A Description of the First Instar Larva of Coptotomus interrogatus (Fabricius) (Coleoptera: Dytiscidae: Coptotominae) with an Emphasis on Cranial Morphology and Comments on the Phylogeny of Basal Lineages of Dytiscidae

First instars hatched from eggs laid by adults of Coptotomus interrogatus (Fabricius) are described and illustrated with an emphasis on cranial morphology. Cranial horns are described as arising from within the frontoclypeal interior and not the distal margin. The distal margin of the frontoclypeus forms a short broadly-rounded nasale with a sub-rectangular labrum inflected onto the epipharynx.

The cranial morphology supports a close phyletic relationship between Coptotominae and Laccophilinae. Larvae of the Nearctic genus Coptotomus Say have a number of characters that represent significant departures from the dytiscid larval bauplan. The most prominent of these is the presence of lateral abdominal gills (Wilson 1923; Bacon et al. 2000).

However, the bifurcate frontoclypeal structure on the first instar (Barman 1972) is different from the short blunt frontoclypeal projection of the mature larva (Wilson 1923; Bacon et al. 2000), representing a developmental dimorphism that may be restricted to larvae of Coptotomus. Other larval characters attributed exclusively to Coptotomus, or occurring only sporadically in other taxa, include reduced abdominal tergal surface areas, extratergal abdominal spiracles (Ruhnau and Brancucci 1984), papillate spiracles, and a hypoprognathous head (Bacon et al. 2000).

Although traditionally assigned to the Colymbetinae (e.g., Hilsenhoff 1980; Larson et al. 2000), Miller (2001) recognized the taxon as a monogeneric subfamily, Coptotominae van den Branden that is closely related to neither the traditional nor revised Colymbetinae in his hypotheses. Studies of dytiscid larvae (e.g., Nilsson 1988; Alarie 1991, 1995, 1998; Alarie et al. 2001; Alarie et al. 2002) have demonstrated the importance of first instar (primary) morphology in the development and testing of systematic hypotheses.

However, the only descriptive information available for the first instar of Coptotomus (sp. ind.) (Barman 1972; Larson et al. 2000) lacks the descriptive detail required for comparative studies. Although Nilsson (1988) included Coptotomus (sp. ind.) in his general evaluation of chaetotaxy of legs of Dytiscidae and related Adephaga, he could only infer primary chaetotaxy of the genus from the third instar material that was available to him.

In this study, the first instar of C. interrogatus (Fabricius) is described, emphasizing its cranial morphology. A cladistic analysis is included to test the analytical efficacy of the selected cranial characters as well as the compatibility of Coptotominae within Miller’s (2001) clade that includes Laccophilinae Gistel, Copelatinae Erichson, Hydrodytinae Miller, and Hydroporinae Aube´. 661 662 Methods and Materials First instar larvae were obtained from eggs deposited by adults collected 14 and 21 II 1998 from temporary habitats (318189170N; 0828279150W) in Ware County, Georgia, U.S.A. Adults were held in a Kewaunee environmental chamber set for a 12 h photoperiod and 188C with fragments of Brazilian elodea (Egeria densa Planch.) as oviposition sites.

Leaves and/or stems with eggs attached onto their surfaces, were removed from the oviposition vessels and transferred into 60 by 15 mm petri dishes for hatching under the above conditions. Newly hatched larvae were preserved in 70 per cent glycerated alcohol with 10 larvae chosen at random for description, unless noted otherwise. Head lengths were measured laterally from the posterior margin of the head capsule to the distal margin of the frontoclypeus, excluding the frontoclypeal (cranial) horns, and head width was measured dorsally at the widest point.

Measurements of other structures were taken along either the greatest lengths or widths. Lengths for legs are sums of the lengths of the individual segments, excluding trochanters and tarsal claws. Primary sensilla of Coptotomus are identified and coded based on descriptions of larvae of Matinae (Alarie et al. 2001) and on analytical systems proposed by Nilsson (1988) for legs, Alarie (1995) for the legs, last abdominal segment and urogomphus, and Alarie (1998) for the head and its appendages.

Cladistic Analysis. The analysis is restricted to larval cranial characters and confined to Miller’s (2001) clade that includes Coptotominae, Laccophilinae, Copelatinae, Hydroporinae, and Hydrodytinae (Clade I). Miller retains the traditional composition for Hydroporinae (Larson et al. 2000) but defines Copelatinae and Laccophilinae differently.

Miller has the Agabetinae as a tribe within the subfamily Laccophilinae so that Lacophilianae includes Agabetes Crotch, Laccodytes Regimbart, and Laccophilus Leach. As proposed by Miller (2001), Copelatinae includes Agaporomorphus Zimmermann, Copelatus Erichson, and Lacconectus Motschulsky with two species of Agaporomorphus removed from Copelatinae and placed in a new genus, Hydrodytes Miller, and subfamily, Hydrodytinae Miller.

The Agabinae Thomson, Colymbetinae Erichson, Lancetinae van den Branden, and Dytiscinae Leach are in a second clade (Clade II), according to Miller. Character Descriptions. Character 1.

Third instar with occipital suture: present (0); absent (1). Mature larvae of Agabus Leach and Ilybius Erichson (Agabinae, Clade II) also lack occipital sutures (e.g., Barman et al. 2000; 2001). Character 2.

Number of first instar lamellate (or peg-like) sensory sensilla (lamellae clypeales): more than four (0); two (1). First instars of Lancetes Sharp included in Miller’s (2001) second clade also have two lamellate sensilla (Alarie et al. 2002 a). Character 3.

First instar labial palpomere sensilla LA 10 and LA 12: present (0); absent (1). Alarie (1991) noted that these sensilla were consistently present on Hydroporinae with the exception of Desmopachria convexa (Aube´). Character 4.

Egg bursters originating medially on the frontoclypeus: near plane formed by the anterior ocelli (0); distinctly posterior to ocelli and proximal to the epicranial suture (1). Character 5. Third instar proventriculus: absent (0); present (1).

Character 6. Nasale extending: only a short distance beyond origins of mandibles (0); well beyond origins of mandibles (1). Character 7.

Maxilla with galea: present (0); absent (1). The galea is absent on Cybister and Megadytes Sharp (Ferreira-Jr 1995). Character 8.

Third instar cranium: prognathic (0); hypoprognathic (1). Character 9. Third instar labrum: distinct and sub-rectangular (0); distinct. and arcuate (1); absent or obscured by extensive sclerotization (2).

Table 1. Character matrix of nine larval cranial characters used in the cladistic analysis (Characters are described in the text). Explanation of symbols: 0 ¼ plesiomorphic state; 1, 2 ¼ apomorphic states.

Characters Taxa 1 2 3 4 5 6 7 8 9 Matinae 0 0 0 0 0 0 0 0 0 Laccophilinae 1 1 1 1 0 0 0 0 0 Coptotominae 1 1 1 0 0 0 0 1 0 Copelatinae 0 0 0 0 1 0 0 0 1 Hydroporinae 0 0 0 0 0 1 1 0 2 Characters one through seven were developed from observations of Coptotomus and from available descriptions of larvae of Laccophilinae (Spangler and Gordon 1973; Alarie et al. 2000; Alarie et al. 2002 a; Alarie et al. 2002 b), Coptotominae (Bacon et al. 2000), Copelatinae (Spangler 1962; De Marzo 1976; Ruhnau and Brancucci 1984; Mashke et al. 2001; Alarie et al. 2002 b), and Hydroporinae (De Marzo and Nilsson 1986; Ruhnau 1986; Alarie 1991 a, b; Alarie et al. 1990; Larson et al. 2000).

Characters described in the above studies were evaluated and/or confirmed on: first instars of Laccophilus faciatus rufus Melsheimer and third instars of Matus bicarinatus (Say), Coptotomus lenticus Hilsenhoff, L. faciatus rufus, L. maculosus Say, Copelatus caelatipennis princeps Young, Neoporus undulatus (Say), N. clypealis (Sharp), Hydroporus signatus Mannerheim, and H. rufilabris Sharp. Assessments of cranial architecture (character 8) and labral morphology (character 9) were confined to those species available in the laboratory.

Hydrodytinae is excluded from the analysis because larvae of this taxon were not available and no larval descriptions were found. Miller (2001) proposed the Matinae van den Branden as the sister group to the remaining Dytiscidae, employing Allomatus nannup Watts, Matus bicarinatus (Say), and M. ovatus Leech as terminal taxa in his analysis. Alarie et al. (2002 a) used the Matinae, represented by A. nannup Watts, M. bicarinatus (Say), and Batrachomatus daemeli, as the out-group in their phylogenetic assessment of Lancetes.

Descriptions available for first instars of M. bicarinatus (Alarie 1998), A. nannup (Alarie and Watts 2003), and third instars of B. daemeli and M. bicarinatus (Alarie et al. 2001) and available third instars of M. bicarinatus were used to determine character polarities (Wiley et al. 1991). Character nine was treated as an unordered branching transformation series with all others as binary transformations. Data (Table 1) were analyzed using MacClade, version 3.07, (Maddison and Maddison 1992) and Winclada (Nixon 1999) with NONA (Goloboff 1995).