Ilybius discedens Sharp

Hicks, B. J. & Larson, D. J., 2000, Descriptions And Recognition Of Larvae Of Some Northern North American Species Of Ilybius Erichson (Coleoptera: Dytiscidae), The Coleopterists Bulletin 54 (1), pp. 36-59 : 40-41

Ilybius discedens Sharp

Instar I. Colour. Head brown with light brown areas; antennae grey with the terminal segment pale. Thorax, abdomen and urogomphi brown to grey. Sterna and legs pale yellow. Head ( Fig. 1B ). HL 5 0.95 mm, HW 5 0.89 mm. Head widened slightly at eyes. Legs ( Fig 4 ). Femora with or without ventral spinulae, if present not numerous and located between AV setae; femur with one proximal seta in PV series. Abdomen ( Fig. 2B ). LAS 0.60 times UT. Siphon long, SI/LAS 5 0.42. Urogomphus with U4 pale and spiniform, U3 and U2 darker and setiform, length of U4/length of U3 5 0.16. Abdomen with terga 1–6 each with two short, pale submedial setiform setae; lateral setae short; segment 7 with two dorsal and two ventral submedial setae and two darker, more elongate lateral setae, these setae much shorter than those of I. biguttulus or I. pleuriticus ; other smaller setae present but not prominent.

Instar III. Colour ( Fig. 6B ). Head brown with light brown areas; appendages light­brown. Thorax and abdomen brown to grey with pale areas laterally except last three segments of abdomen without pale areas. Sterna light grey. Urogomphus brown. Legs light­brown. Head ( Fig. 7B ). HL 5 1.73 mm, HW 5 1.53 mm. Sides of head parallel. Legs ( Fig. 9 ). Femora with secondary dorsal spines as follows: profemur four to six, mesofemur four to five, metafemur five to seven; pro­ and mesofemora with spinulae in the AV series but number and size of spinulae very reduced; metafemur without spinulae in AV series. Abdomen ( Fig. 11B ). Dorsal surface with relatively few spiniform setae. Primary setae of abdominal segments small and pale. LAS/UT 5 0.86.

Ecology. Adults and larvae of I. discedens where collected in small Sphagnum bog pools where the water level remained constant. Instar I larvae hatched from eggs of field collected females in early and middle September. The incubation time for eggs reared in the lab at 23 8 C 6 1 8 was 13.3 6 1.2 days (n 5 37), similar to that reported by Hicks and Larson (1995). The collection records and hatch dates support the life history proposed by Hicks and Larson (1995) for this species.

Figures

Fig. 1. Dorsal surface of the head of instar I larvae of three species of Ilybius Erichson. A I. biguttulus, B I. discedens, C I. pleuriticus.

Fig. 4. Ilybius discedens. Instar I, right legs, anterior and posterior aspects.

Fig. 2. Last three abdominal segments and urogomphi, dorsal aspect, of instar I larvae of three species of Ilybius Erichson. A I. biguttulus, B I. discedens, C I. pleuriticus.

Fig. 6. Dorsal body surface of instar III larvae of three species of Ilybius Erichson. A I. biguttulus, B I. discedens, C I. pleuriticus.

Fig. 7. Dorsal body surface of head of instar III larvae of three species of Ilybius Erichson. A I. biguttulus, B I. discedens, C I. pleuriticus.

Fig. 9. Ilybius discedens. Instar III, right legs, anterior and posterior aspects.

Fig. 11. Last two abdominal segments and urogomphi, dorsal aspect, of instar III larvae of three species of Ilybius Erichson. A I. biguttulus, B I. discedens, C I. pleuriticus.