Studies on the life history of Paederus fuscipes Curtis (Staphylinidae) : Studies on poisonous beetle, III

Abstract

Paederus fuscipes Curtis is the poisonous beetle which frequently causes an acute human dermatitis in Japan as well as in number of foreign countries. The author has made experimental and field observations on the life history, bionomics and distributions of the species in Japan. External structures of each developmental stage have also been fully described. The life cycle of this species comprises egg, larva (two instars), pupa and adult. Adults are about 7mm in length, and look like ants. Free pupae are about 4.5mm in length; body slender and curved ventrally, milky white to orange yellow in colour, with a pair of long seta-like processes on the anterior and posterior margins of pronotum, lateral margins of 1st, 3rd to 7th abdominal segments respectively. The second instar larvae are 4 to 6mm in length; body subcylindrical, white to orange yellow in colour, with a pair of urogomphi. The first instar larvae are 2.2 to 3.4mm in length. Eggs are subspherical, 0.6 to 0.7mm in diameter immediately after the oviposition, but increase in size as the development proceeds. The species is widely distributed throughout Japan, from Hokkaido to Kyushu. It seems to be more abundant in warmer regions. The adults are usually found on soil surface or on grasses in dump places, such as marsh or rice-field. At Narimasu, Tokyo, they could be collected by a light trap during the season from the end of April to the end of October, having the peak in June and July. The nocturnal activity of the adults belongs to the pre-midnight type, i. e. more than half of the total number of a night collection by the light trap was obtained within two and half hours after it had become dark. Eggs are usually laid singly into the crevices on the soil surface. The numbers of eggs deposited by a female were from 18 to 100 with the mean of 52.3. Females which overwintered began to lay eggs from the end of April or from the middle of May to July. Those which emerged in early June continued to lay eggs from July to September. Incubation period of eggs were 3 to 19 days. Hatching rate observed was 96.2%. Larval stage consists of only two instars, the duration being 4 to 22 days for the first instar and 7 to 36 days for the second. Pupation takes place in a chamber constructed beneath the soil surface. Pupae are held up off the wet floor of the chamber by seta-like processes on the pronotum and abdominal segments. Pupal stage was 3 to 12 days. The total development from egg to adult required 22 to 50 days, with an average of 32.5 days. Adults are polyphagous in their food habits, feed mainly on insects, mites and soil nematodes, but also on dacayed vegetables in nature. The food habits of larvae are similar to the adults, mainly carnivorous as a rule, but could experimentally be reared with either beef or cucumber. Adults are eaten by frogs and parasitized by a namatode, a mite (hypopus stage) and Laboulbenia cristata. Eggs and larvae are attacked by various kinds of predacious insects and spiders. The annual life cycle was rather irregular around Tokyo, either one, two or three generations in a year. The adults were the only stage which could be recovered during hibernation. They often form clusters during the overwintering period. The poisonous substance which causes dermatitis could be detected in every part of the body of adults, pupae, larvae and eggs. It is contained in the body fluid, and leaks out only when a part of the body is injured. Among eight species of the genus Paederus found from Japan, four species, i. e. fuscipes, tamulus, poweri and parallelus were found to contain the poisonous substance, but fuscipes seemed to be the only species of practical importance. Out of 68 species of Staphylinidae collected by light traps in Japan, Paederus fuscipes was also found to be the only species of actual importance as the cause of human dermatitis, though a few others were demonstrated to cause slight skin lesions under experimental conditions.