Science of Water : Concepts and Applications

192 The Science of Water: Concepts and Applications
that are hard and leathery and not used in fl ight; the membranous hindwings, which are used for
fl ight, are concealed under the elytra when the organisms are at rest. Only 10% of the 350,000
described species of beetles are aquatic.
Beetles are holometabolous (i.e., they complete metamorphosis). Eggs of aquatic coleopterans
hatch in 1 or 2 weeks, with diapause (i.e., no suspension of development) occurring rarely. Larvae
undergo from three to eight molts. The pupal phase of all coleopterans is technically terrestrial,
making this life stage of beetles the only one that has not successfully invaded the aquatic habitat.
A few species have diapausing prepupae, but most complete transformation to adults in 2–3 weeks.
Terrestrial adults of aquatic beetles are typically short-lived and sometimes nonfeeding, like those
of the other orders of aquatic insects. The larvae of Coleoptera are morphologically and behaviorally
different from the adults, and their diversity is high.
Aquatic species occur in two major suborders, the Adephaga and the Polyphaga. Both
larvae and adults of the six beetle families are aquatic. Dytiscidae (predaceous diving beetles),
Elmidae (riffl e beetles), Gyrinidae (whirligig beetles), Halipidae (crawling water beetles),
Hydrophilidae (water scavenger beetles), and Noteridae (burrowing water beetles). Five families—
Chrysomelidae (leaf beetles), Limnichidae (marsh-loving beetles), Psephenidae (water pennies),
Ptilodactylidae (toe-winged beetles), and scirtidae (marsh beetles)—have aquatic larvae and
terrestrial adults, as do most of the other orders of aquatic insects; adult limnichids, however,
readily submerge when disturbed. Three families have species that are terrestrial as larvae and
aquatic as adults: Curculionidae (weevils), Dryopidae (long-toed water beetles), and Hydraenidae
(moss beetles), a highly unusual combination among insects.
√ Note: Because they provide a greater understanding of the condition of a freshwater body (i.e.,
they are useful indicators of water quality), the focus of discussion here is on the riffl e beetle,
water penny, and whirligig beetle.
Riffl e beetle larvae (most commonly found in running waters, hence the name riffl e beetle) are up
to 3/4 in. long (see Figure 6.24). Their bodies are not only long, but also hard, stiff, and segmented;
they resemble small torpedoes and are usually grayish in color. They have six long segmented legs
on the upper middle section of body; the back end has two tiny hooks and short hairs. Larvae may
take 3 years to mature before they leave the water to form a pupa; adults return to the stream.
Riffl e beetle adults are considered better indicators of water quality than larvae because they
have been subjected to water quality conditions over a longer period. They walk very slowly under
the water (on stream bottom), and do not swim on the surface. They have small oval-shaped bodies
(see Figure 6.25) and are typically about 1/4 in. in length.
Both adults and larvae of most species feed on fi ne detritus with associated microorganisms
that are scraped from the substrate. Others may be xylophagous, that is, wood eating (e.g., Lara,
Elmidae). Predators do not seem to include riffl e beetles in their diet, except perhaps for eggs, which
are sometimes attacked by fl atworms.
The adult water penny is inconspicuous; it resembles circular encrustations on rocks and is often
found clinging tightly in a sucker-like fashion to the undersides of submerged rocks, where they
feed on attached algae. The body is broad, slightly oval and fl at in shape, ranging from 4–6 mm
(1/4 in.) in length. The body is covered with segmented plates and looks like a tiny round leaf
FIGURE 6.24 Riffl e beetle larvae.