Quetotaxia primaria de larvas de Thermonectus succinctus (Aubé) (Coleoptera: Dytiscidae: Dytiscinae), en relación con su hábitat y modo de vida

MICHAT, M. C.; TORRES, P. L. M.

San Miguel de Tucumán, Tucumán, Argentina

Congreso; ACTAS VI Congreso Argentino de Entomología; 2007

Sociedad Entomológica Argentina

The diving-beetle Thermonectus succinctus (Aubé) is placed within the tribe Aciliini, subfamily Dytiscinae, and is the commonest Thermonectus Dejean species in Buenos Aires Province, Argentina. Larvae of the Dytiscidae are characterized by a bentonic lifestyle. Larvae of the Aciliini, however, exhibit an uncommon feature because they are planktonic. They spend much time near the surface film, preying mainly in open waters. Recent papers on larval Dytiscidae have focused on the chaetotaxy. The implementation of a system of nomenclature for primary sensilla allows to establish homologies that enable making hypotheses on possible function and adaptations of the larval structures. Our goals were to analyze the chaetotaxy of selected larval structures of T. succinctus, and to provide hypotheses explaining some chaetotaxic features. Larvae were dissected, mounted on slides and observed with a microscope. Primary setae and pores were distinguished for the maxilla (MX), labium (LA) and legs (CO: coxa, FE: femur, PT: pretarsus, TA: tarsus, TI: tibia, TR: trochanter). Sensilla were coded by two capital letters followed by a number (setae) or a lower case letter (pores). Some chaetotaxic features of the maxillary stipes and the labium of T. succinctus may be related to an accessory function of these organs in feeding. Seta MX4 is inserted close to the internal margin of the stipes, in contrast to other dytiscids in which it is medioventral. Also, a row of additional, spine-like setae oriented inwards is present along the dorsal surface. On the other hand, setae located on the anterodorsal margin of the prementum have been carried forward as a consequence of the development of the median process, with seta LA8 remaining at the apex of median process. Furthermore, there is an additional dorsal pore at each side of the base of median process. The absence of some setae and pores on the legs of T. succinctus larvae is of interest. In most Dytiscidae, tarsal pores TAc, TAd, TAe and TAf, and pretarsal setae PT1 and PT2 might have a role in the sense of contact between the legs and the substrate. The absence of these sensilla in T. succinctus is hypothesized to be a consequence of their planktonic lifestyle, and of a restricted contact with the substrate (bottom or aquatic plants). We could not establish if setae FE4, FE5, FE6, FE8, FE9, FE10 and TI4 are absent or they became elongate, hair-like, and incorporated to the rows of natatory setae.