Predaceous Diving Beetle Larvae

Cybister sp.
Thursday, July 15, 2010

This diving beetle larvae is a fierce little predator. Two conspecific and similarly sized larvae were captured by the Field Methods Class seining on Tuesday in the pond; occupying the same dish for 24 hours resulted in a 50% drop in the population. In addition to cannibalism, these larvae also consume insects, snails, tadpoles, and fish. They are so vicious that the larger larvae earn the name “water tigers.” The adults, similarly fierce, extend their diet to almost all vertebrate and invertebrate prey that they are physically capable of subduing. They have hollow jaws that inject their food with digestive enzymes, predigesting their food. 

The beetles are characteristically streamlined, and generally between 2.5-3.5 cm. They have two rows of punctures on the elytra, distinguishing this genus from similar ones, which only have one row. The males have a modified protarsus (ie forearm) used to grasp the females during mating. The color and other specifics vary with the species; most are black to brown with some lighter accents. The beetles have powerful, fringed hind legs that they move together to propel them through the water. However, their joints are arranged in such a way that, while they are powerful swimmers, they are helpless on land.
The larvae can reach 8 cm, and lack cerci (paired, sometimes pincerlike appendages) on their posterior end. This distinguishes them from the similar genus Dytiscus.
These beetles are descendents of land beetles, and, despite their many adaptations to living in water, are still able to breathe air and fly. Both the beetles and the larvae breathe air. The larvae take in air and store it in their tracheal trunk (windpipe), returning to the surface when they need more. The adult store air bubbles in cavities below their wing covers. However, this makes them too buoyant to swim effectively. They can control this by expanding their abdomen and expelling some of the air, also, they can carry small amounts of water internally, as “ballast,” to counteract the effects of the water.
A second adaptation to swimming makes these beetles water permeable. However, if they are out of the water for too long, they become somewhat waterproof. If they try to reenter the water after this happens, they may become trapped on the surface, unable to break the surface tension. These beetles, however, have specialized glands on the tip of their abdomen that produce “wetting agents,” chemicals that make the exoskeleton more permeable to water, and help the beetle to become submerged. 
These beetles, fierce water predators during the day, often take off and fly at night. They search for moonlight reflections which would indicate other bodies of water for them to take up residence in. However, this also results in an attraction to wet road surfaces, puddles, and artificial lights.
The beetles of the genus Cybister inhabit much of the United States, especially in the south.
Article by Hazel Galloway
See photos of this organism here, courtesy of