Enochrus (Methydrus) japonicus (Sharp, 1873)

Minoshima, Yûsuke N., Iwata, Yasuyuki & Hayashi, Masakazu, 2016, Egg-Case and Pupa ofEnochrus (methydrus) japonicus (Sharp) (Coleoptera: Hydrophilidae), The Coleopterists Bulletin 70 (3), pp. 561-565 : 562-564

Enochrus (Methydrus) japonicus (Sharp, 1873)

( Figs. 1–2 )

Material Examined Five egg-cases, six pupae, Jôganji , Nagaoka-shi, Niigata pref., Honshû, Japan, 27.V.2012 (adults collected in the field), R22 , Y. Iwata leg., reared by Y. Minoshima .

Description of Eggs and Egg-case Eggs light yellowish white in color, oblong-oval, aggregated in a compact group. Egg-case constructed on substrate ( Fig.1A–B ), generally built above water surface. Egg-case composed of 2 layers of silk; inner one covering the eggs; outer one a swollen net of silk, loosely covering inner layer. Mast absent.

Description of Pupa. Color: Slightly greenish, milky white when alive ( Fig.1E ), white in specimens fixed with boiling water, or entirely greyish white or greyish white and partially black in specimens fixed with ethanol. Eyes become red to brown when alive ( Fig.1E ). Body: Broad, abdomen more slender ( Fig.2 ). Head, thorax, and abdomen with styli, without simple setae. Head: Deflexed towards ventral surface, covered by pronotum in dorsal view. Frontoclypeal sulcus partly visible. Head with 2 pairs of short supraorbital styli close to inner margin of each eye. Antennae completely covered by head and pronotum in dorsal view, partially visible in ventral view. Mouthparts visible in ventral view, maxillary palpi long, projecting close to median part of mesotarsi. Thorax: Pro-, meso-, and metathoracic legs visible in ventral view, metathoracic legs partially covered by wingpads, only tibiae and tarsi visible; metathoracic leg strongly bent ventrally. Pronotum with 12 pairs of styli: 5 pairs of styli on anterolateral to anterior margin of pronotum, median 2 pairs long, remaining pairs rather short; 1 rather short pair on anterolateral part; 1 long pair on median part; 5 pairs of rather long styli on posterolateral to posterior margin of pronotum. Meso- and metanota with 1 pair of styli on each side of scutellum. Abdomen: Ninesegmented, attenuate towards apex. Segment 9 with well-developed, non-articulated urogomphi; surface of urogomphi finely denticulate; apex of urogomphi simple, not bifurcate. Abdominal segment 1 with 2 pairs of rather short styli, 1 on median part, 1 on lateral part close to spiracle; segments 2–7 with a transverse row of 4 pairs of rather short styli, 2 pairs on median part, 2 pairs on lateral face, 1 close and posterior to spiracle, 1 on ventral face close to spiracle; segment 8 with 1 pair of short styli.

Biology and Discussion. The egg-case morphology and construction by E. japonicus is unusual compared to other Enochrus species. Females of E. japonicus laid a case on the typical substrate (e.g., leaves) used by Enochrus , however the case was covered with a loose net of silk ( Fig.1A–D ) rather than a dense fabric seen in other species ( Fig.1F ) (e.g., Richmond 1920; Anderson 1976; Archangelsky 1997, 2002; Byttebier and Torres 2009). In contrast, Enochrus (Methydrus) cinctus (Say, 1824) lays eggs on the soil, and the eggs are covered with only a single layer that is a loose net of silk ( Richmond 1920; Archangelsky 1997). Under rearing conditions, egg-cases of E. japonicus were set above or just at the water surface ( Fig.1C–D ). However, because all rearing chambers were filled at a level of ca. 1 cm, preferences for ovipositional sites could not be determined. Other Enochrus species , including other species of the subgenus Methydrus Rey, 1885 , generally lay eggcases below or just at the water surface ( Archangelsky 1997, 2002; Byttebier and Torres 2009).

During the construction of the egg-case, the silk layer was woven with the use of the gonocoxites and gonostyli ( Fig.1D ); the silk was spun from the apex of the gonostylus ( Fig.1D ).

Pupation took place in pupal chambers dug in the soil, like all other species (e.g., Archangelsky 1997). The pupal morphology of E. japonicus is similar to other species of the genus with the only notable difference being the number of styli on the pronotum and abdominal segments, which are known to differ among species (e.g., Byttebier and Torres 2009). For example, number of styli in E. japonicus is identical with that of Enochrus (Methydrus) vulgaris (Steinheil, 1869) , Enochrus (Methydrus) fimbriatus (Melsheimer, 1844) , and Enochrus (Methydrus) maculiceps (MacLeay, 1871) ( Richmond 1920; Anderson 1976; Archangelsky 1999), whereas the number differs from that of Enochrus (Hugoscottia) variegatus (Steinheil, 1869) and Enochrus (Methydrus) barituensis Fernández, 2006 , which have six on abdominal segment 1 (four in E. japonicus ) ( Archangelsky 2002; Byttebier and Torres 2009), and Eochrus (Lumetus) quadripunctatus (Herbst, 1797) , which has 22 on the pronotum (24 in E. japonicus ) ( Hosseinie 1995).

Figures

Fig. 1. Enochrus (Methydrus) japonicus (A–E) and Enochrus (Holcophilydrus) simulans (F), under rearing conditions. A–B) Egg-case, C–D) Egg-case construction by female, E) Live pupa in pupal chamber, F) Egg-case.

Fig. 2. Enochrus japonicus, pupa. A) Dorsal view, B) Lateral view, C) Ventral view, D) Pronotum.