As the immune reactions in human white blood cells of certain substances from insects to defend it when invaded by immune blood cells is increased. We experiment with changes in the total number of blood cells through the blood cells which increases and decreases, as well as to observe whether the immune response through any route is to evaluate what happens. Hemocyte population was analyzed in the last instar larvae of Spodoptera exigua. Granulocyte and plasmatocyte were predominant (>75%) types of hemocytes, whereas spherulocyte, prohemocyte, and oenocytoid hemocytes were observed in small densities (5~10%). Total hemocyte counts (THCs) were varied among different ages (day1-day5) of the last instar, in which day 3 larvae (L5D3) had the maximal density. Upon bacterial challenge to L5D3 larvae, THC was further enhanced within 2 h and then decreased to background level. This rapid THC increase in response to bacterial challenge was inhibited by injection with dexamethasone (1 ㎍ per larva). However, the addition of arachidonic acid reversed the inhibitory activity of dexamethasone and allowed the larvae to increase THC. This THC increase was mediated by cyclooxygenase products, but not by lipoxygenase products.