Ilybius angustior (Gyllenhal)

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 : 41-49

Ilybius angustior (Gyllenhal)

Larval stages of this Holarctic species were described by Nilsson (1981). This species is common across boreal North America. The measurements included here are from direct measurement of reared and collected material. The measurements (means plus standard deviation) of anatomical parts and other diagnostic characters of this species are presented below. Note that the larvae of I. angustior and the equally widespread species I. picipes (Kirby) cannot be separated, therefore this description applies to both.

Instar I. Colour. Head dorsally yellowish­brown with pale yellow areas. Thorax and abdomen brown. Legs and urogomphi pale brown. Head. HL 5 0.96 6 .01 mm, HW 5 0.95 6 .04 mm. Terminal antenna segment (A4) length 5 0.82 of penultimate antenna segment. Sides of head parallel. Legs. Leg length: prothoracic leg 5 1.13 mm, mesothoracic leg 5 1.30 mm, metathoracic leg 5 1.59 mm. Metathoracic leg length/HW 5 1.69. Femora with one prox­ imal seta in the PV series. Ventral spinulae on all tibiae and tarsi. Femora with spinulae in AV series. Abdomen. Terga 1–7 with pale spiniform setae, two submedial and one lateral on each side. Sterna with spiniform submedial setae. LAS 5 0.73 6 0.03 mm; UT 5 1.19 6 0.02 mm. LAS/UT 5 0.61. Siphon long, SI/LAS 5 0.55. Urogomphus with U4 spiniform, pale in colour, length of U4/length of U3 5 0.32; U2 and U3 longer, setiform and darker.

Instar II. Colour. Head and body darker brown with pale spots on both areas. Legs and urogomphi brown. Head. HL 5 1.46 6 0.03 mm, HW 5 1.34 6 0.01 mm. Antenna with A4/A3 5 0.64; number of temporal spines five to seven. Legs. Leg length: prothoracic leg 5 1.61 mm, mesothoracic leg 5 1.85 mm, metathoracic leg 5 2.34 mm. Metathoracic leg length/HW 5 1.75. Tibiae and trasi with ventral spinulae. Spinulae located in the AV series of the femora. Pro and mesotarsi with secondary dorsal setae. Abdomen. Abdomen with primary setae setiform but short and pale. LAS 5 1.17 6 0.04 mm, UT 5 1.56 6 0.13 mm. LAS/UT 5 0.75. Siphon shorter relative to instar 1, SI/LAS 5 0.30.

Instar III. Colour. Head and body dorsally dark brown with pale spots. Legs and urogomphi brown. Head. HL 5 2.01 6 0.04 mm, HW 5 1.85 6 0.04 mm; number of temporal spines five to seven; antenna with A4/A3 5 0.52. Legs. Leg length: prothoracic leg 5 2.23 mm, mesothoracic leg 5 2.65 mm, metathoracic leg 5 3.47 mm; metathoracic leg length/HW 5 1.86. Metafemur with AV spinulae. Abdomen. Terga with setae reduced in size and pale. LAS 5 1.78 6 0.09 mm; UT 5 1.88 6 0.09 mm; LAS/UT 5 0.94. Siphon short, SI/LAS 5 0.18.