T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network

Beetle (Family: Elmidae) Riffle beetles

Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Coleoptera
Suborder: Polyphaga
Infraorder: Elateriformia
Superfamily: Byrrhoidea
Family: Elmidae
Common name: Riffle beetles, Water beetles

Riffle beetles are small aquatic beetles in the family Elmidae of which there are about 1400 species known world-wide, with many more undescribed. The family Elmidae is divided into two subfamilies: Elminae and Larainae. The Elminae are far more abundant and diverse than the Larainae.
Riffle beetles are usually live in cool, rapid streams. They feed on decayed plant material and algae.
The adult beetles are small (1-8 mm long), dark, elongate and sclerotised (hard body). They have relatively long legs with tarsal claws. The slender antennae are 11 segmented and without a distinct club.

The adults are aquatic and have the ability to live underwater because the ventral surface of their body is covered with an extremely dense layer of tiny, specialised, hydrophobic hairs. These hairs trap a layer of air, called a plastron, on the surface of the body, and the beetle uses this for gas exchange. The under the last (9th) abdominal segment (bubble) covers the insect spiracles (valve-like openings in the exoskeleton) so the insect can "breathe" air from the bubble while submerged. The air bubble provides an insect with a supply of oxygen, due to a unique physical property of absorbing oxygen molecules that are dissolved in the surrounding water. In effect, the bubble acts as a "physical gill" -- replenishing its supply of oxygen through the physics of passive diffusion. The larger the surface area of the bubble, the more efficiently this system works. An insect can remain under water as long as the volume of oxygen diffusing into the bubble is greater than or equal to the volume of oxygen consumed by the insect. Unfortunately, the size of the bubble shrinks over time as nitrogen slowly diffuses out into the water. When the bubble's surface area decreases, its rate of gas exchange also decreases. If the bubble becomes too small to keep up with metabolic demands and the insect must renew the entire bubble by returning to the water's surface.

Riffle beetle larvae are strictly aquatic. They are elongated and are up to 16 mm long (most less than 8), and have a head and 3 pairs of legs visible from above. The antennae and mouthparts are shorter than the head. Filamentous gills emerge from under the last (9th) abdominal segment. These can be retracted for protection, or rhythmically expanded and contracted to increase oxygen flow.

Adult riffle beetles mate in the water. The females lay their eggs in crevices in solid objects on the bottom of the stream. The larvae moult up to 8 times before they are ready to pupate. The pupae crawl out of the water, breathes air and pass through a pupal stage in holes along the streams banks. A newly emerged beetle may fly a significant distance before entering the stream. Once entering the aquatic state they no longer fly.

Crystal Maier - Collections Manager of Insects at The Field Museum - spent a month in New Zealand, going from stream to stream in search of beetles that spend their entire lives underwater. How?! Why?! We get answers.

An elmid beetle 

An elmid beetle larvae.