apophysis) are invaginations of the cuticle that form a rod or bar; frequently the external sign of such a structure is a pit. Not all apodemes have externally visible pits to betray their existence. For example, abdominal terga and sterna typically have long, lateral processes to which muscles attach, and these are apodemes; mandibles are another example since they have large apodemes for the attachment of strong mandibular muscles but no pit.

Studying insect segmentation is more complicated than simply seeking suture lines. Except for the most primitive lineages, most insects show some degree of secondary segmentation in the thorax and abdomen (Figure 4.1). Secondary segmentation is often most evident in the abdomen. Terga overlap each other posteriorly, such that the posterior border

4.1. Stages in the development of secondary segmentation in insects.

intersternite

4.1. Stages in the development of secondary segmentation in insects.

of tergum overlaps the anterior border of the tergum behind it. Along the anterior edge of each tergum is a costa called the antecosta (and the associated suture is the antecostal suture; as the name implies, this is a true intersegmental suture). The antecosta, however, is frequently not exactly on the leading edge of the tergum and the sclerotized strip anterior to it is the acrotergite. Immediately anterior to the acrotergite is a membranous region (conjuctiva) marking the region between adjacent terga. By comparing the structure of the terga and sterna as well as the associated internal organs across higher arthropods and throughout the insects, we see that the antecosta marks the true intersegmental boundary and the acrotergite belongs to the preceding body metamere. Thus, the visible membranous regions between terga are a form of secondary segmentation. The primary segmentation is only apparent internally by the arrangement of costae and muscles. Primary segmentation is retained in Onychophora, Tardigrada, and some other Arthropoda as well as in embryonic forms of insects. Expansion of the acrotergite in the thorax of many insects leads to the development of a postnotum. Primitively (or in embryonic forms) each notum has a suture separating it from others and the associated costa is the attachment site for internal muscles. The simplest form of secondary segmentation is where the posterior portion of the notum separates and becomes the acrotergite of the following notum. The next most complicated form of secondary segmentation is that in which the antecostae become developed into phragmata and some thoracic acrotergites expand to form distinct postnota. The ultimate level of complexity is when the postnotal plates and associated phragmata become separated by secondary membranes and appear dissociated upon superficial examination.

In the sterna of pterygote insects, the overlapping pattern in the thorax is just the opposite of the tergal pattern, but this pattern returns to normal in the abdomen (i.e., the posterior border of the sternum is concealed underneath the anterior margin of the immediately following sternum). The sternum of the third thoracic segment, however, overlaps on both ends the preceding and following sterna, and abdominal sterna then return to normal overlapping orientation. Sometimes the sterna are fragmented, reduced, or lost entirely . . . thus confusion reigns! The sternum is subdivided into a eusternum and a spinasternum. The eusternum can be further subdivided into a basisternum (anteriorly) and a sternel-lum (posteriorly), marked externally by a transverse median line, the sternacostal sulcus. The eusternum bears two large sternal apophyses visible externally as the apophyseal pits. These, along with the spina (see discussion that follows) serve as the attachment sites for ventral thoracic muscles. These can either fuse or be attached via muscles to pleural apophyses. On thoracic sterna, the costa marking the inter-segmental line is reduced to a spine, called the spina. The spinasternum is the sclerite that bears the spina; it marks the true intersegmental boundary. When coxae are close to the ventral midline, the sterna tend to be reduced. The sternal apophyses may fuse to form a furca (internally) and leave externally only the apophyseal pit. The eusternum may be reduced to form a tiny furcasternum (= to the portion sometimes called the sternellum) or an internal cryptosternum indicated externally by a single median, longitudinal line called the discrimen.

The advantage of all this complexity created by secondary segmentation is that it provides a series of landmarks for associating structures with their original, ancestral segments (i.e., metameres), important for careful comparative morphology.

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