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A description of the third instar larva of Hydaticus bimarginatus (Say) (Coleoptera: Dytiscidae).

Article Excerpt
ABSTRACT

Larvae collected from a Georgia temporary habitat were identified as Hydaticus bimarginatus based on the presence of trilobed pre-mentum and distribution records. These larvae have well-developed series of natatory sensilla characteristic of nektonic dytiscids with the distribution of the spiniform sensilla corresponding to patterns of previously described larvae of Dytiscinae. Modifications of the protibia and protarsus are described and interpreted as a complex cleaning apparatus.

Key Words: Dytiscidae; Hydaticus bimarginatus; larva; life cycle; morphology; southeastern United States.

INTRODUCTION

The cosmopolitan and speciose genus Hydaticus Leach (1) is represented in Georgia by only two species, H. (G.) bimarginatus (Say) and H. (H.) cinc-tipennis Aube, (2). We have found no descriptive information for immature stages of H. bimarginatus and only a limited amount of information (or none at all) for the remaining Nearctic species. There is a brief description of the mature larva of H. aruspex Clark by Watts (3, as H. stagnalis) and a more informative and complete analysis of sensilla on the legs of the first instar of this Palaearctic species by Nilsson (4). Hilsenhoff (5) presented a key for identification of H. aruspex and H. piceus Le Conte in Wisconsin but the descriptive information provided is limited to comments on the premental anatomy of these species. Dettner (6) noted that second and third instars of European species of the subgenus Guignotites have trilobate labia with labia of larvae of the remaining species bilobate. However, Michat and Torres (7) described larvae of a South American species of Guignotites with two premental lobes, indicating that the number of premental lobes may be employed reliably as a diagnostic character only when dealing with a regional fauna or not at all.

The objectives of this study are to provide a description of the mature larva of Hydaticus bimarginatus, with an emphasis on chaetotaxy of thoracic appendages and cranial anatomy, and to evaluate the life cycle and breeding habits of this species in central Georgia.

MATERIALS AND METHODS

Mature Hydaticus larvae with trilobed labia were collected in Bibb County (Fig. 1), Georgia, U.S.A. between June 30 and September 15, 2004. These larvae were identified as H. (G.) bimarginatus because of the presence of a third (median) premental lobe that is characteristic of larvae of some species in this subgenus (6,7) and because this is the only species of record for Guignotites in Georgia (2).

[FIGURE 1 OMITTED]

Anatomical assessments were of nine larvae, unless otherwise noted. Measurements were obtained from dismembered specimens with head lengths taken dorsally from the posterior margin to the anterior margin of the fron-toclypeus along the coronal suture, excluding frontoclypeal sensilla. Gape was estimated by measuring the ventral distance between the centers of each mandibular ball that articulates with the cranium. All other measurements were taken at the longest or widest aspect. The lengths of the legs are calculated by summing the...

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