Wading for Bugs: Exploring Streams with the Experts

Part VI: Bugs and Beetles on Their Best Behavior

112 PART VI Bugs and Beetles on Their Best Behavior About Beetles (Coleoptera) Beetle species (about three hundred and fifty thousand) make up the largest order of life on earth. Aristotle gave them their name from the Greek words coleo meaning shield, and ptera meaning wing. About five thousand species are aquatic and occupy just about any aquatic habitat imaginable—from running fresh waters to salty marine or estuarine systems—while others are semi-aquatic living along shorelines, deep in stream subsurfaces, in reeds of rooted plants, or in rotting logs. Unlike other aquatic insect orders, beetles are not the dominant organisms in these habitats. They generally live on top of substrates, and the ones that are good swimmers have to return to the surface frequently to replenish their air supplies. They are an ancient group and have invaded aquatic habitats many times over evolutionary history. Beetles diversified greatly during the Jurassic (210 to 145 million years ago), when the whirligig beetles (Gyrinidae), scavenger water beetles (Hydrophilidae) and diving beetles (Dytiscidae) appeared. In short, there has been plenty of time for this order of insects to diversify. Morphology Though their morphologies can vary greatly, many larval beetles are elongate and slightly flattened, a shape that helps them hide in crevices or adhere to rocks. They have distinct heads protected by a heavy cuticle, with strong mouthparts that scrape algae from rocks or capture prey. All aquatic beetles (except weevils) have three pairs of claw-bearing legs on the thorax. Many have gill-like appendages along the abdomen or a pair of hardened appendages at the terminal end. About Beetles 113 Adult beetles are generally long-lived (often an entire season), an unusual trait among aquatic insects. They have a pair of hardened forewings, called elytra, and most have a second pair of wings for flight folded under the elytra. Though some beetle families live their entire lives in the water, most adults take flight for dispersal at least once in their lifetimes. Many adult beetles deploy chemical defenses that are distasteful to potential predators. Beetles have developed several ways to breathe underwater. Some hold air reserves under their wings; some breathe through their cuticle, with or without gills; some possess unwettable hairs that hold air against the body’s undersurface (called a plastron); while still others pierce plant tissues for air. Life History Most aquatic beetles undergo three to eight larval instars. As with the diversity in their morphologies and lifestyles, the time spent as larvae varies between only a few weeks to the majority of the year. For example, water pennies (Psephenidae) are larvae all year, but adults are found for only a brief time in the summer; in contrast, riffle beetles (Elmidae) are collected as larvae and adults almost any time. For many beetles, larval development takes six to eight months, followed by a short pupation in terrestrial habitats such as under stones or logs. Often they emerge as adults in two to three weeks. Bioindicators Beetles can be indicative of particular habitat conditions associated with how they breathe. Those that acquire oxygen through a plastron, such as riffle beetles (Elmidae), require highly oxygenated, usually swiftly moving waters. Those that return to the surface regularly to resupply a bubble held under their wings (like big, predatory dytiscids), must stay in waters near the edge, calm enough for resurfacing. Even more closely tied to stream edges are minute beetles that live in the small spaces between well-aerated sand particles. Other semi-aquatic beetles are associated with muddy substrates. 114 Bugs and Beetles on Their Best Behavior References White, D. S., and R. E. Roughley. 2008 “Aquatic Coleoptera.” In An Introduction to the Aquatic Insects of North America, edited by R. W. Merritt, K. W. Cummins, and M. B. Berg, 571–75. Dubuque, IA: Kendall Hunt. Liebherr, J. K., and J. V. McHugh. 2003. “Coleoptera,” In Encyclopedia of Insects, 209–230. V. H. Resh and R. T. Cardé, eds. Orlando, FL: Academic Press. About True Bugs (Heteroptera) Aquatic true bugs belong to the suborder Heteroptera, order Hemiptera. There is great diversity among these insects, but they are all equipped with a slender sucking tube or “beak” used to extract fluids from animals or plants. The word “bug” derives from the Middle English word “bugge” meaning “spirit” or “ghost,” and was originally associated with the bed bugs that disappeared in the morning after biting their human victims during...