**3. Gustatory system of** *Drosophila*

Although the same taste preferences are shared between *Drosophila* and mammals, the organization of their gustatory systems are rather different. Unlike humans, flies have wide distribution of taste cells over much of the body including many peripheral organs like labellum, legs, wings and genitalia (**Figure 1**). Such a distribution of taste cells enables the fly to gather contact chemosensory information from many reference points that may make contact with their body enabling detection of potential calorie rich foods or toxic compounds [16]. The presence of taste-sensing cells in other tissues provides the safety benefits allowing evaluation of chemicals without the potential hazard of accidental ingestion. The gustatory sensillum or taste bristle are the main sensory unit of all taste organs housing two to four primary gustatory receptor neurons (GRNs) as well as a single mechanosensory neuron (MSN) [2]. The labellum is the main taste organ in *Drosophila* located at the end of the proboscis (equivalent to human tongue). Labial palps contain 31 bristles (sensilla) each that are arranged in a stereotyped pattern. The sensilla are morphologically classified into three types long, intermediate and short (L, I, and S type) based on their shape and location (**Figure 1**) [2, 17, 18]. L- and S-type sensillum house dendrites of four GRNs, and the I type are associated with two GRNs. Electrophysiological investigations suggest each GRN is thought to respond exclusively to either sugar, water, low salt concentration, or high salt concentration and bitter compounds [11, 19–22]. The terminal pore at the tip of the taste bristle (**Figure 1**) allows taste stimuli access to the dendrite of the GRN, which extends into the bristle shaft [23]. In addition to the peripheral taste sensillum on the palps, legs, and wings, taste neurons are also semi-internally or internally located. The first group consists of row of taste pegs that line the inside of the labial palps and are exposed to foods when the fly 'opens' its palps and readies itself for 'sucking up' foods. The internally located group consists of three sensillum clusters that line the pharynx (**Figure 1**). They allow re-evaluation of the food as it passes and enters

the esophagus and the digestive system. Peripheral labial palp GRNs, the internal sensillum, and some leg GRNs project their axons to the sub esophageal zone (SEZ), whereas the wing and a minority of leg GRNs project to the thoracic ganglion (**Figure 2**) [24, 25].

A single MSN and several support cells are also present in the taste sensilla together with the GRNs (**Figure 1**) [26]. These MSNs translate mechanical forces into electrical signals and mediate hearing, positional awareness, and the coordination of movements [27, 28]. The MSNs sense the hardness and viscosity of food [29] similar to the ability of the human tongue to determine the consistency and texture of foods.
