Andogyrus seriatopunctatus Régimbart, 1883

Archangelsky, Miguel & Michat, Mariano C., 2007, Morphology and chaetotaxy of the larval stages of Andogyrus seriatopunctatus Régimbart (Coleoptera: Adephaga: Gyrinidae), Zootaxa 1645 (1), pp. 19-33 : 21-31

Andogyrus seriatopunctatus Régimbart, 1883

Description of first instar larva ( Figs. 1–13 ): TL 7.60–8.30 mm, MW 0.70–0.80 mm, additional measurements and ratios in Table 2. Body elongate, parallel sided, head and pronotum strongly sclerotized, rest of body soft. Head yellowish brown, with dark areas at base of frontoclypeus and around neck; rest of body whitish, with light brown maculae on meso– and metathorax, and abdominal segments, arranged as two pairs of narrow longitudinal pigmented spots (these spots coincide with lightly sclerotized areas, easily seen in live specimens, in preserved specimens they are difficult to visualize).

Head capsule longer than wide, parallel–sided, with distinct narrow neck ( Fig. 1 ). Coronal suture long; frontal sutures U–shaped, extending to base of antennae. Nasale with four short teeth, inner two extending farther than outer two; lateral lobes of epistome well separated, subrectangular with rounded corners. Stemmata present, six on each side of head, easily seen in dorsal and lateral views, arranged in two dorsally converging rows of three stemmata each. Egg–bursters absent.

Antennae long, slender ( Fig. 2 ), four–segmented; A2>A3>A4>A1; A3 with two subapical flat plates on inner margin, distal one interpreted as the sensorium (A3’) which does not protrude; A3 with two minute spinulae (SP) near base of proximal sensorial plate; A4 with one subapical flat sensorial plate on inner margin.

Mandibles shorter than antennae, symmetrical ( Fig. 3 ), mesal margin irregularly serrated, with a very narrow line along inner channel, lacking retinaculum; groove with two channel openings, one at distal two thirds, second at base.

Maxillae long, slender ( Figs. 4–5 ). Cardo subrectangular, flattened, longer than wide; stipes shorter, subtriangular, bearing a hook–shaped lacinia, a two–segmented GA, a short PPF and a three–segmented palpus; inner margin of lacinia coarsely serrated, basal segment of GA short; MP3>MP2>MP1>PPF.

Labium with prementum longitudinally subdivided in two halves, with two–segmented palpi, ligula absent ( Figs. 6–7 ). Membrane at base and distal margin of prementum with group of sharp cuticular spines; palpi long, slender, LP2 subequal to slightly longer than LP1.

Thorax elongate; prothorax convex, longer than wide, with strongly sclerotized pronotum, subdivided by a fine sagittal line; sternal plate subtrapezoidal, entire. Membrane between pronotum and mesonotum with a pair of narrow, transversal sclerites. Meso– and metathorax poorly sclerotized. Pleural areas represented by a small plate subdivided into epipleura and episternum. Legs six–segmented ( Figs. 8–9 ); L3 the longest, L1 the shortest; L3: CO> FE> TA> TI> TR> PT. Legs lacking fringes of swimming hairs.

Abdomen ten–segmented, long, narrow. Segments I–VIII similar in shape and size, bearing a pair of slen- der lateral plumose tracheal gills; segment IX narrower, with two pairs of gills ( Figs. 10–11 ); segment X narrowest, carrying two pairs of apical, sclerotized hooks ( Figs. 10–11 ). Tracheal gills of segments I–VIII subequal in length, those of segment IX longer. Abdominal tergites poorly sclerotized. Urogomphi absent.

Chaetotaxy of first instar larva: Head capsule ( Figs. 12–13 , Table 3): frontoclypeus bears 10 setae (FR1–5, and FR7––11), and six pores (FRa, FRc–f, and FRk); parietal bears 20 setae (PA1–3, PA6–15, PA17– 19, and four stout additional setae) and nine pores (PAb, PAf–j, PAm, and PAo–p). Additional inconspicuous and fine (hair–like) setae present on the head capsule; they can be easily distinguished from the larger, spine– like ancestral setae homologized and numbered above. These additional setae can also be recognized because of their narrow socket (about half the width of a regular socket), and because they do not always appear in the same location on both sides of the head; they were not numbered.

Antenna ( Fig. 2 , Table 4): first antennomere bears five pores (ANa–e); second antennomere bare; third antennomere bears two setae (AN2–3), one pore (ANf), and two minute spinulae (SP) near base of proximal sensorial plate; fourth antennomere bears pore ANg.

Mandible ( Fig. 3 , Table 4): pores MNa–c present; dorsal surface and outer margin of mandible covered by numerous minute setae; inner margin of mandible bears several setae on distal third, one of these setae can be interpreted as MN2, the remaining setae are considered additional since they are absent in other adephagan; the presence of seta MN1 is ambiguous (see “Comments on homology decisions concerning chaetotaxy”, below).

* See text for comments on MN1 and MN2.

Maxilla ( Figs. 4–5 , Table 4): cardo bears seta MX 1; stipes bears five setae ( MX 2–6) and two pores (MXa–b); first segment of galea bears two setae ( MX 7 and an additional seta), second segment of galea bears three setae ( MX 8–9 and an additional distal seta) and two pores (MXd and MXh); palpifer bears seta MX 10, first palpomere bears two pores (MXe–f) and seta MX 13, second palpomere bears two setae ( MX 11–12) and pore MXg, third palpomere bears pore MXj and seta MX 14. Third palpomere bearing several minute pores as shown in Figs. 4–5 .

Labium ( Figs. 6–7 , Table 4): Prementum bears five setae (LA1–5) and one pore (LAa); first palpomere bears seta LA9 and pore LAb; second palpomere bears pore LAc, and one seta (probably LA12).

Legs ( Figs. 8–9 , Table 5): coxa bears 21 setae (CO1–18, and three additional setae), and two pores (COa, COd); trochanter bears six setae (TR1 and TR3–7), and 8 pores (TRa–g, and one additional pore); femur bears six setae (FE1–6) and pore FEb; tibia bears seven setae (TI1–7) and pore TIa; tarsus bears six setae (TA1–6), and six pores (TAa–f); pretarsus bears two setae (PT1–2).

Abdomen: chaetotaxy of segments VIII–X as in Figs. 10–11 .

Comments on homology decisions concerning chaetotaxy: As stated above, no chaetotaxic ground pattern has been suggested for the family Gyrinidae so far, therefore some setae and pores were difficult to homologize. The homologization of setae and pores is based on comparisons made with other adephagan families: Carabidae , Dytiscidae , Paelobiidae and Aspidytidae ( Bousquet & Goulet 1984; Alarie 1991, 1995, 1998; Alarie et al. 1990, 2004; Alarie & Bilton 2005). First and third instar larvae of the genus Dineutus , and third instar larva of Gyrinus argentinus were also used to corroborate setal/pore positions. Here we discuss those choices we had to make with setae and pores that were not homologized straightforward.

Two of the additional setae of the parietal are dorsal, and form a group with PA8 and PA9 ( Fig. 12 ), these are also present in Dineutus . Considering their position in relation to the stemmata we have numbered them as is shown in Fig. 12 . This is consistent with the position of these setae in Carabidae , which have seta PA8 behind the stemmata and seta PA9 at the level of the stemmata. Of the remaining additional setae one is dorso- lateral and the other is ventrodistal ( Figs. 12–13 ); the dorsolateral seta is located among the stemmata (it is also present in Dineutus ), in the same position as pore PAe of Carabidae ( Bousquet & Goulet 1984) ; the ventrodistal additional seta is also present in a similar position in Dineutus .

The mandibles in Carabidae and many Dytiscidae have two setae: MN1–2, MN1 is more basal than MN2. The position of MN1 is always on the outer margin of the mandible; it is also present in Paelobiidae ( Alarie et al. 2004) and Aspidytidae ( Alarie & Bilton 2005) . MN2 is more variable, it is absent in Paelobiidae and Aspidytidae ( Alarie et al. 2004; Alarie & Bilton 2005), and in many Dytiscidae it has a pore–like appearance; in other Dytiscidae and Carabidae it is present, but its position is variable, it could be on the dorsal side, close to the inner margin ( Colymbetinae , Lancetinae ; Alarie 1998; Alarie et al. 2002; Michat 2005), or on the outer margin (several Laccophilinae ; Alarie et al. 2000). In Carabidae MN 2 is dorsal, close to the inner margin ( Bousquet & Goulet 1984). A. seriatopunctatus has five setae on the inner margin, arranged in two groups ( Fig. 3 ): three distal and two at about the distal third. The first instar larva of Dineutus has three setae arranged in a distal group of two setae, the remaining seta is placed at about the distal fourth of the mandible (this based on a single specimen). Andogyrus and Dineutus also have numerous minute setae on the dorsal and outer margins of the mandible, and this poses a problem in homologization, and several options come into view. A possible interpretation is that MN1–2 are present as one of those minute setae. Another explanation would be that MN2 is present as one of the five setae on the distal inner margin; since it is impossible to tell which one, the conclusion is that this group has MN2 and four additional setae (MN2 and two additional setae in Dineutus ). A third interpretation would be that both MN1–2 are absent (that means that the distal inner setae should all be considered as additional).

The inspection of a third instar larva of Gyrinus argentinus showed several interesting features that help to clarify this situation, but since no first instar larvae were available our interpretations should be taken with caution. Pores MNa–c are present; the basal half of the outer margin shows four setae, two closer to the base, and two near mid–length; one of these is probably MN1, the other three could be interpreted as additional or secondary. The distal third of the inner margin bears two long setae; one of these could be MN2, and the other is additional or secondary. Finally, no minute setae as those of Andogyrus and Dineutus are present, suggest- ing that the presence of these minute setae could probably be a character of Enhydrini .

Based on the above observations, we assume that the most parsimonious interpretation is that MN2 is present (it could be any of the five inner setae, but it is impossible to tell which one, therefore we conclude that this group has MN2 and four additional setae); the situation of MN1 is more obscure, we could assume that it is present (any of the minute setae on the outer margin), or that it is absent.

In the maxilla the second segment of the galea has an additional seta (also present in Dineutus ), the two basal ones are interpreted by us as MX 8 and MX 9, and the distal seta is considered as additional since this is the situation found on carabid larvae where MX 8 is basal and MX 9 is subapical ( Bousquet & Goulet 1984). In Dineutus MX 9 is subapical, while the additional seta is apical. This contrasts with those dytiscid larvae with galea in which these two setae are usually apical or subapical ( Alarie 1998; Alarie et al. 2002). Pores MXd and MXh posed no homologization problem, but in Dineutus MXh is absent. In A. seriatopunctatus pores MXf and MXe were difficult to homologize. MXf is considered the distal one, while MXe is the basal one, both appearing on the inner margin. In carabid and dytiscid larvae both pores (MXe–f) are distal, but in carabids MXe is on the outer margin, and MXf on the ventral margin; in dytiscids ( Colymbetinae , Lancetinae ) MXf is on the inner margin and MXe on the outer margin; in Paelobiidae MXe is on the outer margin and MXf on the ventral face; in Aspidytidae MXe is close to the outer margin, and MXf is apical. The pore MXg, on second palpomere, has a distal position in Carabidae , Dytiscidae , Paelobiidae , and Aspidytidae ; in A. seriatopunctatus the only pore present in this segment is basal, therefore we homologize and consider it as MXg. The seta MX 14, on the third palpomere, has a different position in A. seriatopunctatus from that found in the other families; carabid larvae lack this seta ( Bousquet & Goulet 1984), in dytiscids ( Colymbetinae , Lancetinae ), Paelobiidae and Aspidytidae this seta is located close to the center of the segment, on the inner margin. Since that is the only seta present on the last palpomere, the most parsimonious explanation is to consider it as MX 14.

Antennae and labium posed no problem in homologization. The only difference found is that the larva of Dineutus has an additional pore close to the base of the first labial palpomere.

The legs presented no problem with the homologization. Nevertheless, a couple of comments should be made after comparisons with the first instar larva of Dineutus . The three additional setae observed on the coxa of A. seriatopunctatus are absent in Dineutus ; on the other hand the additional pore on the trochanter of A. seriatopunctatus is present in Dineutus .

Description of second instar larva: As instar I except for the following. Coloration: Pigmented areas on base of FR and around neck darker; maculae on meso– and metathorax, and abdominal segments darker. Measures and ratios: summarized in Table 2. Chaetotaxy: Head capsule with numerous secondary (hair– like) setae; all primary setae present. MN with two subapical secondary setae on inner margin. Cardo with several fine secondary setae; stipes and PPF with short cuticular spines on outer margin. Number and position of secondary setae on legs summarized in Table 6.

Description of third instar larva ( Fig. 14 ): As instar II except for the following. Measures and ratios: summarized in Table 2. Chaetotaxy: Antenna with pore ANf closer to subapical flat sensorial plates. Number and position of secondary setae on legs summarized in Table 6.

Bionomical notes: Larvae of the gyrinid Andogyrus seriatopunctatus have a fairly fast larval development, being recorded mostly in medium to large size rivers from December to April ( Bachmann 1961, 1966; Archangelsky 2004). In the Percey River first instar larvae appear by mid December, and they can be found for only a few days since by the end of the month only second instar larvae are present. The duration of the second instar is also short and by mid January most of the larvae collected in this river are third instars. Pupae can be found by the end of March and also in April. In more northern and warmer watercourses, such as the upper Chubut River, the life cycle starts a couple of weeks earlier.

Larvae are predatory and very active, they can be found all along the cross–section of the river, but they are easier to collect near the shore where the current is slower. They usually hide among small rocks and peb- bles, and when disturbed they actively swim upstream producing a strong undulating movement of the body.

Comparative notes with A. buqueti . Comparison of both species is based on information from the literature (Arndt et al. 1993). Larvae of both species seem to be very similar. Very few differences could be found. Among the most evident ones is the ratio FRL/COL, which is about 1.8 for A. buqueti and under 1.6 for A. seriatopunctatus . The lacinia in A. buqueti is longer than the galea; they are subequal in A. seriatopunctatus . The length of A4 is also distinctive, being longer than A 3 in A. buqueti , and subequal to A 3 in A. seriatopunctatus . Also, the maculae on the head capsule of third instar larvae are different, A. buqueti shows maculae on the lateral margins behind the stemmata, on the neck, and one on the frontoclypeus, which extends into the parietale along the coronal suture. Third instar larvae of A. seriatopunctatus have maculae on the neck and on the basal half of the frontoclypeus.

Chaetotaxic characters were not compared in detail due to the fact that only the most important chaetotaxal characters of A. buqueti were mentioned (sensilla were not numbered on the illustrations).

Figures

FIGURE 14. Andogyrus seriatopunctatus, third instar larva, head capsule, dorsal view. Scale bar: 1 mm.

FIGURES 1–7. Andogyrus seriatopunctatus, first instar larva. 1, Head capsule, dorsal view; 2, antenna, dorsal view; 3, mandible, dorsal view; 4, maxilla, dorsal view; 5, maxilla, ventral view; 6, labium, ventral view; 7, labium, dorsal view. Additional setae marked with a black square. Scale bars, Fig. 1: 0.5 mm; Figs 2–7: 0.1 mm.

FIGURES 8–9. Andogyrus seriatopunctatus, first instar larva. 8, Metathoracic leg, anterior view; 9, metathoracic leg, posterior view. Additional setae marked with a black square. Scale bar: 0.2 mm.

FIGURES 10–11. Andogyrus seriatopunctatus, first instar larva. 10, Abdominal segments VIII–X, dorsal view; 11, abdominal segments VIII–X, ventral view. Scale bar: 0.5 mm.

FIGURES 12–13. Andogyrus seriatopunctatus, first instar larva. 12, Head capsule, chaetotaxy, dorsal view; 13, head capsule, chaetotaxy, ventral view. Additional setae marked with a black square.