Hemiosus dejeanii (Solier, 1849)

Archangelsky, Miguel & Fernández, Liliana A., 2005, Description of New Neotropical Berosini Larvae: Derallus paranensis and Hemiosus dejeanii (Coleoptera: Hydrophilidae), The Coleopterists Bulletin 59 (2), pp. 211-220 : 216-219

Hemiosus dejeanii (Solier, 1849)

( Figs. 10–16 )

Material Examined. ARGENTINA, Chubut province: Percey River and Trevelin , pools by the side of the river, 400 m, 438059280S, 718289180W, 9.II.2003, M. Archangelsky coll. (63 larvae) .

Description. Third instar larva. Length: 5.1 to 7.9 mm. Color whitish, with sclerotized parts brown; non-sclerotized integument covered by microscopic spines, denser and rougher on dorsal side.

Head capsule subquadrate ( Fig. 10 ), dorsal surface brown, ocular areas and ventral surface lighter; occipital foramen wide, dorsal part of cervix with two small subtriangular cervical sclerites. Frontoantennal sutures vestigial, subparallel, coronal suture absent. Six stemmata on each side of head close to base of antennae.

Clypeolabrum asymmetrical ( Figs. 10–11 ); nasale convex, with five or six small teeth pointing to right side, and six short setae present along outer margin. Right epistomal lobe with tuft of short and fine spines (pubescence); left epistomal lobe large, covering basal third of mandible, with nine stout setae on inner margin, outer margin pubescent.

Antennae three-segmented ( Fig. 14 ). First segment as long as other two combined, with strong subapical hyaline seta on inner margin; second segment with two outer setae and one inner seta, also bearing a small outer sensorium; third segment the smallest, carrying five apical setae or sensoria, two long and three short.

Maxillae longer than antennae ( Figs. 10 and 15 ). Cardo small, irregularly shaped; stipes the longest segment, with a row of five slender setae on inner margin and five long setae on outer margin; outer margin with three short cuticular projections on basal third. Palp four-segmented; first segment short, with three setae and a small inner appendage with two short sensoria and one long seta; second segment short, bare; third segment the longest, with two preapical setae; fourth segment with one long seta at base on inner margin, and several short setae and sensoria on apex.

Mandibles asymmetrical ( Figs. 12–13 ). Right mandible lightly serrated on distal margin, with two hooked inner teeth, distal one largest; base of mandible with sharp spine on inner margin, pointing forward. Left mandible serrated on distal half, with three inner teeth or projections; distal tooth with strong spines on inner margin, middle projection with four or five points on apex, basal tooth subtriangular, with several sharp inner toothlets; base of mandible with sharp spine on inner margin, pointing forward.

Labium small ( Fig. 16 ). Submentum large, subpentagonal; mentum small, as a short ring, with two lateral setae; prementum subquadrate, with four spines, two on ventral face and two apical ones. Ligula short but easily differentiated. Palp two-segmented, first segment short, bare, second segment longer, with five apical setae and sensoria; intersegmental membrane with few short cuticular spines.

Prothorax slightly wider than head capsule. Pronotum with two large plates separated by a fine sagittal line; prosternum subrectangular, with incomplete sagittal line on posterior half. Mesonotum with two pairs of transverse subtriangular sclerites, anterior pair smaller and narrow, posterior pair large; metanotum lacking sclerites. Legs five segmented, moderately long and visible in dorsal view.

Abdomen ten-segmented, tapering towards distal end, membranous areas covered by dense pubescence. Segments I to VII similar in shape, each subdivided by transverse fold; segment VIII smaller, with two small and narrow, lightly sclerotized tergites; segment IX partially covered by preceding segment, with two small and irregular tergites and small circular sternite; segment X strongly reduced. Segments VIII and IX each with short pair of lateroposterior lobes. Pleural areas slightly lobed. Urogomphi reduced.

Nine pairs of spiracles, one on mesothorax and eight abdominal. Thoracic and first seven abdominal pairs of spiracles non-functional. Last abdominal pair with closing apparatus, enclosed within poorly developed spiracular atrium.

Comparative Notes with Second Instar Larvae. Besides the size (length of larvae II: 3.7 to 4.7 mm), only two differences could be found between larvae II and III. Larvae II have the ecdysial line (frontal arms) functional, extending from the base of the head to the base of the antennae; the second difference is the width of the first and second antennomeres, wider in larvae II than in larvae III. Since only two instars of this species are known, no comparative table of measures between both instars was prepared.

Bionomical Notes. Adults and larvae were collected from marginal pools at the sides of Percey River, and also at the margins of the river in places with algae, where the water ran slower. Adults are active swimmers among aquatic plants, algae, and rocks; larvae, in contrast, are sluggish and are associated with the bottom substrate. The aquatic Coleoptera community at the collecting sites included adults and larvae of the genera Andogyrus (Gyrinidae) , Rhantus , Lancetes , Laccophilus and Liodessus (Dytiscidae) , Tropisternus and Enochrus (Hydrophilidae) , Gymnochthebius (Hydraenidae) , and Heterocerus (Heteroceridae) . During the months of February and March only second and third instar larvae were collected; first instar larvae are probably present earlier, between November and January.

Comparative Notes with Other Hemiosus Larvae. The larvae of two other Hemiosus species are known, H. bruchi and H. multimaculatus . Following Orchymont (1940) and based on adult characters Oliva (1994), in her generic revision of Hemiosus for South America, mentions two species groups: one with pronotum entirely melanic (maculatus -group) and the other with pronotum testaceous at least on posterior edge and angles ( dejeanii -group). H. bruchi belongs to the maculatus -group, while H. multimaculatus and H. dejeanii belong to the dejeanii -group. A morphological comparison among the three known Hemiosus larvae shows that the larva of H. dejeanii is closer to that of H. multimaculatus than to the larva of H. bruchi . Similarities and differences between all three larvae are listed in Table 4.

Within Hemiosus , differences in larval morphology agree thus far with the subdivision of Hemiosus in two species groups, albeit the number of known species is low. We believe that as more larvae of this genus are described, larval characters could serve to subdivide the genus into well-defined species-groups or subgenera.

Figures

Figs. 10–16. Hemiosus dejeanii, third instar larva, dorsal view. 10) head capsule; 11) clypeolabrum; 12) left mandible; 13) right mandible; 14) right antenna; 15) right maxilla; 16) labium. Scale bars: Figures 10 ¼ 0.2 mm, Figures 11–16 ¼ 0.1 mm.