Ilybius Erichson Larvae

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 : 38-39

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 membra­ nous; 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).