From their annelid and marine arthropod ancestors, insects and their terrestrial relatives have inherited a segmented body. Between an anteriormost (acron) and pos-teriormost segment (telson), a varying number of segments are interposed, depending on the group. There were originally 18 (or 19, if a second antennal segment is recognized) in insects, 19 in chelicerates, and as many as 100 in myriapods. These were more or less equal in form and in the possession of a pair of walking appendages in the first terrestrial arthropods, much like modern-day centipedes. Evolution eventually favored the fusion of adjoining segments (a process called tagmosis) for various functional purposes (e.g., flight in higher insects), and body regions were formed. Of these, insects display a triple set, including the head (composed of the acron plus four or five highly fused original segments), a thorax (of three segments, pro-, meso-, and metathorax) and abdomen (with eleven segments, the posteriormost being highly modified into genitalia).

Arachnids and myriapods show different patterns of fusion. In the former, the head is undefined and its segments totally incorporated into the thorax (céphalothorax), which itself may also join into the abdomen, as in mites and ticks. Segmentation in these anterior two regions is concealed by a shield or carapace and is evident ventrally only by the serial set of appendages. The abdomen is either undefined or formed from several segments. Myriapods display only a well-developed head and uniformly segmented thorax-abdomen, with each segment bearing similar legs.

Uniquely, the insect thorax may bear a pair of wings on the meso- or metathorax, or both, but never on the prothorax. Thus, according to the appendages they possess, the three body regions are specialized for separate functions, the head for ingestion and perception, the thorax for locomotion, and the abdomen for metabolic processes and reproduction.

The Head and Its Appendages

The head (Matsuda 1965) is the most highly modified body region, being a separate organ (except in arachnids), in which the primitive segmentation is almost obliterated. It is normally a rigid capsule, containing the main perceptive and integrative neural elements of the animal as well as ingestive organs.

The many sensory appendages of the head include the antennae in insects, centipedes, and millipedes, all with one pair. Arachnids lack antennae, their place usually being assumed by the pedipalps that have become antennalike. However, in some arachnids, the pedipalps take other forms and functions, as the claws of scorpions or walking legs in sun spiders.

Around the mouth, modified segmented appendages serve as jaws or stylets for chewing or imbibing liquids and bear food-tasting and smelling organs called palpi. In arachnids, these organs are the chelicerae, with the basic scissor form, but they are used directly in feeding by tearing or stabbing the food, not chewing. The chelicerae may lose the movable element and become a piercing needle in mites, especially parasitic ones, and in spiders they are modified into fangs. Arachnids use the inner portion of the leg coxae to scoop liquid nutrient into the simple mouth. Among insects and millipedes, there are two pairs of jaws, the anterior mandibles and behind them the maxillae; centipedes have two sets of maxillae. In insects, the mandibles and maxillae may retain a primitive toothed or molar form for biting and chewing solid foods, or they may be greatly elongated and bladelike or hypodermiclike for piercing and siphoning. The labium may form only a supportive sheath around the latter or be itself spongy and absorptive and act directly in food collection. Flexibility in adaptation of mouth parts has been a major factor in the success of insects as a group, the variety of morphological types making possible an enormous diversity of food niches and feeding strategies.

Although not of appendicular origin, the eyes are of major sensory importance to the head capsule (Horridge 1975). There is a pair of larger multifaceted compound eyes in adult insects laterally and usually one to three smaller, single-faceted simple eyes medially on top of the head. In other terrestrial arthropod groups and immature insects, only simple eyes (ocelli) are present, either in lateral clumps on the sides of the head (millipedes) or on the back of the cephalothorax (arachnids). Eyes are also often absent altogether (many centipedes).

The structure and function of the compound eyes are complex. They bulge out on either side of the head to give a wide range of vision in all directions. Each is an aggregation of similar rod-shaped facets called ommatidia, the number of which varies from one per eye in some ants to over 10,000 in dragonflies. Each ommatid-ium is composed of elongate sensory cells containing light-sensitive pigments, these concentrated toward the center (thus seen as a dark rod, called the rhabdome) and exposed on the exterior through a capping, duplex lens that gathers and focuses light. There are also cells with diffuse pigment around the lens. The sensory cells are nerve cells and are connected directly to the brain, there being no optic nerve in insects.

There are many variations in the detailed structure of the ommatidium, such as the "apposition" versus the "superposition" types. In the former, the rhabdome is long, and the diffuse pigment cells isolate each ommatidium. In the latter, the rhabdome is short, and the screening pigment moves depending on the amount of light in the environment. Image formation is believed to be basically different in the two types, but little is certain about this aspect of eye function. It is known that insects generally have good visual acuity and light level accommodation. Wavelength discrimination varies considerably, with a tendency toward the ultraviolet in many species (Silberglied 1979). Many insects, such as bees and butterflies, have good color vision and can orient by polarized light.

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