Lagynotomus and Aelia which belong to the Family Pentatomidae (s. str.) are represented in Japan each by one species, Lagynotomus elongatus (DALLAS) and Aelia fieberi SCOTT. The former species is one of the most important pests of the rice and other cereal plants. The latter is also known to attack sometimes these plants. Lagynotomus is closely related to Aenarai which is represented in Japan by only one species, Aenaria lewisi (SCOTT), the larval stages of which were already reported by the author in 1958. The developmental stages of the two species, Lagynotomus elongatus (DALLAS) and Aelia fieberi SCOTT, together with their ecological notes, are given in this paper.<br>The diagnoses of the Genus Lagynotomus<br>Egg: Short-elliptical, slightly depressed, pale yellow or yellowish white at first. Chorion pale white, subhyaline, surface with microscopically granulous structures which are connected by irregular lines. Micropylar projections nearly subhyaline, rather short-capitate. Egg-burster blackish brown, well-chitinized, T-shaped, with membraneous appendages which are mostly dark except the transparent centre. Operculum distinct. Eggmass ordinarily consisting of about 10∼16 eggs, arranged in one or two, rarely three rows.<br>Larvae: Stigmata placed interior to connexiva of abdominal segments from second to eighth, the eighth pair conspicuously smaller than the others. Body nearly short-ovoid in the 1st instar, ovoid in the 2nd, elongate-ovoid in the 3rd, fusiform in the 4th and 5th instars. Head short, arched in the 1st instar, rather long in the 2nd to 5th instars; median lobe considerably wide and longer than lateral lobes in the 1st and 2nd instars, comparatively wide and almost as long as lateral lobes in the 3rd, rather narrow and more or less shorter than lateral lobes in the 4th, considerably narrow and shorter than lateral lobes in the 5th instar; lateral lobes not angular apically, more or less produced laterally in front of eyes in the 2nd to 5th instars. Abdominal dorsal plates absent in the first and second segments; the plate of the anterior odoriferous gland orifices wider than the other of the median orifices, fusiform, and conspicuously constricted medially in the 1st instar, while the former is rather narrower than the latter, fusiform, and considerably constricted centrally in the 2nd to 5th instars. Lateral margins of thorax serrated, clearly in the 2nd to 4th instars, slightly in the 5th instar. Body sparsely with short hairs. Head and thorax very sparsely with black punctures in the 1st instar, rather sparsely with fuliginous or black punctures in the 2nd and rather densely with black or fuliginous punctures and reddish dots in the 3rd to 5th instars except the lateral parts of thorax in the 4th and 5th instars. Abdomen through all instars samely punctate as thorax and in the 2nd to 5th instars scattered with reddish dots. Body without conspicuously coloured markings.<br>The diagnoses of the Genus Aelia<br>Egg: Elliptical, somewhat wider towards above, very light brown at first. Chorion extremely light brown, with a brownish reticulation which is furnished with light brown or darkish small spines. Micropylar projections white except the bases which are nearly subhyaline, rather short-capitate. Egg-burster well-chitinized, T-shaped, mostly black except the brownish superior portion of axis, with membraneous appendages which are mostly transparent except the dark inferior ends. Egg-mass ordinarily consisting of about 12 eggs, arranged in two rows.<br>Larvae: Stigmata placed interior to connexiva of abdominal segment from second to seventh. Bodies rather long-elliptical in the 1st to 3rd instars, rather fusiform in the 4th and 5th instars, and with a luster and short hairs sparsely through all instars. Head wide, lunate in the 1st and 2nd instars, semicircular in the 3rd instar, rather conical in the 4th and 5th instars