Ilybius Erichson

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 : 38-56

Diagnosis of Ilybius Erichson Larvae

The genus Ilybius belongs to the subfamily Colymbetinae , tribe Agabini ( Nilsson and Roughley 1997) . Larvae of Ilybius can be recognized on the basis of the following combination of characters:

General. body elongate with head narrower than thorax or abdomen; clypeus lacking an anterior projection; antenna segment 3 at most three times as long as antenna segment 4 which lacks an appendage; legs long and with two claws of equal length; last abdominal segment lacking lateral setal fringes; and apex of abdomen with two elongate urogomphi.

Instar I. Head large in proportion to body; lacking temporal spines, with distinct egg bursters on frontoclypeus; antenna segment 4 pale. Femur with one or two dorsal setae, if single, seta attached distad to middle, ancestral primary setae and pores as described by Nilsson (1988) with the inclusion of other primary setae. Abdomen segments 1 to 7 with ventral surface membranous; last segment with siphon well developed and protruding; urogomphus with four setae in each a proximal and an apical group; proximal group with U2 to U4 attached close together, U4 smaller and paler than U2 or U3.

Instars II and III. Most specimens with a faint to distinctive dorsal colour pattern, best developed on head. Head with temporal spines, but number of spines variable. Femur with four to ten dorsal setae, the most distal being 0.6 to 0.75 of length from base. Abdomen segments 1 to 6 with ventral surface membranous, segment 6 with ventral membranous area very wide, ventral postero­submedial setae within membranous area; segments 7 and 8 completely sclerotized ventrally and cylindrical; last segment with protruding siphon not as prominent as in instar I; urogomphus with 2 setal groups; proximal group with U2 to U4 attached close together.

Distinguishing between larvae of Ilybius and its sister genus, Agabus Leach , can be difficult. Galewski (1987) and Nilsson (1987) compared the characters of larvae of the two genera and characters useful for their separation are presented in Table 1 .

Habitat. All species are lentic, occurring in pools amongst emergent plants. The adults and larvae occur in the same habitat. Adults possess an ovipositor adapted for placing eggs endophytically. Life histories vary geographically with northern European species being semivoltine ( Nilsson 1986) while Wisconsin and Newfoundland species primarily univoltine ( Hilsenhoff 1986; Hicks and Larson 1995). Northern North American species have a larger proportion of the population with semivoltine life histories than the more southern species ( Hicks and Larson 1995).

Descriptions of Immature Stages

Eggs. The colour of newly layed eggs was creamy white in all species studied. This colour did not change during the incubation period, however the brownish mandibles of the larva were visible through the egg shell 1 to 2 days prior to hatching. Eggs are ovoid in shape with rounded ends and broadest in the centre. The size and shape of eggs varies between species, but generally the length is greater than twice the width ( Table 2 ).

Larvae. Diagnostic characters only are presented in the following larval descriptions. Since instars II and III are similar in appearance, we have only given the description of instar III larvae. Descriptive statistics of measurements and ratios for all instars, and numbers of secondary leg setae for instars II and III are given in Table 3 . Measurements represent mean values.

Keys to Larvae of Known North American Ilybius Key to the larval instars (Modified from Galewski 1966)

1 Head without temporal spines; abdomen segment 7 with ventral side membranous, not sclerotized; siphon elongate, SI/LAS> 0.35 ------------ Instar I

1 ' Head with temporal spines; abdomen segment 7 with ventral side sclerotized; siphon not as elongated, SI/LAS <.33 ------------------------------------------- 2

2(1) Mandibles without emargination on their outer edge ( Fig. 12A ) ----------------------------------------------------------------------------------------------------------------------------- Instar II

2 ' Mandibles with a distinct emargination on their outer edge ( Fig. 12B ) ----------------------------------------------------------------------------------------------------------------------------- Instar III

Key to the Instar I Larvae

1 Large larvae (HW> 1.10 mm); dorsal surface of head and abdomen with a conspicuous pattern of contrasting pale and dark longitudinal stripes- ---------------------------------------------------------------------------------- I. pleuriticus LeConte

1 ' Smaller larvae (HW <1.10 mm); dorsal surface of head and abdomen without above pattern- -------------------------------------------------------------------------------------------- 2

2(1) Metafemur with one proximal PV seta ( Fig. 5 ) ------------------------------------------- 3

2 ' Metafemur with two proximal PV setae ( Fig. 3 ) -- I. biguttulus Germar

3(2) Urogomphus short, at most 2 X LAS- -------------------------------------------------------------- 4

3 ' Urogomphus long, greater than 2 X LAS- --------------- I. vittiger (Gyllenhal)

4(3) Abdomen terga 1–7 with dark spiniform setae; urogomphus with U 3 short relative to U 4, U 4/ U 3> 0.30 ------------------------------------------------------------------ 5

4 ' Abdomen terga 1–7 with pale setiferous setae ( Fig. 2B ); urogomphus with U 3 longer relative to U 4, U 4/ U 3 = 0.16 ----------- I. discedens Sharp

5(4) Urogomphus short ( UT <1.21mm) and just over 1.5 times the LAS (LAS/ UT = 0.61); ultimate antenna segment (A4) 0.82 of penultimate antenna segment(A3) ----------------------------------------------------- I. angustior Gyllenhal

5 ' Urogomphus long ( UT > 1.40 mm) and 2 times the LAS (LAS/ UT = 0.53); ultimate antenna segment (A4) 0.67 of penultimate antenna segment(A3) ---------------------------------------------------------------------------- I. subaeneus Erichson

Key to Instar II and Instar III Larvae

1 Head and dorsal body surface with a conspicuous pattern of contrasting pale and dark longitudinal stripes ( Fig. 6C ) ------- I. pleuriticus LeConte

1 ' Head and dorsal body surface without above pattern ( Fig. 6A ) ---------- 2

2(1) Protibia without dorsal setae- ----------------------------------------------------------------------------- 3

2 ' Protibia with dorsal setae- ----------------------------------------------------------------------------------- 5

3(2) Metafemur with spinulae in AV series; pro­ and meso­tarsi with secondary dorsal setae- ----------------------------------------------------------------------------------------------- 4

3 ' Metafemur without spinulae in the AV series; pro­ and meso­tarsi without secondary dorsal setae ( Fig. 9 ) ------------------------------- I. discedens Sharp

4(3) Large larvae (instar II, HL> 1.4 mm; HW> 1.3 mm; LAS> 1.0 mm; instar III, HL> 1.9 mm; HW> 1.8 mm; LAS> 1.65); siphon relatively long (instar II, SI/LAS =.30; instar III, SI/LAS =.18); urogomphus small relative to LAS (instar II, LAS/ UT = 0.75; instar III, LAS/ UT = 0.94) ------------------------------------------------------------------------------------ I. angustior Gyllenhal

4 ' Smaller larvae (instar II, HL <1.4 mm; HW <1.2 mm; LAS <1.0 mm; instar III, HL<1.9 mm; HW <1.8 mm; LAS <1.65); siphon relatively short (instar II, SI/LAS =.17; instar III, SI/LAS = 0.10); urogomphus longer relative to LAS (instar II, LAS/ UT = 0.52; instar III, LAS/ UT = 0.72) ----------------------------------------------------- I. vittiger (Gyllenhal)

5(2 ') Instar II with urogomphus short (1.98 ± 0.14 mm); instar III with LAS short (1.79 ± 0.07mm)and urogomphus short (2.62 ± 0.2mm) ------------- ----------------------------------------------------------------------------------------------------- I. biguttulus Germar

5 ' Instar II with urogomphus long (2.15–2.22 mm); instar III with LAS long (2.25–2.30 mm) and urogomphus long (3.00– 3.07 mm) ----------------- -------------------------------------------------------------------------------------------------- I. subaeneus Erichson

Figures

Fig. 12. Left mandible of instar II (A) instar III (B) larvae of Ilybius.

Fig. 5. Ilybius pleuriticus. Instar I, right legs, anterior and posterior aspects.

Fig. 3. Ilybius biguttulus. 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. 9. Ilybius discedens. Instar III, right legs, anterior and posterior aspects.