Salivary gland morphology and regulation in orthopteroid insects : teleogryllus commodus (walker) and gastrimargus musicus (fabricius)

Abstract

This study focused on the structure and regulation of the salivary glands in relation to feeding by two native insects of Australia, Teleogryllus commodus (Walker) (black field cricket) and Gastrimargus musicus (Fabricius) (yellow-winged grasshoppers). The majority of the thesis examined the changes in the salivary glands of black field crickets under laboratory controlled experimental conditions. The last data chapter examined the salivary glands of the yellow-winged grasshopper in response to feeding when they were captured in the field. The morphology of the cricket salivary gland is similar to that of previously described orthopterans, consisting of a series of acini on both left and right sides of the thorax connected by secondary ducts leading to two main ducts that lead to the pharynx. Each acini consists of an outside group of parietal cells, small cuboidal cells, and an inside group of zymogen cells, larger columnar cells. To examine the effect feeding had on the gland morphology, crickets were continuously allowed access to food and water (FED crickets) or held without food for 24, 48 or 72 h (UNFED crickets). The mass of animals held without food decreased over time, as did the crop masses and salivary glands masses. Examination of the glands using standard histological techniques indicated that zymogenic and parietal cells vary in staining patterns depending upon feeding status. The tissue sections of the salivary glands from FED+ (fed cricket without food in the crop upon dissection) crickets had more intense staining with PAS-alcian blue compared with tissue sections from glands either from FED- (fed crickets with food in the crop upon dissection), or UNFED (no access to food but water present) groups. Using antibodies for serotonin, dopamine and tyrosine hydroxylase, the pattern of staining for these compounds varied with feeding state. In the salivary glands from fed animals, the parietal cells on the outside surface of the acini had both serotonin and dopamine present. However, in unfed animals the serotonin was restricted to the centre line of the acini, and dopamine appeared to be present in the columnar zymogen cells. Serotonin staining was detected in the nerves throughout the glands while dopamine had patchy staining on the glands. Incubation of the salivary glands of unfed crickets with serotonin and dopamine (1 micromole per litre) showed that these chemicals can induce the swelling of the glands to the size of fed crickets. The incubating saline becomes cloudy with serotonin stimulation while the saline containing dopamine remained clear. Analysis of the glands for content of serotonin and dopamine using HPLC indicated that glands removed from unfed crickets had lower contents for both serotonin and dopamine than glands removed from fed crickets. Salivary glands removed from field-captured yellow-winged grasshoppers were heavier in animals with food in their crops than glands removed from grasshoppers with had empty crops. As most animals had empty crops, our capture technique may interfere with feeding activity or may cause a change in glands as a result of flight.