4. Sensory systems and behavior

In the opening chapter of this book we suggested that the success of insects derives at least in part from their ability to sense and interpret their surroundings and to discriminate on a fine scale. Insects can identify and respond selectively to cues from a heterogeneous environment. They can differentiate between hosts, both plant and animal, and distinguish among many micro- climatic factors, such as variations in humidity, temperature, and air flow.

Sensory complexity allows both simple and complex behaviors of insects. For example, to control flight, the aerial environment must be sensed and appropriate responses made. Because much insect activity is nocturnal, orientation and navigation cannot rely solely on the conventional visual cues, and in many night- active species odors and sounds play a major role in communication. The range of sensory information used by insects differs from that of humans. We rely heavily on visual information and although many insects have well-developed vision, most insects make greater use of olfaction and hearing than humans do.

The insect is isolated from its external surroundings by a relatively inflexible, insensitive, and impermeable cuticular barrier. The answer to the enigma of how this armored insect can perceive its immediate environment lies in frequent and abundant cuticular modifications that detect external stimuli. Sensory organs (sensilla, singular: sensillum) protrude from the cuticle, or sometimes lie within or beneath it. Specialized cells detect stimuli that may be categorized as mechanical, thermal, chemical, and visual. Other cells (the neurons) transmit messages to the central nervous system (section 3.2), where they are integrated. The nervous system instigates and controls appropriate behaviors, such as posture, movement, feeding, and behaviors associated with mating and oviposition.

This chapter surveys sensory systems and presents selected behaviors that are elicited or modified by environmental stimuli. The means of detection and, where relevant, the production of these stimuli are treated in the following sequence: touch, position, sound, temperature, chemicals (with particular emphasis on communication chemicals called pheromones), and light. The chapter concludes with a section that relates some aspects of insect behavior to the preceding discussion on stimuli.

Chapter 4