Changes in metabolic substrate utilization patterns resulting from exposure to varying humidity and dietary regimens were examined in the adult, male American cockroach. A series of respirometric experiments were conducted to monitor changes in respiratory quotient (RQ), oxygen quotient (QO₂), food and water consumption and body weight. Also, a series of radiolabel experiments, utilizing injected ¹⁴C-glucose and ¹⁴C-palmitic acid, were conducted to determine radiolabel content (after exposure to varying humidity and dietary regimens) in whole body, feces, ¹⁴CO₂, hemolymph, fat body tissue and cuticle. The results of these two series of experiments were compared to determine the overall effects of humidity and diet on catabolic substrate utilization pattern changes in the American cockroach.

In these experiments, variation in dietary regimens appeared to play a more significant role than humidity variations with respect to significant differences observed in insect RQ, QO₂, body weight and radiolabel incorporation in whole body, ¹⁴CO₂ , hemolymph, and fat body tissue. However, humidity significantly affected radiolabel incorporation in fat body tissue in the glucose experiments and ¹⁴CO₂ in the palmitic acid experiments.

RQ and QO₂ data from the respirometric experiments and ¹⁴CO₂ data from the radiolabel experiments suggested that cockroaches deprived of food, but provided water to drink, change their metabolic substrate utilization pattern from initial catabolic substrates such as carbohydrates to lipid-based substrates. Also, cockroaches deprived of either water (FO) or food and water (OO) appeared to change metabolic substrate utilization patterns toward carbohydrates, but the final carbohydrate metabolic substrates are still unknown.

Cockroaches provided water, but no food, showed a change from initial RQ values (low humidity OW = 0.92 and medium humidity OW = 0.86) to terminal RQ values (low humidity OW = 0.71 and medium humidity OW = 0.68) representative of lipid-based substrate metabolism. There was also an unexplained decrease in QO₂ (initial low humidity OW = 0.45 ul/mg/hr and terminal low humidity OW = 0.28 ul/mg/hr; initial medium humidity OW = 0.35 ul/mg/hr and terminal medium humidity OW = 0. 30 ul/mg/hr). Also, cockroaches deprived of water (FO) or food and water (OO) showed an overall experimental rise in RQ values (initial low humidity FO = 0.87; and terminal low humidity FO = 0.87; initial medium humidity FO = 0.82 and terminal medium humidity FO = 0.85: initial low humidity OO = 0.79 and terminal low humidity OO = 0.88; initial medium humidity OO = 0. 75 and terminal medium humidity OO = 0.87). In addition, these same insects showed an overall experimental increase in QO₂ (initial low humidity FO = 0. 34 ul/mg/hr and terminal low humidity FO = 0.43 ul/mg/hr; initial medium humidity FO = 0.45 ul/mg/hr and terminal medium humidity FO = 0.49 ul/mg/hr: initial low humidity OO = 0.37 ul/mg/hr and terminal low humidity OO = 0.53 ul/mg/hr; initial medium humidity OO = 0.32 ul/mg/hr and terminal medium humidity OO = 0.45 ul/mg/hr), and in the radiolabelled glucose study OO insects showed an almost complete cessation of ¹⁴CO₂ release at 2 days (d). Terminal body weights differed significantly among cockroaches provided with both food and water (controls = FW), only water (OW), only food (FO) and no food or water (OO). Insects deprived of either water (FO) or food and water (OO) had significantly lower terminal body weights than those insects provided either water or food and water. Whole body water content increased and dry weight decreased in insects provided only water to drink.