Wading for Bugs: Exploring Streams with the Experts

23. Secrets of an Infrequent Flyer

132 23 Secrets of an Infrequent Flyer Mark P. Miller Streams of the Arizona White Mountains have marvelous aquatic insect assemblages. This high-elevation (5,000+ feet), heavily forested region in a remote section of eastern Arizona is an aquatic entomologist’s dream. Textbook representatives of numerous well-known mayflies, caddisflies, stoneflies, and dragonflies are extremely abundant. When I conducted fieldwork in the area, it seemed like every new rock we picked up held yet another new organism to collect. One of the true bugs I was studying, the one-centimeter- Secrets of an Infrequent Flyer 133 long Ambrysus thermarum (Hemiptera: Naucoridae) seemed most conspicuous, provided that we sampled correctly. After placing a kick net in a fast-flowing riffle and jostling some rocks around upstream, those stout, oval-shaped bugs were washed downstream by the current into our net. They were extremely active; as soon as we put our samples in a sorting tray, A. thermarum were swimming about wildly, seeking refuge from their exposed situation. “Do they fly?” my companions asked on one of my first expeditions to the White Mountains. “I don’t know, but that’s something I’d really like to try to find out,” I replied. I’d already explained to my friends that I wanted to understand how aquatic insects disperse across the terrestrial environment. In essence, I wanted to develop a feel for how fragmented or connected different headwater streams are from the perspective of individual aquatic insect species. We already know, for many species, that juveniles disperse downstream with the current, and that adults disperse back upstream by flight (when they are capable). But do flying adults make extensive use of areas outside of the stream corridor? In particular, might A. thermarum fly to disperse? If so, then high rates of overland dispersal may lead to increased interactions among aquatic insect populations from different isolated headwaters. My main study approach involved using genetic markers and population genetic analyses. If there were strong genetic differences within the stream network, then gene flow and dispersal were probably low. In contrast, weak genetic differences would indicate high gene flow and greater dispersal among populations. Conceptually, the approach is simple, and in practice, the approach is reasonably robust. However, using only markers and population analyses seemed slightly insufficient to me. I also wanted firsthand observations of adults flying through the air to help confirm patterns identified by genetic analyses. I established a formal set of study sites in the mixed pine and spruce forests adjacent to the small streams that captured my interest. These second and third order streams were downstream of the headwaters (which we classify as first order streams). The stream sites were well shaded by adjacent forests. A. thermarum 134 Bugs and Beetles on Their Best Behavior became one of the four species that I tried to find flying through the forest’s open understory next to these streams. At the onset, I knew that this species could prove difficult to understand. Their morphology and life history provided no clear clues about how they dispersed. Juveniles are wingless and spend all of their time in the water. And like many other true bugs, adults have fullyformed wings. This attribute, at least superficially, suggested that the species is capable of flying over land. However, naucorids display an unusual behavior that allows them to complete their life cycle entirely under water. While they requireatmosphericoxygentobreathe,theyhavenoneedtoleave the streams where they live to collect it. Instead, naucorids use an extremely specialized technique for storing air. Periodically, an adult will briefly extend the tip of its abdomen just above the water surface to gather a small air bubble that becomes trapped beneath its elytra, the rigid, modified forewings that cover and protect the hind wings. These air bubbles that naucorids collect are called sub-elytral air stores. Once replenished, the sub-elytral air store stays in contact with openings on the insect’s abdomen (its spiracles) and allows the bug to breathe while submerged. Naucorids are efficient predators, and in the streams that I studied, there are ample soft-bodied prey items (other aquatic invertebrates) for them to consume throughout their life cycle. Food is not in short supply. Thus, adult A. thermarum hunt and feed exclusively within the stream; even copulation and egg laying occur completely underwater. Given the lifestyle of these bugs, I addressed my initial question: do they fly? I used two...