Regulation of Tarsal Taste Threshold in the Blowfly, Phormia Regina

The nutritional condition of adult blowflies (Phormia regina Meigen) affects their readiness to respond with proboscis extension when their tarsi contact food stimuli. Thresholds are high in sated flies (100–1000 mmol−1 sucrose) and low in starved flies (1–10 mmoll−1 sucrose). Two feeding regimes employing aqueous sucrose were used to reveal factors regulating tarsal taste threshold in this insect: long-term feeding (ad libitum) and single meals administered to starved flies. A positive logarithmic relationship was found between crop weight and tarsal taste threshold, expressed as mean acceptance threshold, in flies fed aqueous sucrose ad libitum for 4 days. Threshold changes after a single meal were positively correlated to both concentration and volume of the sugar solution fed. Thresholds observed in flies fed a single meal were not as high as those in ad libitum-ied flies having the same crop volume. Nerve-transection experiments demonstrated that the median abdominal nerve plays no direct role in threshold regulation in either single-meal-fed or ad libitum-fed flies. Transection of the recurrent nerve (RN), however, significantly attenuated the post-feeding rise in tarsal threshold in starved flies fed a single meal and markedly reduced threshold in sated flies fed ad libitum. Thresholds for RN-transected flies subjected to either feeding regime were still significantly higher than thresholds for starved flies. Haemocoel-injected D-glucose did not significantly elevate threshold in starved flies. These observations establish that the RN plays an important role in the regulation of tarsal taste threshold in blowflies. The effect of the RN on threshold depends largely on the prior feeding activity of the flies. It appears, however, that other factors, in addition to the recurrent nerve, affect taste threshold after feeding.