Contact Chemoreception in Haematophagous Tabanids: A Study of the Distribution and Structure of Tarsal and Labellar Taste Receptors and Their Sensitivity to Sugars and Other Chemicals

Abstract

<p>A study of the tarsi and mouthparts of females in three representative genera of tabanids revealed that the ventro-lateral surface of the fore-tarsi and the aboral margin of the labella were the principal loci of contact chemical sensilla. Of the four main types of aboral labellar trichodes (A,B,C and D), two (B and C) proved to be gustatory and then only at the tip. Two other types of trichodes (E and F) were identified on the anterior aspect of the labella. These were different from the aboral setae (type A,B,C, and D) in shape, size and function. </p><p>Extension of the proboscis was taken as the index of positive tarsal stimulation and the criterion of positive labellar stimulation was the spreading of the labellar lobes in an extended proboscis. This was observed to be the same in free as well as attached experimental flies. Newly emerged females showed similar response.</p><p>The frequency method generally employed in psycho-physical studies for estimating thresholds in mammals was successfully extended to tabanids. Using this method the stimulative effectiveness and threshold of various sugars for the tarsal and labellar taste sensilla was determined. Comparison was made between the frequency method and the ascending method of estimating thresholds with regard to sucrose. The effect of starvation on responsiveness of tarsal and labellar contact chemoreceptors was determined. Newly emerged flies became increasingly sensitive to sucrose during the duration of tests when they were strictly maintained on water diet. </p><p>Intergenerlc differences and similarities were seen in the behaviour immediately before feeding. The flies fed on "dry" sucrose as well as on solutions of appropriate concentrations. By analysing the crop contents of wild-caught tabanids, it was established that they fed on sugars and these findings were related to tabanid feeding behaviour and adaptation in nature.</p><p>Using whole blood, sugars and blood-sugar mixtures at various concentrations and proportions (in the case of blood-sugar mixtures), their dispatch to crop and/or mid-gut was studied. In addition, select group of amino acids and nucleotides were also tested to determine if these acted as feeding stimulants for the deer-flies.</p><p>Wild-caught females of deer-flies lived on dry sucrose and water and on 1.OH glucose solution longer than on distilled water alone or without food and water.</p>