CPIE Project

Page A12

Key to Identification of Aquatic Coleoptera

Dytiscid beetle from Waiulaula Gulch, Ouli BEETLES  Order Coleoptera. Adult aquatic beetles are air-breathing. The dytiscid pictured here is obtaining air through the posterior tip of its abdomen, seen just breaking the surface as the animal hangs head down in the water. The hind legs are adapted for swimming, being more or less flattened and heavily fringed with hairs. Adult beetles are characterized by the front pair of wings hardened as elytra that cover and protect both the hind pair of wings and the abdomen. Beetles are most typically found in ponds of various sizes. Adult aquatic beetles are capable of flying (the elytra open sufficiently for the wings to unfold and beat) and distribute to isolated bodies of water by this means.

When completed, the following key will cover both adult and immature stages, both of which are usually aquatic. The larvae somewhat resemble very small damselfly nymphs, although there is minimal taxonomic data on the immature stages of the Hawaiian species.

Antennae long and filiform (narrow, filament-like); maxillary palps short. In the water, frequently seen hanging head downward at the surface, or swimming by moving both hind legs together
131b Antennae short and club-shaped; maxillary palps as long or longer than the antennae. In the water air carried in a film on the ventral surface; swimming by moving hind legs alternately. Water scavenger beetles
Adult beetle about 1 cm long or slightly more. Males (only) with as many as 30 sucker discs on the underside of fore and middle tarsi
Rhantus pacificus Boisdduval
132b Adults smaller, less than 0.5 cm long and closer to 0.25 cm. Males with far fewer sucker discs on tarsi [133]

144a (131) Underside of thorax having a sternal keel, projecting backwards as a long spine. Moderately large, 8-9.5 mmm long [145]
144b Underside of thorax may have a sternal process, but this not a continuous keel terminating in backward-pointing spine; OR posterior segment only projecting as a short spine [146]
145a (144) Pronotum and elytra uniformly black with a metalic hue
Tropisternus salsamentus Fall
145b Pronotum and elytra metalic black with yellow lateral margins
Tropisternus lateralis humeralis Motschulsky
146a (144) Larger species, typically greater than 7.5 mm long. Elytra with 10 regular series of tiny punctures [147]
146b Smaller, less than 8 mm long. Elytra with only sutural stria posteriorly, otherwise without striae or lines of punctures [148]
147a (146) Elytra with numerouis punctures forming distinct, well-defined striations (impressed lines). Endemic
Limnoxenus nesiticus (Sharp)
147b Elytra punctures not forming distinct striae (lines) except along posterior edge of suture. Endemic
Limnoxenus semicylindricus (Eschscholtz)
148a (146) Middle segment of thorax with a sharp keel along midline of underside. First segment of middle and posterior tarsi much shorter than second segment. { Color yellowish to yellowish brown with a black head and 3-4.1 mm long.
Enochrus sayi Gundersen
148b Middle and posterior segments of thorax with convex processes. First segment of middle and posterior tarsi at least as long as second segment. { Color of elytra mostly black, except posterior quarter yellowish red, 4-6.5 mm long. [149]
149a (148) Elytra entirely black. Beetle 5.5-6.5 mm long. Indigenous(?)
Coelostoma fabricii (Montrouzie)
149b Elytra black except at posterior end, which is yellowish red in color. Beetle 4.0-4.7 mm long. This species may not be established (Hansen, 1995)
Coelostoma segne Balfour-Browne

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Arthropods, Insects