414 neuromodulators are released from the tip of an adjacent neuron (less commonly as a neurohormone released into the hemolymph) and act on the presynaptic or postsynaptic membrane adjacent to, but not within, the synaptic gap or neuromuscular junction. Their effects include reduction in the amount of neurotransmitter released and inhibition of the action of the neurotransmitter. Amines, especially octopamine, and some neuropeptides (e.g., proctolin) are likely to be important neuromodulators, though in many instances definitive evidence is still lacking. A probable neuromodulator of a special kind may be nitric oxide. This very short-lived, rapidly diffusing gas was discovered in nervous tissues of locusts, honey bees, and Drosophila in the early 1990s. Production of nitric oxide is especially rich in interneurons in the antennal and optic lobes, as well as in antennal chemosensory cells of some species, following appropriate olfactory and visual stimulation, suggesting that this unconventional neuromodulator may have roles in olfactory information processing, olfactory memory, and vision (Muller, 1997; Bicker, 1998).

In insects reflex responses are segmental, that is, a stimulus received by a sense organ in a particular segment initiates a response that travels via an interneuron located in that segment's ganglion to an effector organ in the same segment. This is easily demonstrated by isolating individual segments. For example, in an isolated thoracic segment preparation of a grasshopper, touching the tarsus causes the leg to make a stepping movement. Of course, in an intact insect such a stimulus also leads to compensatory movements of other legs to maintain balance or to initiate walking, activities that are coordinated via association centers in the subesophageal ganglion. Touching the tip of the isolated ovipositor in Bombyx, for example, initiates typical egg-laying movements, provided that the terminal ganglion and its nerves are intact. In other words, each segmental ganglion possesses a good deal of reflex autonomy.

Nervous activity of the type described above, which occurs only after an appropriate stimulus is given, is said to be exogenous. However, an important component of nervous activity in insects is endogenous, that is, does not require sensory input but is based on neurons with intrinsic pacemakers. Such neurons (non-spiking neurons) possess specialized membrane regions that undergo periodic, spontaneous changes in excitability (permeability) and where impulses are thereby initiated. A wide variety of motor responses are organized, in part, by endogenous activity. For example, ventilation movements of the abdomen are initiated by endogenous activity in individual ganglia. Even walking and stridulation are motor responses under partially endogenous control (Huber, 1974). An obvious question to ask, therefore, is "Why don't insects walk or stridulate continuously?" The answer is that these and all other motor responses are "controlled" by higher centers, specifically the brain and/or subesophageal ganglion. These association centers assess all information coming in via sensory neurons and, on this total assessment, determine the nature of the response. In addition, the centers coordinate and modify identical segmental activities, such as ventilation movements, so that they operate most efficiently under a given set of conditions.

Early evidence for the role of the brain and subesophageal ganglion as coordinating centers came from fairly crude experiments in which one or both centers were removed and the resultant behavior of an insect observed. More recent experiments involving localized destruction or stimulation of parts of these centers have confirmed and added significantly to the general picture obtained by earlier authors. To illustrate the complexity of coordination and control of motor activity, walking will be used as an example. This rhythmic stepping movement of each leg is controlled by a network of non-spiking neurons (called the central pattern generator and located in each half ganglion) whose endogenous activity sends signals (via motor neurons) alternately to the extensor and flexor leg muscles (Chapter 14, Section 3.2.1). The signals may be excitatory or inhibitory and, in effect, serve to switch on or off the muscles. Intraganglionic and intersegmental coordination among the central pattern generators, and ultimately leg movements, is achieved via normal interneu-rons. This is readily shown by cutting even one connective of the pair between adjacent ganglia when coordinated stepping is disrupted. Though the overall control of walking, that is, starting, stopping, turning, and change of speed, resides in the brain, the subesophageal ganglion is also involved. Removal of the latter, for example, sensitizes some insects so that they walk incessantly in response to even slight stimuli. In the brain the mushroom bodies and central body play major roles in the regulation of walking. Impulses originating in the mushroom bodies inhibit locomotor activity, presumably by decreasing the excitability of the subesophageal ganglion. Moreover, reciprocal inhibition may occur between the mushroom body on each side of the brain, and this is the basis of the turning response. In contrast, the central body appears to be an important excitatory system in locomotion, because its stimulation evokes fast running, jumping, and flying in some species. As yet, however, the interaction between these two cerebral association centers is not understood.

Superimposed on the central control of walking is the influence of sensory stimuli received by the insect; that is, the insect adjusts its walking pattern to suit environmental conditions such as movement uphill or downhill, along a slope, or over rough terrain. To this end, the legs are equipped with a variety of mechanoreceptors that provide information on their position, loading, and movement (Chapter 12, Section 2). In a walking Colorado potato beetle, contact between the antennae and an obstacle causes the insect to modify its body angle. The extent to which the body angle is changed is proportional to the height of the obstacle, allowing the beetle to extend the reach of the prothoracic leg so as to step up on to the obstacle. Insects that use running to escape predators receive information via other sensory pathways. For example, in the cockroach escape reaction, even the slightest air movements stimulate hairs on the cerci that are both velocity- and direction-sensitive. The information received travels via giant axons in the ventral nerve cord to the thoracic ganglia to initiate both the running and the turning away responses within 0.5 msec of the stimulus being received.

At the outset, insect behavior is dependent on the environmental stimuli received, though, as noted earlier, not all behavior patterns originate exogenously; many common patterns have a spontaneous, endogenous origin. Axons may be branched; synapses may be convergent or divergent; temporal or spatial summation of impulses may occur at synapses; neurons may be excitatory or inhibitory in their effects. Thus, an enormous number of potential routes are open to impulses generated by a given set of stimuli. The eventual routes taken and, therefore, the motor responses that follow, depend on the size, nature, and frequency of these stimuli.

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