Water scavenger beetles

Overview

Tropisternus is a large genus of water scavenger beetles. There are currently 70 recognized Tropisternus species worldwide, all of them in North and South America. There are 15 Tropisternus species that have been reported in North America north of Mexico, and at least 4 species in Minnesota.

Tropisternus beetles are ubiquitous throught the Americas and are tolerant of a wide range of ecological conditions. The vast majority of species are most frequently and abundantly collected in standing water (lentic) environments, including both permanent and semi-permanent ponds, swamps and marshes, and sewage lagoons. They are also found in rain pools and shallow depressions that are seasonally or temporarily filled with water (ephemeral pools). They are even found in mixed fresh water and salt water (brackish) pools. Some species are found in slow-moving water with silty or sandy bottoms (lotic-depositional) environments, including slow streams or stream margins and in shallow areas near the shores of large lakes (littoral zones). One species, Tropisternus lateralis, is one of the first water scavenger beetles to colonize newly formed small bodies of water, including wheelbarrows and buckets partially filled by rain, and even tire track puddles.

Adults feed on algae, animal remains, and other biological material.

Description

Tropisternus are small to medium-sized beetles. Adults are 5⁄16″ to ⅝″ (7 to 15 mm) in length. The body is broadly oval or boat-shaped (navicular) when viewed from above, and strongly convex above while flat below. Most species are dark brown to black and glossy. Some species have a metallic or olive-green sheen, and many have a light-colored border on the lateral margins of the pronotum and the outer margins of the elytra.

The antennae are short and clubbed. The last three antennal segments form a club that fits into a cup-like preceding segment (the cupule). The mouthparts are directed downwards. The finger-like sensory organs attached to the “second jaws” (maxillary palps) are noticeably longer than the antennae.

The upper plate on the first segment of the thorax (pronotum) is at least as wide and often wider than the head. The plates on the underside of the thorax, the mesosternum and metasternum, form a long, continuous, sharp, and acute spine or keel that extends backward between the middle and hind legs, often reaching the abdomen.

The wing covers (elytra) are widest at the middle.

The legs are adapted for swimming. The last part of each leg (tarsus) has five visible segments (a 5-5-5 tarsal formula). On the hind legs, the tarsi are flattened and distinctly fringed with long swimming hairs.