Insect Behavior

Insect behavior (Matthews and Matthews 1978) is a rapidly developing field of study that attempts to explain both the complex anatomical and physiological bases and higher, integrative mechanisms for activity. Only short-term, decisively determined actions are recognized in this framework. Long-lasting, slowly induced actions, such as diapause or maturation, are considered physiologic or developmental phenomena (see other parts of this chapter).

Physiochemically and anatomically, insects possess the same elements that control behavior in all animals. Foremost of these is the nervous system (Roeder 1963), including its sensory component, but the muscular and hormonal components play an essential, if secondary, part. It is the degree of complexity of the first that determines the levels on which lines of action lie.

A key element of the nervous system in determining behavior is the associative (ad-justor, internuncial) neuron, which intercedes between receptor (efferent) and effector (afferent) neurons and has the capacity to redirect and modify otherwise simple reflex reactions. Large numbers of these form masses (neuropiles) in the brain and ventral ganglia and serve as centers of neural integration. These are something like the cortex or gray matter of the human brain and define the overall function of a ganglion. They represent the main areas where activities are generated and organized. A major such center is the corpus pe-dunculatum ("mushroom-shaped body"), believed to be the site of summation of simultaneous excitation from all sources. It tends to be small in arthropods with simple behavior, large in those with complicated lives, such as the social Hymenoptera. These cells both stimulate and inhibit.

Endocrine secretions are not only caused to flow in response to nervous command but are actually part of the nervous system in the form of neurosecretory cells. These cells produce hormones that move along the axons and direct other nerve and endocrine tissues to emote.

Of course, activity is finally the result of muscular contraction. I nsects and their relatives may have very large numbers of discrete muscle bundles that predicate a likewise elaborate system of efferent nerves. It is fortunate that a lack of obstructive connective tissue in these animals makes it possible to dissect and experiment to determine pathways relatively easily. The largest nerves lead to the most active locomotor organs, the wings and legs. Other major efferents control the mouthparts, antennae, cerci, genitalia, and numerous other muscularized structures.

The insect behaviorist looks for chains or pathways of stimulation-integration-action to explain activities (Browne 1974). The latter can be considered to be composed of bits or units that meld together into sequences first, then complexes or systems. The simplest movements have the simplest nerve control and fewest muscles involved. The most complex systems have very large numbers of pathways and processes and are so complicated that it is possible to analyze them only in general. An understanding of the way the whole insect acts requires an extension of the rudimentary functioning of the neural, hormonal, and muscular elements. This extension progresses along a scale of in creasing complexity, beginning with so-called automatic or instinctive behavior and terminating with learned activity.

The simplest instinctive actions are reflex arcs, so-called knee-jerk responses, where a part of the body reacts directly to a stimulus without the intercession of an association nerve. An example is the retraction of the tarsus from a hot surface. A step up from this level occurs when the whole body is coordinated but by nonmodifiable reactions. Where only a single action is identifiable, such as movement away from or toward light or touching or shunning other individuals or objects, the behavior is called a taxis or tropism. Such behavior may be positive or negative. The attraction of moths to artificial light, the catatonic freezing or "death feigning" display many species use to escape harm, and the following of odor trails by dung beetles to find food for their young are specific examples.

A series of these tropistic elements may be strung together, one triggering the next to form a fixed action pattern. These may take up a sizable part of the behavioral repertoires of most insects. Pupation in giant silk moth larvae offers an appropriate example: changes in photoperiod or some internal stimulus causes them to cease feeding. This initiates defecation and a wandering, searching activity, leading to the discovery of a suitable pupation site. Even if the latter is not found, the larvae will begin to spin silk and form a cocoon of a specific shape in which it finally settles and pupates. This sequence follows the same steps regardless of changes in external stimuli (unless acute) and does not vary according to any information learned by the individual.

Insects and other terrestrial arthropods are capable of limited learning (Alloway 1972), defined as any relatively permanent change in behavior that results from practice. Such learning is of a low order and often short lived, but it is often essential to the animal's existence. At least two types have been seen, classical Pavlovian conditioning and, much more commonly, instrumental conditioning, where reinforcement stimuli direct the performance of the insect. The latter is a characteristic especially of social insects, like the honeybee, which can be trained artifically to fly to a colored surface by food offerings. Under natural conditions, this ability is important in recruiting foragers and in efficient utilization of a flower nectar food source. Some forms, such as cockroaches and ants, facilitate to mazes. The vast majority of these arthropods, however, probably are capable of virtually no learning whatever.

The complexity of some behavior in insects, particularly social insects, most especially ants, whose lives parallel our own in some ways, has suggested to some the possibility of the existence of intelligence. As possessed by higher vertebrates, including ourselves, no such high degree of learning and reasoning can be truly ascribed to these creatures. All activity, regardless of how cunning and comprehending it seems, can be explained on the basis of fixed action sequences, with very limited learning. The nesting of digger wasps (Ammophila) is a classic example: the female wasp first digs a burrow in sandy soil which it then closes over at the mouth. It then leaves to search for prey, captures it, and returns to the location of the burrow. To do this, it has had to learn a few landmarks by which it navigates. Their misplacement, however, may lead the digger wasp to conclude wrongly on the exact location. The nest, when found, is opened and the prey packed within, an egg is laid on it, and the female exits, closes the nest permanently, and leaves to repeat the process elsewhere. All of these are innate, unmodi-fiable acts.

The remarkable thing about insect behavior is that it may be highly complicated, comparable in this respect alone to vertebrates, yet it is nearly all controlled by instinctive mechanisms. Fundamental life processes are thus served efficiently, although automatically and unswervingly, and have contributed to their success as a group.

It is useful to segregate and classify the kinds of motivation driving the insect body because it is often found that single action sequences operate within them. The following are only representative, as many examples fit into the categories given; additional types will appear in the main text of this book.

1. Alimentation. Finding food and feeding involve specific movements, often elaborate. Mosquitoes respond to visual and odor cues to find warm-blooded hosts and then follow tactile stimuli to select a proper station and find a capillary. Internal pressure from expansion of the stomach causes cessation of feeding and induces flight.

2. Survival. Its host, discovering a mosquito in the act of feeding, will attempt to destroy or remove it. The insect displays flight as a survival act, an extremely common one with winged types. Other survival-related behavior is shelter seeking, catalepsis, and biting. Most protective coloration is accompanied by postures that enhance deception or warning patterns.

3. Aggression. Both intra- and interspecific agonistic (fighting) behavior occurs in insects, including male-male competition for females, as in the horned scarabs. Bees may grapple for a nectary or over territory and females. Raiding for food, such as found in many ants, should not be confused with aggression, although the results are the same. The vanquished colony is perceived as food, not as a rival faction.

4. Sex. This essential, overriding drive in all organisms has led to some of the most incredibly complex and even bizarre activities in all groups of terrestrial arthropods. These are divided into mate finding, courtship, copulation, and insemination (Thornhill and Al-cock 1983).

5. Brood care. Parental behavior occurs in relatively few insects and other terrestrial arthropods and is a precursor to social organization in general. It greatly increases survivability and is necessary for the maintenance of colonies.

6. Intraspecific communication. The ways in which information is transmitted between individuals of the same species are tremendously varied, employing visual, chemical, auditory, tactile, and other methods. The use of airborne pheromones seems to dominate, although nutritive chemicals, ingested by the receiver (trophallaxis), are transmitted among members of social insect colonies. Sound also ties many nonsocial types together.

7. Tool using. It is an amazing fact that a few insects actually use tools—in an instinctive way, of course. The prime example is the pebble employed by digger wasps to tamp the soil plug of their burrow nests.

8. Construction. Many types form structures from a variety of building materials, both extraneous (mud, paper, wood) and intrinsic (silk). Architecture may be elaborate and the size and strength of many edifices prodigious. A high level of cooperation can be required between members of social forms to put up nests. Individual efforts are also intricate and consistent with regard to geometry and engineering.

9. Migration. A large number of species regularly move from one territory to another, some even on long-established and precise migratory routes. Unidirectional flight is a conspicuous manifestation of this behavior, and it is most conspicuous in larger, active forms such as butterflies and day-flying moths.

Some behavioral traits apparently not found in insects and their relatives are play, expression of grief or sorrow, and humor. These are characteristics of a vertebrate cerebrum and set these higher creatures apart from insects and other arthropods, which function as virtual automatons.

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