Tuatara: Volume 15, Issue 1, May 1967
The identification of New Zealand Water Beetles (Dytiscidae)
The identification of New Zealand Water Beetles (Dytiscidae)
Insects are primarily terrestrial animals but one of the conspicuous features of this class is its adaptability to a wide range of habitats and even within New Zealand there are many aquatic and semiaquatic forms (Wise 1965). Among them are the Dytiscidae, the carnivorous water beetles, the first New Zealand species of which was recorded in 1846. By 1893 fourteen species were described and by 1920 this country was attributed with nineteen species. The early lists however relied on research carried out during the last century which was, of necessity, based on very few specimens and consequently it failed to take into account variation within the species. Today, with improved techniques and adequate series of specimens, specific variability can be assessed and synonymy in the classification of the family can be established. A full systematic revision has been carried out by the present author (Ordish 1966), and the reader is referred to it for detailed descriptions, anatomical details, and discussions of synonymy and distribution. It is the aim of this paper to provide a ready means of identification of the New Zealand Dytiscidae, to illustrate the morphology referred to in the key, and to present some notes on the general biology of this family.
The most distinctive feature of this family of beetles is the ged plate-like post coxa which occupies a large area of the ventral surface leaving the trochanter as the first movable segment of the leg. This, together with the undivided eyes and eleven-segmented antennae, serves to differentiate this from other families of the Coleoptera.
Key to the Water Beetles (Dytiscidae) of New Zealand
|1||Small species, 5 mm long or less; scutellum not visible dorsally, prosternal process deflected posteriorly; tarsi with four or five segments Subfamily Hydroporinae||2|
|Larger, more than 5 mm long; scutellum visible; prosternal process on one plane; all tarsi with five segments||—8|
|2 (1)||2.5 to 3.0 mm long; pronotum with a conspicuous longitudinal groove on each side which is continued on to the base of each elytron||—3|
|Not less than 4 mm long; pronotum and elytra without longitudinal grooves||—6|
|3 (2)||Head smooth, without a line running from eye to eye; dorsal and ventral surfaces of the body uniformly and densely punctured||Genus Huxelhydrus Sharp One species. H. syntheticus Sharp|
|Head with cervical stria present as a line running from eye to eye; ventral surface of the body sparsely and unevenly punctured||Genus Liodessus Guignot 4|
|4 (3)||2.5 mm long; prosternal process broad but apically acute; colour uniform or not uniform; intercoxal process of the metasternum not deeply grooved||—5|
|3.0 mm long; prosternal process lanceolate but apically rounded; colour uniform; intercoxal process of the metasternum deeply grooved (Kermadecs)||L. oliveri Ordish|
|5 (3)||Colour not uniform; aedeagus of the male evenly curved when viewed from the side; surface of the eye curved dorsally and obviously protruding from the margin of the head when viewed from above||L. plicatus (Sharp)|
Fig. 2: Lancetes lanceolatus, ventral aspect showing principal sclerites.
Abbreviations in Figure 2.
A2-A7, Abdominal sternites; AM, Angle of metasternum; C1, Anterior coxa; C2, Mid coxa; C3, Hind coxa; CLY, Clypeus; CL, Coxal line; CN, Coxal notch; E, Epipleura of elytron; EP, Prothoracic episternum; EPP, Position of epipleural pit; ExC, External lamina of hind coxa; F3, Posterior femur; G, Gena; GU, Gula; IC, Interior lamina of hind coxa; IP, Intercoxal process; L, Labrum; LG, Ligula; LP, Labial palp; M, Mentum; MD, Mandible; ME, Mesoepimeron; MEP, Mesoepisternum; MS, Mesosternum; MET, Metasternum; METW, Metasternal wing; MG, Maxillary galea; MX, Maxillary stipes; MT, Metaepisternum; MXP, Maxillary palp; P, Prosternal process; PN, Inflexed margin of pronotum; PR, Anterior band of prosternum; TA1-TA5, Posterior tarsal segments; T3, Posterior tibia; TR3, Posterior trochanter.
|Colour uniform dark brown except for anterior pronotum; aedeagus with apex obviously deflected when viewed from the side; eyes surface flat and not disrupting the curvature of the head when viewed from above||L. deflectus Ordish|
|6 (2)||Fourth segment of the front tarsus much shorter than the third segment; epipeural pit (page 7) present on the ventral shoulder of the elytra||Genus Hyphydrus Illiger
One species H. elegans (Montrouzier)
|Fourth segment of front tarsus at least as long as the third segment; epipleural pit absent||Genus Antiporus Sharp 7|
|7 (6)||5.0 mm long; elytra usually partially striped, never uniform in colour; male with hind femur bearing an inwardly directed protruberance which is acute and slightly recurved; anterior tarsal claw of the male without a tooth at its mid point||A. strigosulus (Broun)|
|4.2 mm long; elytra typically uniform dark brown, occasionally with lighter lateral patches; male with hind femur bearing an inwardly directed protruberance which is angular and never recurved; anterior tarsal claw of the male with a tooth at its mid point in addition to the basal tooth||A. wakefieldi (Sharp)|
|8 (1)||Medium size, less than 12 mm long; inner eye margins excised when viewed from the front||Subfamily Colymbetinae 9|
|Larger, more than 12 mm long; inner eye margins entire when viewed from the front||Subfamily Dytiscinae 12|
|9 (8)||6 mm. long, elongate; dorsal surface sculptured with short longitudinal grooves (striole) visible at x15; metacoxal lines very close posteriorly||Genus Copelatus Erichson One species, C. australis (Clark)|
|Larger, not less than 8 mm long; dorsal surface smooth; metacoxal lines widely separated||—10|
|10 (9)||Elytra longitudinally striped and truncate at the apex||Genus Lancetes Sharp One species L. lanceolatus (Clark)|
|Elytra of uniform colour and rounded apically||Genus Rhantus Larcordaire 11|
|11 (10)||12 mm long; inner tarsal claw of the male longer and less curved than the outer claw; basal segment of the male anterior tarsus very compressed (common and widespread)||—R. pulverosus (Stephens)|
|10 mm long; inner anterior tarsal claw of male shorter than the outer claw; basal segment of male anterior tarsus very broadly dilated but not compressed||R. plantaris Sharp|
|12 (8)||Metacoxal lines present; posterior femur elongate; front of the head with a yellow or brown triangular mark between the eyes||Genus Dytiscus Linnaeus One species D. semisulcatus Muller|
|Metacoxal lines absent; posterior femur short; front of the head uniform in colour||Genus Homoeodytes Regimbart|
|13 (12)||Scutellum black with greenish reflections, not differing in colour from the rest of the dorsal surface; under surface black||H. hookeri (White)|
|Scutellum reddish brown at least in the centre; under surface yellowish brown||H. scutellaris (Germar)|
As will be seen from the foregoing key, identification at species level is often facilitated by determining the sex of the specimen. This is readily accomplished in the genera Homoeodytes, Dytiscus, Rhantus, Lancetes and Copelatus because in these the males have the three basal segments of the anterior tarsus much broadened so that together they form a disc, typically bearing suction cups (palettes) on the under surface. The anterior tarsi of the females of these genera are narrow and cylindrical. Males of Hyphydrus elegans may be recognised by their black anterior tarsal segments, those of the female being light brown. Within New Zealand, males of the genus Antiporus are readily recognised by the presence of an inwardly directed triangular protruberance on the hind femur, but separation of the sexes of members of Liodessus and Huxelhydrus is not so easy. Accurate determination can be done only by dissection but usually males of these two genera are shiny in appearance and narrower in the body, while the females are broader, and the upper surface is dulled by microsculpture. Sexing of the members of these two genera, however, is not necessary for identification.
The key also makes reference to the epipleural pit, a term which requires elaboration. The epipleura is the deflexed lateral page 7 margin of the wing case which thus attains a ventral position. In the genus Hyphydrus the basal portion of the epipleura is separated by a conspicuous transverse ridge and the basal portion thus confined is termed the epipleural pit.
Morphologically the Dytiscidae show a close relationship with the carnivorous ground beetles, the Carabidae, and possibly they have evolved from this group. While the similarities are quite apparent, so too are the modifications to meet a rather demanding habitat. These are apparent both in structure and in behaviour. For most aquatic carnivorous animals there is survival value in effective swimming and in the dytiscids this is achieved by the use of greatly modified hind legs which serve as oars. Complementing this is a streamlining of body form to give a continuous oval outline; firm application and interlocking of the head, thorax and abdomen to give rigidity, and a smooth surface to provide minimum resistance to movement. Like the ground beetles they are nocturnal and remain hidden during the day. They are, however, air breathing and this requires periodic visits to the surface. Most dytiscids can remain submerged for about twenty minutes because they retain a considerable quantity of air beneath the wing cases in addition to the bubble of air often seen attached to the end of the abdomen. Replenishment at the surface is very rapid and usually takes about three seconds.
Dytiscid eggs are laid in the water and the larvae undergo four moults during growth, and feed on a variety of aquatic life including smaller examples of their own kind. Those larvae whose biology has been studied have been found to have hollow mandibles through which digestive enzymes are injected into prey thus providing an external digestion. Later the resultant fluid is pumped back through the mandibles into the alimentary canal. The available evidence would suggest that a given species has a favoured food species but field observations of this type are difficult. In captivity the larvae of Homoeodytes hookeri show a marked preference for Anisops assimilis, a backswimmer, when offered a range of food, and consume an average of one insect every twenty four hours. The mature larva leaves the water and burrows into the ground where it excavates an oval pupal chamber. The emerging adult returns to the water and may remain there, although for many species dispersal by nocturnal flight is not unusual.
Habitat preference, of course, varies from species to species, but generally speaking the still pond is the favoured site although some species are equally at home in the quieter backwaters of streams. The greatest numbers, however, occur in temporary and permanent ponds that have the organic content to support the animal life on which the dytiscids feed. Lakes and streams on the other hand usually have a modest quota of beetles. The significance of pH values has yet to be investigated but alkaline conditions appear to be the most favourable.
In contrast to the moderate flexibility of habitat of the New Zealand water beetles, Huxelhydrus syntheticus is essentially a river-bed species inhabiting the margin of larger rivers, and in particular the pools in marginal shingle. Copelatus australis is known only from Ruatoria on the east coast and Mokohinau Island in the Hauraki Gulf, and consequently little is known of its habitat. With regard to geographic origin there is an apparent Australian element in the presence of the genus Homoeodytes and Antiporus, but other possibilities cannot be assessed until more is known of this group in the Pacific region.