*2.3.5 Transit and excretion*

Nutrient extraction and utilization may get affected by the passage of food along the alimentary canal and by its subsequent excretion. Transport of food to travel the entire length of the digestive tract takes less than 1 hour [105] in flies. As suggested, the amount of food retained in the crop is much larger in starved flies then refed flies than in flies fed *ad libitum* [2, 105]. Starvation also lowers defecation rate long before the gut is emptied [14]. Chronic food deprivation during the larval life has been shown to subsequently increase excretion in adult flies [106]. The hindgut may contribute to the pH adjustment of excreta, which may help offset the excess acid produced [14]. Changes in intestinal fluid retention are likely to involve the distal part of the hindgut (rectum and/or rectal glands), as known for its role in water reabsorption in other insects [8], and may help maximize absorption at a time of high nutritional demand. Such a mechanism is partly mediated by the sex peptide transferred by males during copulation [14, 107], affecting the HGN1 (Hindgut Neuron1) subset of hindgut-innervating neurons [14]. Further investigations are required to clarify the connections between intestinal fluid retention, absorption, peristalsis, and excretion where crop may prove to be a key organ, given that its differential peristalsis and engorgement can determine whether food is temporarily stored or released into the midgut for digestion and absorption [4]. Apart from affecting the nervous system, mutations in the *drop-dead* gene are also associated with increased crop size, reduced transfer of ingested food from the crop to the midgut, and reduced defecation [108, 109].
