Abstract: The energy density,water content and their variations of adult coleopteran insects were studied by determining carnivorous Carabidae and herbivorous Tenebrionidae in arid environment.The energy density of carnivorous Carabidae (22.64 J/mg AFDW) was significantly higher than herbivorous Tenebrionidae (20.55 J/mg AFDW),but their difference in water content (54.96% and 63.28% respectively) was converse (P<0.05).The energy density in these two families of beetles correlated negatively to their water content (r=-0.5962,P<0.05).Both the energy density and water content of these coleopteran insects exhibited significantly seasonal variations,in which the energy density was found maximum in autumn (22.80 J/mg AFDW) and minimum in spring (20.42 J/mg AFDW),conversely,the water content was lowest in autumn (54.93%) and highest in spring (61.88%) (P<0.05).The energy density of different parts of body in these beetles also showed significant variations in which the energy density was highest in abdomen (22.65 J/mg AFDW) and then decreased in thorax (21.33 J/mg AFDW),head (20.89 J/mg AFDW) and legs (20.60 J/mg AFDW),and similarly,found maximum in autumn (22.30 J/mg AFDW) and minimum in spring (19.46 J/mg AFDW).The energy density of Carabidae and Tenebrionidae insects correlated positively to the energy density in their abdomen,thorax,head and legs (r values were 0.897,0.846,0.718 and 0.604 respectively,P<0.05) in which the correlation coefficients of abdomen and thorax were significant higher than head and legs (P<0.05).This results implied that main reason for the high energy density in beetles was the accumulated lipid in abdomen and thorax.In addition,the factors that affect the variations of energy density and water content,and the effects of these variations in these prey beetles on the food selection of insectivorous predators were also discussed.