Chostonectes Sharp, 1882

Tsyrlin, Edward, Carew, Melissa & Alarie, Yves, 2020, Re-description of larvae of Chostonectes nebulosus (Macleay, 1871) (Coleoptera: Dytiscidae, Hydroporinae, Hydroporini, Sternopriscina) with an identification key to the known larvae of Chostonectes Sharp, 1882, Zootaxa 4718 (3), pp. 436-446 : 444

Key to instar III and instar II of known larvae of Chostonectes Sharp, 1882

1. Frontoclypeus with egg-bursters; head width smaller than 0.4 mm .......................................... instar I

- Frontoclypeus lacking egg-bursters; head width greater 0.4 mm ................................................. 2

2. Thoracic and abdominal spiracles absent.............................................................. instar II

- Thoracic and abdominal spiracles present............................................................ instar III

1 Head capsule with dark lateral bands running over frontoclypeus; lateral projections and lateral edges of nasale dark; dorsally, no distinct narrowing at base of siphon; head length less than 2.0 times greater than length of last abdominal segment.................................................................................................... C. gigas

- Head capsule with distinct dark bands outlining edge of frontoclypeus without crossing it; lateral projections and lateral edges of nasale pale; distinct narrowing of last abdominal segment separating its proximal part from siphon ( Fig. 3B ); head length more than 2.2 times greater than length of last abdominal segment.............................................. 2

2 Width of spatulate apex of nasale more than 2 times (more than 1.5 times for instar II) greater than minimum width of nasale ( Fig. 2C ); less than seven tubercles on ventral side of lateral projections of nasale......................... C. nebulosus

- Width of spatulate apex of nasale less than 1.5 times (less than 1.4 times for instar II) greater than minimum width of nasale; more than eight ventral tubercles on lateral projections of nasale ( Fig. 3C )............................... C. johnsonii

Figures

FIGURE 2. A—selected head measurements and features of Chostonectes larvae. WSAN—width of spatulate apex of nasale; MWN—minimum width of nasale; B—mandible measurements; C—head capsule of C. nebulosus instar III showing distinctive colour pattern and distribution of setae; D—posterior view and E—anterior view of middle leg of C. nebulosus instar III; E—details of femur.

FIGURE 3. A—neighbour joining tree for 12 sequences of a 658 bp fragment of CO1 using Kimura 2-parameter distance and 1000 bootstrap replicates. Dorsal view of head illustrating distinctive features of each species outlined in text; B—dorsal view of last abdominal segment of Chostonectes nebulosus instar III, showing shape of siphon and distribution of setae; C—side view of lateral projections of nasale of Chostonectes nebulosus and C. johnsonii.