Heterosternuta, STRAND

Alarie, Yves & Longing, Scott D., 2010, Descriptions of the Second and Third Instars of Heterosternuta sulphuria (Matta and Wolfe) (Coleoptera: Dytiscidae: Hydroporinae), with A Key For Identification of the Known Larvae of Heterosternuta Strand, The Coleopterists Bulletin 64 (1), pp. 85-90 : 87-89

GENUS HETEROSTERNUTA STRAND

Diagnostic Combination. The known second and third instars of Heterosternuta may be distinguished from those of other genera of Hydroporini that have been well studied by the following combination of characters: sclerites of body predominantly brownish; stemmata present; antennomere 2 without secondary setae; prementum with lateral spinulae; legs without natatory setae; spinulae present on ventral margin of protibia and protarsus, but absent from both meso- and metatibiae and meso- and metatarsi of third instar; spiracular openings absent on mesothorax and abdominal segments 1–7; siphon of third instar cone-shaped, highly constricted posterior to insertion of urogomphi, and short; urogomphomere 1 with secondary setae.

KEY TO THIRD INSTARS OF FOUR SPECIES OF THE GENUS HETEROSTERNUTA

Key to Instars

1. Frontoclypeus with 2 egg bursters .... Instar I

1’. Frontoclypeus smooth, lacking egg bursters........................................................................2

2. HL <0.80 mm; HW <0.60 mm; MP2/MP1> 1.00; LP2/LP1> 1.00; L3 <1.70 mm; LAS <0.30 mm ............................................. Instar II

2’. HL> 0.90 mm; HW> 0.80 mm; MP2/MP1<0.90; LP2/LP1<0.90; L3> 2.00 mm; LAS> 0.40 mm .......................................... Instar III

Key to Species

1. L3/HW ≥ 2.80; HW ≤ 1.00 mm .................... .................................................... H. wickhami

1’. L3/HW ≤ 2.80; HW ≥ 1.00 mm..................2

2. U1/LAS> 2.20; U1+U2’/LAS> 3.70; L3> 2.30 mm; U1> 0.80 mm ..... H. diversicornis

2’. U1/LAS <1.90; U1+U2’/LAS <3.50; L3 <2.30 mm; U1 <0.80 mm .............................3

3. L3/HW <2.50 ........................ H. cocheconis

3’. L3/HW> 2.70 ........................... H. sulphuria

DESCRIPTIONS OF THE LARVAE OF HETEROSTERNUTA SULPHURIA (MATTA AND WOLFE) ( Figs. 1–4)

First instar. No specimens were available for study.

Second instar (n = 2). Measurements and ratios characterizing the body shape are shown in Table 1. Color: Body predominantly brown. Head capsule predominantly yellow, brownish over posterior portion of frontoclypeus and mesally over parietale; head appendages pale yellow; thoracic and abdominal terga brown; legs creamy white to pale yellow; urogomphus creamy white to pale yellow, darker over proximal part of urogomphomere 1. Legs: Number and position of secondary setae are as indicated in Table 2.

Third instar (n = 4) ( Figs. 1–4). Measurements and ratios characterizing the body shape are shown in Table 1. Color: Darker than second instar; head capsule predominantly dark yellow, slightly darker mesally over frontoclypeus; head appendages dark yellow; abdominal and thoracic terga predominantly dark brown; thoracic terga with several yellowish maculae; last abdominal segment dark yellow over siphon; legs dark yellow; urogomphus dark brown, yellowish proximally over a short distance. Legs: Number and position of secondary setae are as indicated in Table 2.

Habitat. Cobble margin of riffles.

Distribution. Known only from Arkansas (Matta and Wolfe 1981).

TAXONOMIC NOTES

The larvae of H.sulphuria that we describe and document in detail resemble those of other species of the genus recently described ( Alarie 1991a; Alarie 1992; Alarie and Fritz 1998). Indeed, larvae of H. diversicornis , H. wickhami , H. cocheconis , and H. sulphuria are rather similar morphologically, showing a narrow range of morphologies, which is similar to the condition observed with adults ( Larson et al. 2000). A close similarity between larvae of these four species is confirmed also by leg chaetotaxy ( Table 2) as no significant differences may be reported in regard to the number of secondary setae among the specimens studied. Larvae of H. sulphuria are characterized by the absence of meso- and metatibial PD secondary setae (present in every other species of Heterosternuta studied) ( Table 2). Such character state must be interpreted with caution, however, as the relative position of the secondary setae along the dorsal margin of tibiae and tarsi of larvae of Heterosternuta may be difficult to determine according to the relative orientation of legs.