Generic keys for the identification of larval Dytiscidae from Argentina (Coleoptera: Adephaga)

Claves genéricas para la identificación de las larvas de Dytiscidae de la Argentina (Coleoptera: Adephaga)

Mariano C. Michat ^{*, **}, Miguel Archangelsky^*** and Axel O. Bachmann²

* CONICET; e-mail: marianoide@gmail.com
** Laboratorio de Entomología, Dpto. de Biodiversidad y Biología Experimental, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Av. Int. Güiraldes s/n, Ciudad Universitaria, 1428 Buenos Aires, Argentina.
*** CONICET-LIESA, Laboratorio de Investigaciones en Ecología y Sistemática Animal, Universidad Nacional de La Patagonia, San Juan Bosco, Sarmiento 849, 9200, Esquel, Chubut, Argentina; e-mail: hydrophiloidea@yahoo.com.ar

ABSTRACT: Generic keys for the identification of larvae of Dytiscidae from Argentina are presented. A total of 27 dytiscid genera are recognized in Argentina, included in 16 tribes and seven subfamilies. Twenty-two of the 27 genera are included in the keys. The remaining fi ve genera could not be included because their larvae are unknown (Bidessonotus Régimbart, Brachyvatus Zimmermann, Hemibidessus Zimmermann, Neobidessus Young) or remain poorly known (Cybister Curtis). The keys are presented in English and Spanish. Morphological as well as chaetotaxic characters were included, and an emphasis was made to include easily seen and identifiable characters. However, due to the small size of many dytiscid larvae, some of the characters mentioned in the keys (especially the chaetotaxic ones) are difficult to visualize through the observation with a stereoscopic microscope. Therefore, the use of a compound microscope for the identifi cation is recommended.

KEY WORDS: Diving-beetles; Dytiscidae; Larvae; Keys; Argentina.

RESUMEN: Se presentan claves genéricas para la identificación de las larvas de Dytiscidae de la Argentina. Un total de 27 géneros de Dytiscidae son reconocidos en la Argentina, comprendidos en 16 tribus y siete subfamilias. Veintidós de los 27 géneros fueron incluidos en las claves. Los restantes cinco géneros no pudieron ser incluidos porque sus larvas son desconocidas (Bidessonotus Régimbart, Brachyvatus Zimmermann, Hemibidessus Zimmermann, Neobidessus Young) o son conocidas muy imperfectamente (Cybister Curtis). Las claves se presentan en inglés y español. Se incluyeron caracteres morfológicos y de la quetotaxia, y se puso énfasis en incluir caracteres fácilmente visualizables e identificables. Sin embargo, debido al pequeño tamaño de muchas larvas de Dytiscidae, algunos de los caracteres mencionados en las claves (especialmente los de la quetotaxia) son difíciles de visualizar a través de la observación con el microscopio estereoscópico. Por lo tanto, se recomienda el uso del microscopio compuesto para la identificación.

PALABRAS CLAVE: Escarabajos buceadores; Dytiscidae; Larvas; Claves; Argentina.

Recibido: 01-04-2008;
Aceptado: 08-07-2008

INTRODUCTION

Diving beetles (Dytiscidae) are a very specious and diverse group of aquatic coleopterans. The family is cosmopolitan, with representatives all around the world, from both polar regions to the equator (Galewski, 1971). However, the greater number of species is found in tropical and subtropical areas (Nilsson, 2001; Ribera et al., 2004; Balke, 2005). Some species have a very wide distributional range, including several zoogeographic regions; others, on the contrary, have a very restricted distribution, or are endemic of islands or particular areas.
Some others have a circumpolar distribution, and even others inhabit great altitudes up to 4700 masl in mountainous areas. Dytiscids exploit almost every continental waters (Wilson, 1923; Trémouilles, 1998; Balke, 2005). In general, they are found in larger numbers in lentic habitats, both permanent and temporary. In lotic habitats they are most frequently found in the wetted margins, where the water current is slow or absent and there is an accumulation of sediments and organic debris. They are always less abundant in waters with large populations of fish (Watts, 1978). Diving beetles live in association with the aquatic vegetation, and for that, are much more numerous in the marginal zones of the water bodies, where the water is shallow and the emergent and/or submergent vegetation is particularly abundant. They also use the terrestrial vegetation when inhabiting floodplain ponds or temporary rain pools. Both larvae and adults are active throughout the year, though are much less numerous in the winter than in the summer (Watts, 1978). The larvae pass through three morphologically similar instars except for the size. They are predaceous and generally very voracious, eating practically every prey (most commonly arthropods or annelids) of a suitable size (Balke, 2005).
The family Dytiscidae is included in the order Coleoptera, suborder Adephaga. With approximately 4000 described species (Nilsson, 2001, 2003, 2004; Nilsson & Fery, 2006; Jäch & Balke, 2008), dytiscids are the most specious group of aquatic beetles in the world. The classification followed here is that of Miller (2001, 2002, 2005) and Nilsson (2001) (Table I), in which the family is divided in 10 subfamilies, 27 tribes and about 175 genera. About half of the species are included in the morphologically diverse subfamily Hydroporinae (some 2000 species). The rest is divided in the remaining nine subfamilies as follows (approximate numbers): Agabinae (370 species), Colymbetinae (130 species), Copelatinae (540 species), Coptotominae (5 species), Dytiscinae (380 species), Hydrodytinae (4 species), Laccophilinae (400 species), Lancetinae (22 species) and Matinae (8 species). Seven (70%) out of the 10 subfamilies are present in Argentina, only Coptotominae, Hydrodytinae and Matinae are absent. Similarly, 16 (59%) out of the 27 tribes and 27 (15%) out of the 175 genera are represented in Argentina (Table I). Dytiscidae includes some 113 species in Argentina (approximately 3% of the total number of species) (Table I), though almost surely this number will be increased with further studies. Twenty-three (81%) out of the 27 Argentinean dytiscid genera are known as larvae (Table I). Only four genera remain unknown: Bidessonotus Régimbart, Brachyvatus Zimmermann, Hemibidessus Zimmermann and Neobidessus Young, all included in the tribe Bidessini. A special case is the genus Cybister Curtis, which is cosmopolitan in distribution and the larva is known only from European material.

Table I. Genera of Dytiscidae, number of species in Argentina, larvae described, and source. I-III under the column "Larvae described" refer to larval instars. Information in the column "N° species" was taken from Young (1974, 1981), Trémouilles & Bachmann (1980, 1981), Bachmann & Trémouilles (1981, 1982), Trémouilles (1984, 1989, 1995, 1996, 1998, 2000), Trémouilles et al. (1995), Balke (1992), Miller (2000, 2005), Nilsson (2001), Torres et al. (2008), Michat, pers. obs.

Besides their inherent biological interest, dytiscids play an important role within the aquatic ecosystems, as predators of species of economic or medical importance and as indicators of environmental conditions (Larson, 1975). Also, they are a component in the diet of different groups. Diving beetles are a very interesting group from a limnological point of view, though rarely appear in large biomasses (Trémouilles et al., 1995). The deficient knowledge of the larvae of aquatic beetles in general, and of Dytiscidae in particular, has serious consequences for other disciplines of biological investigation, such as studies on biodiversity, ecology, and limnology. These problems are due in many cases to the impossibility of identifying the larvae collected in the fi eld (Archangelsky, 1997).
One of the most important problems derived from the deficient knowledge on the larvae of Dytiscidae in Argentina is the lack of keys of identification. As a consequence, the dytiscid fauna from Argentina has been usually identified with keys from other regions of the world, constructed for a different fauna and therefore problematic when applied to our fauna. Among the most common issues are: 1) the genus to be identified is not present in the key (this is a serious problem because it frequently leads to an erroneous identifi cation), 2) the Argentinean genera are mixed with many others that are not present here and make the identification process difficult, and 3) some diverse and widespread genera may show morphological variation (or may be paraphyletic), and those species from other zoogeographical regions may present characters that differ from those of Argentinean species. To fill this gap, in the present paper we provide generic keys for the identification of the larvae of Dytiscidae from Argentina.

MATERIAL AND METHODS

The material used for preparing the keys was collected in several collecting trips to different parts of Argentina (1997-2008). The visited provinces were Buenos Aires, Chubut, Córdoba, Corrientes, Entre Ríos, Formosa, Jujuy, La Rioja, Misiones, Salta and Tucumán. For the genera which we did not have access to material (Derovatellus Sharp and Notaticus Zimmermann), data were taken from the literature. As mentioned above, 27 dytiscid genera are known from Argentina (Table I). Few other genera, known from bordering countries, could be present here. However, as their presence has not been documented so far, they are not treated in this paper.
Larvae were cleared in lactic acid, dissected and mounted on glass slides with polyvinyl-lacto-glycerol or Hoyer's medium. Observation (at magnifications up to 1000×) and drawings were made using an Olympus CX31 compound microscope equipped with a camera lucida. Drawings were scanned and digitally edited. The material is held in the larval collections of M. C. Michat (Laboratorio de Entomologia, Universidad de Buenos Aires, Argentina) and M. Archangelsky (LIESA, Universidad Nacional de La Patagonia, Esquel, Argentina).
The nomenclature used in the keys for structures as well as for setae and pores follows that employed in previous papers dealing with the larval morphometry and chaetotaxy of members of the family Dytiscidae (Nilsson, 1988; Alarie & Harper, 1990; Alarie et al., 1990, 2000, 2001, 2002; Alarie, 1991, 1995, 1998; Alarie& Michat, 2007a; see also references in Table I). The reader is referred to those papers for a complete guide of the morphometric and chaetotaxic terms commonly applied to dytiscid larvae.

RESULTS

Key to larval instars
1. Egg bursters present (Figs. 1-18); temporal spiniform setae absent ................................................................................. Instar I Egg bursters absent; temporal spiniform setae generally present (Figs. 49-51, 54, 60-64) .................................................. 2

2. Spiracles on lateral margin of mesothorax and abdominal segments I-VII absent ……......................................…...... Instar II Spiracles on lateral margin of mesothorax and abdominal segments I-VII present (Fig. 66) ..................................... Instar III

Figs. 1-12. Dytiscidae, first-instar larva, head, dorsal view. 1, Laccornellus lugubris (Aubé); 2, Amarodytes duponti (Aubé); 3, Anodocheilus maculatus Babington; 4, Hypodessus cruciatus (Régimbart); 5, Liodessus flavofasciatus (Steinheil); 6, Pachydrus obesus Sharp; 7, Vatellus haagi Wehncke; 8, Desmopachria concolor Sharp; 9, Hydrovatus caraibus Sharp; 10, Megadytes glaucus (Brullé); 11, Thermonectus succinctus (Aubé); 12, Hydaticus tuyuensis Trémouilles. AMF: anterior margin of frontoclypeus (margen anterior del frontoclípeo), EB: egg bursters (ovirruptores), LPF: lateral projections of frontoclypeus (proyecciones laterales del frontoclípeo), NA: Nasale, OS: occipital suture (sutura occipital). Fig. 1 modified from Alarie & Michat (2007b); Fig. 2 modified from Michat & Alarie (2006); Fig. 3 modified from Michat & Torres (2006b); Fig. 4 modified from Michat & Alarie (2008); Fig. 5 modified from Alarie et al. (2007); Fig. 6 modified from Michat & Torres (2008); Fig. 7 modified from Michat & Torres (2005a); Fig. 8 modified from Michat & Archangelsky (2007); Fig. 9 modified from Michat (2006a); Fig. 10 modified from Michat (2006b); Fig. 11 modified from Michat & Torres (2005b); Fig. 12 modified from Michat & Torres (2006a).

 

Figs. 13-30. Dytiscidae, first-instar larva. 13-18, head, dorsal view: 13, Rhantus calileguai Trémouilles; 14, Bunites distigma (Brullé); 15, Copelatus sp.; 16, Leuronectes curtulus Régimbart; 17, Laccophilus obliquatus Régimbart; 18, Lancetes marginatus (Steinheil). 19-24, antenna, ventral view: 19, Hypodessus cruciatus; 20, Anodocheilus maculatus; 21, Vatellus haagi; 22, Derovatellus floridanus Fall; 23, Laccophilus obliquatus; 24, Lancetes marginatus. 25-26, mandible, dorsal view: 25, Copelatus sp.; 26, Leuronectes curtulus. 27-28, maxilla, ventral view: 27, Laccornellus lugubris; 28, Anodocheilus maculatus. 29-30, maxilla, dorsal view: 29, Copelatus sp.; 30, Laccophilus obliquatus. ALF: anterolateral lobes of frontoclypeus (lóbulos anterolaterales del frontoclípeo), CD: cardo, OS: occipital suture (sutura occipital), SP: spinula (espínula), ST: stipes (estipes). Fig. 14 modified from Michat (2005); Figs. 17, 23, 30 modified from Michat (2008); Figs. 18, 24 modified from Michat et al. (2005); Fig. 19 modified from Michat & Alarie (2008); Figs. 20, 28 modified from Michat & Torres (2006b); Fig. 21 modified from Michat & Torres (2005a); Fig. 22 modified from Spangler (1966); Fig. 27 modified from Alarie & Michat (2007b).

Figs. 31-48. Dytiscidae, first-instar larva. 31-36, labium, dorsal view: 31, Laccornellus lugubris; 32, Anodocheilus maculatus; 33, Desmopachria concolor; 34, Hydrovatus caraibus; 35, Thermonectus succinctus; 36, Hydaticus tuyuensis. 37, Thermonectus succinctus, habitus, lateral view. 38, Hydaticus tuyuensis, habitus, dorsal view. 39-40, metathoracic leg, anterior view: 39, Bunites distigma; 40, Rhantus calileguai. 41-42, Celina parallela (Babington), last abdominal segment: 41, dorsal view; 42, lateral view. 43-48, last abdominal segment, dorsal view: 43, Thermonectus succinctus; 44, Rhantus calileguai; 45, Bunites distigma; 46, Leuronectes curtulus; 47, Laccophilus obliquatus; 48, Lancetes marginatus. MPP: median process of prementum (proceso medio del prementum), PRMT: prementum, SP: spinula (espínula), TT: tracheal trunks (troncos traqueales). Fig. 31 modified from Alarie & Michat (2007b); Fig. 32 modified from Michat & Torres (2006b); Fig. 33 modified from Michat & Archangelsky (2007); Fig. 34 modified from Michat (2006a); Figs. 35, 37, 43 modified from Michat & Torres (2005b); Figs. 36, 38 modified from Michat & Torres (2006a); Figs. 39, 45 modified from Michat (2005); Figs. 41-42 modified from Michat et al. (2007); Fig. 47 modified from Michat (2008); Fig. 48 modified from Michat et al. (2005).

 

Figs. 49-60. Dytiscidae, third-instar larva, head, dorsal view. 49, Hypodessus cruciatus; 50, Amarodytes duponti; 51, Pachydrus obesus; 52, Hydrovatus caraibus; 53, Desmopachria concolor; 54, Vatellus haagi; 55, Megadytes glaucus; 56, Megadytes magnus Trémouilles & Bachmann; 57, Megadytes robustus (Aubé); 58, Megadytes marginithorax (Perty); 59, Thermonectus succinctus; 60, Hydaticus tuyuensis. NA: nasale, OS: occipital suture (sutura occipital), TSS: temporal spiniform setae (setas espiniformes temporales). Fig. 49 modified from Michat & Alarie (2008); Fig. 50 modified from Michat & Alarie (2006); Fig. 51 modified from Michat & Torres (2008); Fig. 52 modified from Michat (2006a); Fig. 53 modified from Michat & Archangelsky (2007); Fig. 54 modified from Michat & Torres (2005a); Figs. 55, 56, 57 modified from Michat (2006b); Fig. 58 modified from Ferreira-Jr. (1995); Fig. 59 modified from Michat & Torres (2005b); Fig. 60 modified from Michat & Torres (2006a).

Figs. 61-77. Dytiscidae, third-instar larva. 61-64, head, dorsal view: 61, Copelatus sp.; 62, Rhantus calileguai; 63, Laccophilus obliquatus; 64, Lancetes marginatus. 65, Notaticus fasciatus Zimmermann, labium, ventral view. 66, Rhantus calileguai, body, lateral view. 67-68, prothoracic leg, anterior view: 67, Pachydrus obesus; 68, Hydrovatus caraibus. 69-70, prothoracic leg, posterior view: 69, Leuronectes curtulus; 70, Laccophilus obliquatus. 71, Megadytes glaucus, urogomphi, ventral view. 72-77, last abdominal segment and urogomphi, dorsal view: 72, Thermonectus succinctus; 73, Agaporomorphus sp.; 74, Rhantus calileguai; 75, Leuronectes curtulus; 76, Laccophilus obliquatus; 77, Lancetes marginatus. OS: occipital suture (sutura occipital). Figs. 63, 70, 76 modified from Michat (2008); Figs. 64, 77 modified from Michat et al. (2005); Fig. 65 modified from Miller et al. (2007); Fig. 67 modified from Michat & Torres (2008); Fig. 68 modified from Michat (2006a); Fig. 72 modified from Michat & Torres (2005b).

 

Key to instar I
1. Nasale present (Figs. 1-9) ..................... 2 Nasale absent (Figs. 10-18) ................ 12

2. Occipital suture present (Fig. 2) ........... 3 Occipital suture absent (Figs. 1, 3-9) ..................................................................... 4

3. Tracheal trunks not projected from the apex of the siphon; pore PAk absent ……................… Amarodytes Régimbart Tracheal trunks projected from the apex of the siphon (Figs. 41-42); pore PAk present …………………………..….. Celina Aubé

4. Nasale subtriangular (Figs. 1-5) ............ 5 Nasale with lateral margins subparallel (Figs. 6-9) ............................................. 8

5. Cardo not fused to stipes (Fig. 27); lateral margins of prementum with elongate spinulae (Fig. 31); seta AB15 present; pore ABc present …...................... Laccornellus Roughley & Wolfe Cardo fused to stipes (Fig. 28); lateral margins of prementum without spinulae (Fig. 32); seta AB15 absent; pore ABc absent ….............................................. 6

6. Third antennomere with a ventroapical spinula (Fig. 19); distribution: Salta Province ……….. Hypodessus Guignot Third antennomere without a ventroapical spinula (Fig. 20) …...................……..... 7

7. Length of last abdominal segment more than 0.65 times head width; length of urogomphus more than 3.20 times head width; length of first urogomphomere less than 1.20 times length of second ………...…....... Anodocheilus Babington Length of last abdominal segment less than 0.60 times head width; length of urogomphus less than 2.80 times head width; length of first urogomphomere more than 1.30 times length of second ………….................... Liodessus Guignot

8. Nasale with lateral projections (Figs. 6-7) …......................................................... 9 Nasale without lateral projections (Figs. 8-9) …................................................ 11

9. Lateral projections of nasale short, unifid apically (Fig. 6) …..................................................…..…….......... Pachydrus Sharp Lateral projections of nasale very elongate, bifid apically (Fig. 7) ….....………...... 10

10. Length of fourth antennomere 1/4 length of third (Fig. 21) .……....... Vatellus Aubé Length of fourth antennomere almost 1/2 length of third (Fig. 22) ….…..........…........... Derovatellus Sharp

11. Prementum much longer than broad (Fig. 33); second labial palpomere broad, robust, rounded apically (Fig. 33); pores PAm and PAo present …................................... Desmopachria Babington Prementum about as long as broad (Fig. 34); second labial palpomere slender, pointed apically (Fig. 34); pores PAm and PAo absent ….. Hydrovatus Motschulsky

12. Anterior margin of frontoclypeus deeply sinuate (Figs. 10, 55-58); urogomphi reduced to small ventral tubercles (Fig. 71) ...……........ Megadytes Sharp ….. 13 Anterior margin of frontoclypeus evenly curved (Figs. 11-18); urogomphi well developed (Figs. 72-77) …................. 16

13. Median projection of frontoclypeus sharp apically, almost glabrous (Fig. 57); mandible regularly curved, without a ring of setae on distal third (Fig. 57) ………………....... M. (Trifurcitus) Brinck Median projection of frontoclypeus truncate apically, with numerous apical setae (Figs. 55-56, 58); mandible with distal third more strongly projected inwards, bearing a ring of elongate setae (Figs. 55-56, 58) …................................ 14

14. Emargination between median projection and each lateral projection of frontoclypeus very narrow (Fig. 58) …................................... M. (Megadytes) Sharp Emargination between median projection and each lateral projection of frontoclypeus wide (Figs. 55-56) ........ 15

15. Lateral projections of frontoclypeus simple (Fig. 56) …….............................................................. M. (Bifurcitus) Brinck Lateral projections of frontoclypeus bifid (Fig. 55) ….....................……………..…. M. (Paramegadytes) Trémouilles & Bachmann

16. Last abdominal segment with a lateral row of natatory setae (Fig. 43); prothorax with a ventral sclerite …..................... 17 Last abdominal segment without a lateral row of natatory setae (Figs. 44-48); prothorax without ventral sclerites .... 19

17. Median process of prementum absent (Fig. 65) ….............................................................…...... Notaticus Zimmermann Median process of prementum present (Figs. 35-36) …................................... 18

18. Body bent medially, gibbous in lateral view (Fig. 37); the two anterodorsal stemmata very large (Fig. 11); median process of prementum simple (Fig. 35) …....……….......... Thermonectus Dejean Body not bent medially, not gibbous in lateral view (Fig. 38); the two anterodorsal stemmata similar to the others (Fig. 12); median process of prementum bifi d (Fig. 36) ………........……… Hydaticus Leach

19. Occipital suture present (Figs. 13-14) …..…………............................................ 20 Occipital suture absent (Figs. 15-18) ...................................................….......... 21

20. Frontoclypeus and parietals with additional setae (Fig. 14); tibia with additional anterodorsal setae (Fig. 39); last abdominal segment with many additional setae (Fig. 45); distribution: Jujuy and Tucumán Provinces, at altitudes higher than 2000 m ….…….. Bunites Spangler Frontoclypeus and parietals without additional setae (Fig. 13); tibia without additional anterodorsal setae (Fig. 40); last abdominal segment without additional setae (Fig. 44); distribution: all throughout Argentina, from sea level to high altitudes …...……………...…….. Rhantus Dejean

21. Mandibles without closed channel, with robust denticles on internal margin (Fig. 25); internal margin of stipes with three robust denticles (Fig. 29); anterolateral lobes of frontoclypeus inconspicuous (Fig. 15) .…………........... Copelatus Erichson Mandibles with closed channel, without robust denticles on internal margin (Fig. 26); internal margin of stipes without robust denticles (Fig. 30); anterolateral lobes of frontoclypeus well developed (Figs. 16-18) ....................................... 22

22. Last abdominal segment elongate, longer than head width (Fig. 73)……….............Agaporomorphus? Zimmermann Last abdominal segment short, shorter than half of head width (Figs. 46-48) ……… 23

23. Anterior margin of frontoclypeus with 10 spatulate setae (Fig. 16); last abdominal segment with a robust additional seta on lateral margin (Fig. 46) …….………………... Leuronectes Sharp Anterior margin of frontoclypeus with two spatulate setae (Figs. 17-18); last abdominal segment without additional setae (Figs. 47-48) ……...................... 24

24. Frontoclypeus truncate posteriorly, with the egg bursters placed on posterolateral angles (Fig. 17); length of fourth antennomere 1/3 length of third (Fig. 23); pore ABc absent ….................................................................... Laccophilus Leach Frontoclypeus acute posteriorly, with the egg bursters placed on lateral margins (Fig. 18); length of fourth antennomere similar to that of third (Fig. 24); pore ABc present….......…......................... Lancetes Sharp

Key to instars II-III
1. Nasale present (Figs. 49-54) …............. 2 Nasale absent (Figs. 55-64) ….............. 12

2. Tracheal trunks projected from the apex of the siphon (Figs. 41-42) …….............………........... Celina Aubé Tracheal trunks not projected from the apex of the siphon ................................ 3

3. Nasale subtriangular (Figs. 49-50) .......... 4 Nasale with lateral margins subparallel (Figs. 51-54) ………............................... 8

4. Cardo not fused to stipes (Fig. 27); lateral margins of prementum with elongate spinulae (Fig. 31) …..…....................................... Laccornellus Roughley & Wolfe Cardo fused to stipes (Fig. 28); lateral margins of prementum without spinulae (Fig. 32) …..…...................................... 5

5. Third antennomere with a ventroapical spinula (Fig. 19) ….....................…...... 6 Third antennomere without a ventroapical spinula (Fig. 20) …............................... 7

6. Pore PAk present; larva predominantly testaceous to light brown, colour pattern of the head as in Fig. 49; distribution: Salta Province ..............………………..…….......................... Hypodessus Guignot Pore PAk absent; larva predominantly dark brown, colour pattern of the head as in Fig. 50; distribution: Misiones Province …………........... Amarodytes Régimbart

7. Length of last abdominal segment more than 0.85 times head width; length of first urogomphomere less than 0.95 times length of second .............................................……........ Anodocheilus Babington Length of last abdominal segment less than 0.70 times head width; length of first urogomphomere more than 1.05 times length of second …... Liodessus Guignot

8. Occipital suture absent (Figs. 51-52) … 9 Occipital suture present (Figs. 53-54) …. 10

9. Nasale with lateral projections (Fig. 51); femora with natatory setae on ventral margin (Fig. 67) ….....................................................……...... Pachydrus Sharp Nasale without lateral projections (Fig. 52); femora without natatory setae on ventral margin (Fig. 68) ................................……...........… Hydrovatus Motschulsky

10. Nasale without lateral projections (Fig. 53); prementum much longer than broad (Fig. 33) ……… Desmopachria Babington Nasale with lateral projections (Fig. 54); prementum about as long as broad ….……......................................……......... 11

11. Length of fourth antennomere 1/5-1/6 length of third (Fig. 21) ………………................... Vatellus Aubé Length of fourth antennomere somewhat more than 2/5 length of third (Fig. 22) …………………....... Derovatellus Sharp

12. Anterior margin of frontoclypeus deeply sinuate (Figs. 55-58); urogomphi reduced to small ventral tubercles (Fig. 71) …........…….... Megadytes Sharp ….. 13 Anterior margin of frontoclypeus evenly curved (Figs. 59-64); urogomphi well developed (Figs. 72-77) …................. 16

13. Median projection of frontoclypeus sharp apically, almost glabrous (Fig. 57); mandible regularly curved, without a ring of setae on distal third (Fig. 57) ………………….... M. (Trifurcitus) Brinck Median projection of frontoclypeus truncate apically, with numerous apical setae (Figs. 55-56, 58); mandible with distal third more strongly projected inwards, bearing a ring of elongate setae (Figs. 55-56, 58) …............. 14

14. Emargination between median projection and each lateral projection of frontoclypeus very narrow (Fig. 58) …........................ M. (Megadytes) Sharp Emargination between median projection and each lateral projection of frontoclypeus wide (Figs. 55-56) …..... 15

15. Lateral projections of frontoclypeus simple (Fig. 56) ….........................................………........... M. (Bifurcitus) Brinck Lateral projections of frontoclypeus bifid (Fig. 55) ………………………….…….... M. (Paramegadytes) Trémouilles & Bachmann

16. Last abdominal segment with a lateral row of natatory setae (Fig. 72); prothorax with a ventral sclerite ….......... 17 Last abdominal segment without a lateral row of natatory setae (Figs. 73-77); prothorax without ventral sclerites …... 19

17. Median process of prementum absent (Fig. 65) …........... Notaticus Zimmermann Median process of prementum present (Figs. 35-36) ….................. 18

18. Body bent medially, gibbous in lateral view (Fig. 37); the two anterodorsal stemmata very large (Fig. 59); median process of prementum simple (Fig. 35) ….……………......Thermonectus Dejean Body not bent medially, not gibbous in lateral view (Fig. 38); the two anterodorsal stemmata similar to the others (Fig. 60); median process of prementum bifid (Fig. 36) …….…….............. Hydaticus Leach

19. Occipital suture present (Figs. 61-62)...20 Occipital suture absent (Figs. 63-64) ..... 22

20. Mandibles without closed channel, with robust denticles on internal margin (Fig. 25); internal margin of stipes with three robust denticles (Fig. 29); anterolateral lobes of frontoclypeus inconspicuous (Fig. 61) ..........................……………. Copelatus Erichson Mandibles with closed channel, without robust denticles on internal margin (Fig. 26); internal margin of stipes without robust denticles; anterolateral lobes of frontoclypeus well developed (Fig. 62) .....…………….. 21

21. Head width less than 2.30 mm in instar II and less than 3.50 mm in instar III; distribution: all throughout Argentina, from sea level to high altitudes …...……...……............ Rhantus Dejean Head width more than 2.50 mm in instar II and more than 3.70 mm in instar III; distribution: Jujuy and Tucumán Provinces, at altitudes higher than 2000 m ............. Bunites Spangler

22. Length of last abdominal segment greater than head width (Fig. 73) ………............ Agaporomorphus? Zimmermann Length of last abdominal segment smaller than head width (Figs. 75-77)….......…...…....................................... 23

23. Urogomphi short, less than twice head width, without secondary setae (Fig. 75); tibia and tarsus without natatory setae (Fig. 69) …..................... Leuronectes Sharp Urogomphi elongate, more than twice head width, with numerous secondary setae (Figs. 76-77); tibia and tarsus with natatory setae on dorsal margin (Fig. 70) ….…………………….......………........ 24

24. Lateral margin of parietal with 2-3 elongate, spiniform, temporal setae (Fig. 63); urogomphi less than 3.30 times head width ……….......... Laccophilus Leach Lateral margin of parietal with several (more than 6) short, spiniform, temporal setae (Fig. 64); urogomphi more than 3.60 times head width …..........……….......... Lancetes Sharp

Clave para estadios larvales
1. Ovirruptores presentes (Figs. 1-18); setas espiniformes temporales ausentes …........................................................... Larva I Ovirruptores ausentes; setas espiniformes temporales generalmente presentes (Figs. 49-51, 54, 60-64) ................................... 2

2. Espiráculos ausentes en el margen lateral del mesotórax y de los segmentos abdominales I-VII …….................. Larva II Espiráculos presentes en el margen lateral del mesotórax y de los segmentos abdominales I-VII (Fig. 66) …............................................................ Larva III

Clave para larvas I
1. Nasale presente (Figs. 1-9) …............... 2 Nasale ausente (Figs. 10-18) …............ 12

2. Sutura occipital presente (Fig. 2) …...... 3 Sutura occipital ausente (Figs. 1, 3-9) …. 4

3. Troncos traqueales no proyectados desde el ápice del sifón; poro PAk ausente ……… Amarodytes Régimbart Troncos traqueales proyectados desde el ápice del sifón (Figs. 41-42); poro PAk presente …….....… Celina Aubé

4. Nasale subtriangular (Figs. 1-5) …..... 5 Nasale con márgenes laterales subparalelos (Figs. 6-9) …................ 8

5. Cardo no fusionado al estipes (Fig. 27); márgenes laterales del prementum con espínulas largas (Fig. 31); seta AB15 presente; poro ABc presente …....................... Laccornellus Roughley & Wolfe Cardo fusionado al estipes (Fig. 28); márgenes laterales del prementum sin espínulas (Fig. 32); seta AB15 ausente; poro ABc ausente ................................ 6

6. Espínula ventroapical del tercer antenómero presente (Fig. 19); distribución: provincia de Salta ............................................. Hypodessus Guignot Espínula ventroapical del tercer antenómero ausente (Fig. 20) …........... 7

7. Longitud del último segmento abdominal más de 0.65 veces el ancho de la cabeza; longitud del urogonfo más de 3.20 veces el ancho de la cabeza; longitud del primer urogonfómero menos de 1.20 veces la longitud del segundo ….................................................. Anodocheilus Babington Longitud del último segmento abdominal menos de 0.60 veces el ancho de la cabeza; longitud del urogonfo menos de 2.80 veces el ancho de la cabeza; longitud del primer urogonfómero más de 1.30 veces la longitud del segundo ….............................. Liodessus Guignot

8. Nasale con proyecciones laterales (Figs. 6-7) …................................................ 9 Nasale sin proyecciones laterales (Figs. 8-9) …................................................ 11

9. Proyecciones laterales del nasale cortas, simples apicalmente (Fig. 6) ….................................................. Pachydrus Sharp Proyecciones laterales del nasale muy largas, bífidas apicalmente (Fig. 7) …. 10

10. Longitud del cuarto antenómero 1/4 de la longitud del tercero (Fig. 21) …........................................................ Vatellus Aubé Longitud del cuarto antenómero casi 1/2 de la longitud del tercero (Fig. 22) …......................................... Derovatellus Sharp

11. Prementum mucho más largo que ancho (Fig. 33); segundo palpómero labial ancho, robusto, redondeado apicalmente (Fig. 33); poros PAm y PAo presentes …............................................... Desmopachria Babington Prementum aproximadamente tan largo como ancho (Fig. 34); segundo palpómero labial delgado, aguzado apicalmente (Fig. 34); poros PAm y PAo ausentes …........................................................ Hydrovatus Motschulsky

12. Margen anterior del frontoclípeo profundamente sinuado (Figs. 10, 55-58); urogonfos reducidos a pequeños tubérculos ventrales (Fig. 71).... Megadytes Sharp …......... 13 Margen anterior del frontoclípeo regularmente curvado (Figs. 11-18); urogonfos bien desarrollados (Figs. 72- 77) …............................................... 16

13. Proyección media del frontoclípeo aguzada apicalmente, casi glabra (Fig. 57); mandíbula regularmente curvada, sin un anillo de setas en el tercio distal (Fig. 57) …..... M. (Trifurcitus) Brinck Proyección media del frontoclípeo truncada apicalmente, con muchas setas apicales (Figs. 55-56, 58); mandíbula con el tercio distal más fuertemente proyectado hacia adentro, llevando un anillo de setas largas (Figs. 55-56, 58) ….............................................. 14

14. Emarginación entre la proyección media y cada proyección lateral del frontoclípeo muy angosta (Fig. 58)................................................ M. (Megadytes) Sharp Emarginación entre la proyección media y cada proyección lateral del frontoclípeo ancha (Figs. 55-56) ....... 15

15. Proyecciones laterales del frontoclípeo simples (Fig. 56) …............................................... M. (Bifurcitus) Brinck Proyecciones laterales del frontoclípeo bífidas (Fig. 55) …………………….... M. (Paramegadytes) Trémouilles & Bachmann

16. Último segmento abdominal con una hilera lateral de setas natatorias (Fig. 43); protórax con un esclerito ventral .....17 Último segmento abdominal sin una hilera lateral de setas natatorias (Figs. 44-48); protórax sin escleritos ventrales ....... 19

17. Proceso medio del prementum ausente (Fig. 65) …......... Notaticus Zimmermann Proceso medio del prementum presente (Figs. 35-36) …..................18

18. Cuerpo arqueado medialmente, giboso en vista lateral (Fig. 37); los dos stemmata anterodorsales muy grandes (Fig. 11); proceso medio del prementum simple (Fig. 35) …............... Thermonectus Dejean Cuerpo no arqueado medialmente, no giboso en vista lateral (Fig. 38); los dos stemmata anterodorsales similares a los demás (Fig. 12); proceso medio del prementum bífido (Fig. 36) ........................................... Hydaticus Leach

19. Sutura occipital presente (Figs. 13-14)…...................................................... 20 Sutura occipital ausente (Figs. 15-18) .............................................................. 21

20. Frontoclípeo y parietales con setas adicionales (Fig. 14); tibia con setas adicionales anterodorsales (Fig. 39); último segmento abdominal con muchas setas adicionales (Fig. 45); distribución: provincias de Jujuy y Tucumán, en altitudes mayores a 2.000 m …................................................. Bunites Spangler Frontoclípeo y parietales sin setas adicionales (Fig. 13); tibia sin setas adicionales anterodorsales (Fig. 40); último segmento abdominal sin setas adicionales (Fig. 44); distribución: en toda la Argentina, desde el nivel del mar hasta grandes altitudes ….............................................................. Rhantus Dejean

21. Mandíbulas sin surco cerrado, con dentículos robustos en el margen interno (Fig. 25); margen interno del estipes con tres dentículos robustos (Fig. 29); lóbulos anterolaterales del frontoclípeo inconspicuos (Fig. 15) ............................................................. Copelatus Erichson Mandíbulas con surco cerrado, sin dentículos robustos en el margen interno (Fig. 26); margen interno del estipes sin dentículos robustos (Fig. 30); lóbulos anterolaterales del frontoclípeo bien desarrollados (Figs. 16-18) ................. 22

22. Último segmento abdominal largo, de longitud mayor que el ancho de la cabeza (Fig. 73) ………....................................................... Agaporomorphus? Zimmermann Último segmento abdominal corto, de longitud menor que la mitad del ancho de la cabeza (Figs. 46-48) ................ 23

23. Margen anterior del frontoclípeo con 10 setas espatuladas (Fig. 16); último segmento abdominal con una seta adicional robusta en el margen lateral (Fig. 46) ........................... Leuronectes Sharp Margen anterior del frontoclípeo con dos setas espatuladas (Figs. 17-18); último segmento abdominal sin setas adicionales (Figs. 47-48) ............... 24

24. Frontoclípeo truncado posteriormente, con los ovirruptores ubicados en los ángulos posterolaterales (Fig. 17); cuarto antenómero de 1/3 de la longitud del tercero (Fig. 23); poro ABc ausente ...................................... Laccophilus Leach Frontoclípeo aguzado posteriormente, con los ovirruptores ubicados en los márgenes laterales (Fig. 18); cuarto antenómero de longitud similar al tercero (Fig. 24); poro ABc presente ………………............…. Lancetes Sharp

Clave para larvas II-III
1. Nasale presente (Figs. 49-54) …........2 Nasale ausente (Figs. 55-64) ........... 12

2. Troncos traqueales proyectados desde el ápice del sifón (Figs. 41-42) ………............................................... Celina Aubé Troncos traqueales no proyectados desde el ápice del sifón ….............. 3

3. Nasale subtriangular (Figs. 49-50) ...... 4 Nasale con márgenes laterales subparalelos (Figs. 51-54) ................... 8

4. Cardo no fusionado al estipes (Fig. 27); márgenes laterales del prementum con espínulas largas (Fig. 31) ..................... Laccornellus Roughley & Wolfe Cardo fusionado al estipes (Fig. 28); márgenes laterales del prementum sin espínulas (Fig. 32) …....... 5

5. Espínula ventroapical del tercer antenómero presente (Fig. 19) ........... 6 Espínula ventroapical del tercer antenómero ausente (Fig. 20) ....... 7

6. Poro PAk presente; larva predominantemente testácea a marrón clara, patrón de coloración de la cabeza como en la Fig. 49; distribución: provincia de Salta ................................... Hypodessus Guignot Poro PAk ausente; larva predominantemente marrón oscura, patrón de coloración de la cabeza como en la Fig. 50; distribución: provincia de Misiones .... Amarodytes Régimbart

7. Longitud del último segmento abdominal más de 0.85 veces el ancho de la cabeza; longitud del primer urogonfómero menos de 0.95 veces la longitud del segundo ….................... Anodocheilus Babington Longitud del último segmento abdominal menos de 0.70 veces el ancho de la cabeza; longitud del primer urogonfómero más de 1.05 veces la longitud del segundo …................................................ Liodessus Guignot

8. Sutura occipital ausente (Figs. 51-52)…............................................................ 9 Sutura occipital presente (Figs. 53-54) ... 10

9. Nasale con proyecciones laterales (Fig. 51); fémures con setas natatorias en el margen ventral (Fig. 67) ........................................... Pachydrus Sharp Nasale sin proyecciones laterales (Fig. 52); fémures sin setas natatorias en el margen ventral (Fig. 68) ….................... Hydrovatus Motschulsky

10. Nasale sin proyecciones laterales (Fig. 53); prementum mucho más largo que ancho (Fig. 33) ......... Desmopachria Babington Nasale con proyecciones laterales (Fig. 54); prementum aproximadamente tan largo como ancho ............................ 11

11. Longitud del cuarto antenómero 1/5-1/6 de la longitud del tercero (Fig. 21)……......................................... Vatellus Aubé Longitud del cuarto antenómero algo más de 2/5 de la longitud del tercero (Fig. 22) ……………………… Derovatellus Sharp

12. Margen anterior del frontoclípeo profundamente sinuado (Figs. 55- 58); urogonfos reducidos a pequeños tubérculos ventrales (Fig. 71) …................. Megadytes Sharp ….......... 13 Margen anterior del frontoclípeo regularmente curvado (Figs. 59-64); urogonfos bien desarrollados (Figs. 72- 77) ............................................. 16

13. Proyección media del frontoclípeo aguzada apicalmente, casi glabra (Fig. 57); mandíbula regularmente curvada, sin un anillo de setas en el tercio distal (Fig. 57) ….................................. M. (Trifurcitus) Brinck Proyección media del frontoclípeo truncada apicalmente, con muchas setas apicales (Figs. 55-56, 58); mandíbula con el tercio distal más fuertemente proyectado hacia adentro, llevando un anillo de setas largas (Figs. 55-56, 58) ..................................... 14

14. Emarginación entre la proyección media y cada proyección lateral del frontoclípeo muy angosta (Fig. 58) ............................................... M. (Megadytes) Sharp Emarginación entre la proyección media y cada proyección lateral del frontoclípeo ancha (Figs. 55-56) …15

15. Proyecciones laterales del frontoclípeo simples (Fig. 56) …............................... M. (Bifurcitus) Brinck Proyecciones laterales del frontoclípeo bífidas (Fig. 55) ………….……….... M. (Paramegadytes) Trémouilles & Bachmann

16. Último segmento abdominal con una hilera lateral de setas natatorias (Fig. 72); protórax con un esclerito ventral ....... 17 Último segmento abdominal sin una hilera lateral de setas natatorias (Figs. 73-77); protórax sin escleritos ventrales …..................................... 19

17. Proceso medio del prementum ausente (Fig. 65) …................................................................... Notaticus Zimmermann Proceso medio del prementum presente (Figs. 35-36) ......................18

18. Cuerpo arqueado medialmente, giboso en vista lateral (Fig. 37); los dos stemmata anterodorsales muy grandes (Fig. 59); proceso medio del prementum simple (Fig. 35) ….............. Thermonectus Dejean Cuerpo no arqueado medialmente, no giboso en vista lateral (Fig. 38); los dos stemmata anterodorsales similares a los demás (Fig. 60); proceso medio del prementum bífi do (Fig. 36) ............................... Hydaticus Leach

19. Sutura occipital presente (Figs. 61-62)…...................................................... 20 Sutura occipital ausente (Figs. 63-64) .................................................................. 22

20. Mandíbulas sin surco cerrado, con dentículos robustos en el margen interno (Fig. 25); margen interno del estipes con tres dentículos robustos (Fig. 29); lóbulos anterolaterales del frontoclípeo inconspicuos (Fig. 61) .......................... Copelatus Erichson Mandíbulas con surco cerrado, sin dentículos robustos en el margen interno (Fig. 26); margen interno del estipes sin dentículos robustos; lóbulos anterolaterales del frontoclípeo bien desarrollados (Fig. 62) .......................... 21

21. Ancho de la cabeza menor que 2.30 mm en la larva II y menor que 3.50 mm en la larva III; distribución: en toda la Argentina, desde el nivel del mar hasta grandes altitudes .....................................….. Rhantus Dejean Ancho de la cabeza mayor que 2.50 mm en la larva II y mayor que 3.70 mm en la larva III; distribución: provincias de Jujuy y Tucumán, en altitudes mayores a 2.000 m …......................................... Bunites Spangler

22. Longitud del último segmento abdominal mayor que el ancho de la cabeza (Fig. 73) ........... Agaporomorphus? Zimmermann Longitud del último segmento abdominal menor que el ancho de la cabeza (Figs. 75-77) ….......... 23

23. Urogonfos cortos, menos del doble del ancho de la cabeza, sin setas secundarias (Fig. 75); tibia y tarso sin setas natatorias (Fig. 69) …............................ Leuronectes Sharp Urogonfos largos, más del doble del ancho de la cabeza, con muchas setas secundarias (Figs. 76-77); tibia y tarso con setas natatorias en el margen dorsal (Fig. 70) ….............. 24

24. Margen lateral del parietal con 2-3 setas espiniformes temporales largas (Fig. 63); urogonfos de menos de 3.30 veces el ancho de la cabeza ..... Laccophilus Leach Margen lateral del parietal con varias (más de 6) setas espiniformes temporales cortas (Fig. 64); urogonfos de más de 3.60 veces el ancho de la cabeza …........... Lancetes Sharp

DISCUSSION

When using the keys presented here, the following considerations should be kept in mind:
1. The larvae of four genera of the tribe Bidessini (Bidessonotus, Brachyvatus, Hemibidessus and Neobidessus) are unknown. In Argentina, the genus Bidessonotus is known only from Jujuy Province, where Torres et al. (2008) reported the finding of an unidentified species. The only species of Brachyvatus present in Argentina (B. acuminatus (Steinheil)) is known only from Buenos Aires and Entre Ríos Provinces (Trémouilles, 1995; Torres et al., 2007). However, it is possible that this species is also present in other provinces of NE Argentina. Hemibidessus is represented in Argentina by a single species (H. spangleri Miller), recorded from Buenos Aires and Santa Fe Provinces (Trémouilles, 1995; Miller, 2000). A single species of Neobidessus (N. curticornis (Régimbart)) is present in Argentina, in Salta Province (Young, 1981; Trémouilles, 1995, 1998). As the four genera mentioned above belong to the tribe Bidessini, probably their larvae are morphologically very similar to those of the other Bidessini genera (included in the keys). As a consequence, when a larva of Bidessini coming from any of the provinces mentioned above is identified, the possibility that it belongs to one of the unknown genera should not be discarded.
2. The genus Cybister could not be included in the keys due to the lack of material and of adequate larval descriptions in the literature. Only C. puncticollis (Brullé) has been recorded from Argentina, from Chaco, Formosa and Santa Fe Provinces (Trémouilles & Bachmann, 1980; Michat, pers. obs.). The larvae of Cybister will surely be identified as Megadytes Sharp with the keys presented here, and within Megadytes, will probably be identified as any of the subgenera with the exception of Trifurcitus Brinck.
3. For some genera we did not have access to material of all larval instars (Table I). However, those genera were included in all the keys, extrapolating characters from the known instars. This procedure is supported by the great stability exhibited by the characters used among the larval instars.
4. Even though distributional data for the Argentinean genera of Dytiscidae are not yet known in detail, this aspect can be used as an additional criterion to help in the identification process. Therefore, in some couplets distributional data were used in combination with morphological data to separate genera.
5. Due to the small size of many dytiscid larvae, it is very useful that the identification be done using a compound microscope rather than a stereoscopic microscope. Unfortunately, this procedure requires an additional effort because the larvae need to be mounted on slides, and preferably cleared previously (see Material and Methods). However, it is impossible to visualize some of the characters mentioned in the keys (especially the chaetotaxic ones) through the observation with a stereoscopic microscope. In the construction of the keys, an emphasis was given to include easily seen and identifiable characters, but this was impossible in some cases due to the great morphological similarity of some groups.
6. The genus Agaporomorphus was included in the keys on the basis of some larval specimens collected in Misiones Province, where the genus is known to be present. However, according to the size of the larvae, the possibility that they belong to a recently discovered (Michat, pers. obs.), unidentified genus of the subfamily Laccophilinae instead of Agaporomorphus cannot be discarded. For this reason, a question mark is included in the keys after Agaporomorphus. Further sampling is needed before making a defi nitive conclusion about this matter.

ACKNOWLEDGEMENTS

Thanks to P. L. M. Torres for her valuable suggestions and comments on the manuscript. Field and laboratory work by M. C. Michat and A. O. Bachmann was supported in part by grant PIB X846 from the Universidad de Buenos Aires, and grant PIP 02541/00 from CONICET. Field and laboratory work by M. C. Michat was also partly supported by a postgraduate scholarship from CONICET. This is contribution n° 33 from LIESA.

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