Odour detection in the desert locust, Schistocerca gregaria: antennal structure and function

Abstract

Insects are among the most successful organisms in the animal Kingdom. Their capacity to survive and reproduce depends greatly on their ability to identify and respond selectively to cues from a heterogeneous environment. They can identify conspecifics and mates, differentiate between hosts, both plant and animal, and distinguish between many microclimatic factors such as variation in humidity, temperature and airflow. All these factors are attributed to the complexity of their sensory systems that facilitate both simple and complex behaviours. The insect is, however, isolated from its external surroundings by a cuticular structure that is relatively impermeable to chemical sensation. But insects can perceive signals from their environment since the cuticle has been modified at certain points to give rise to sensory detectors for external stimuli. Many sensory organs are housed in hair-like structures, known as sensilla, that protrude from the cuticle. These structures participate in the detection of stimuli that can be categorised as mechanical, thermal or chemical. Receptor neurones associated with these structures convey messages from outside of the insect body to the central nervous system (CNS), where signals are integrated. The fully integrated signal can subsequently participate, together with internal stimuli, in the moulding of appropriate behaviours such as posture, movement, feeding and behaviours associated with mating and oviposition. Information regarding the chemical environment of an animal is collected by the olfactory and gustatory systems. In land-living animals, the difference between the systems is the phase in which the molecules occur. The gustatory system detects molecules dissolved in liquid or waxes, while the olfactory system detects molecules in gaseous phase. In order to understand how chemical signals are detected and analysed by the olfactory system, and how these signals ultimately affect insect behaviour, it is important to study the anatomical and functional organization of the olfactory pathway involved in odour processing. This has been the main aim of this thesis. The morphological features of antenna! sensilla and olfactory receptor neurones associated with them were investigated by scanning (SEM) and transmission (TEM) electron microscopy and by activity dependent staining of neurones, whereas the physiological characteristics of olfactory receptor neurones were determined using electrophysiological methods.