Wading for Bugs: Exploring Streams with the Experts

22. Riding the Current for the Riverine Backswimmer

127 22 Riding the Current for the Riverine Backswimmer Michael Bogan Our raft was gliding nearly silently over the roiling, chocolatebrown water, when I noticed a slight disruption on the left side of the boat. We were on the third day of an eight-day rafting trip down one hundred and twenty miles of wild river in a roadless area of central Sonora, Mexico. The majority of our ragged group of eighteen people in four rafts and two kayaks were biologists of one variety or another; our goal was to document as many species as possible during the voyage. My specialty is aquatic insects, and one of the bugs I was searching for was a strange type of backswimmer known as Martarega mexicana. When I noticed the hundreds of tiny splashes on the left side of the boat, I knew that I’d found my bug. Unlike most backswimmers commonly found in ponds and lakes around the world, Martarega wouldn’t be caught dead in still water. This genus of backswimmer is exclusively found in 128 Bugs and Beetles on Their Best Behavior medium- to large-sized streams from the southwestern United States to central South America. In the runs and eddies of these streams, Martarega come together by the hundreds to form what have been described as “large, gregarious schools.” They also have the unique habit, for aquatic insects anyway, of jumping out of the water when threatened by fish or avian predators—in our case they had been threatened by our large raft of biologists. As the current carried our raft downstream into the next set of whitewater rapids, I managed to swing my net through the leaping school of Martarega and toss some into a collection vial. Then I grabbed tightly to the boat as we plunged into the rapids. The continuous downstream drive of the current was essential for our riverine voyage, and it is also essential in delivering meals to hungry beaks of Martarega. Unlike most backswimmers, Martarega do not have large, powerful rear legs capable of bursts of speed to capture mobile aquatic prey. Instead, Martarega’s hind legs have the same type of “low mechanical advantage” that horses do, perfect for long periods of sustained running (or swimming) instead of short sprints. This design allows Martarega to swim against a current without tiring, and in that current they wait for food to be carried to them. Sometimes they will eat aquatic insect larvae, but, when given a choice in an experimental study, their favorite food was ants! In most tropical and subtropical streams, there is likely no shortage of ants to accidentally fall out of the leafy stream canopy and get carried to the waiting arms of Martarega. In addition to having leafy canopies, most subtropical streams also experience intense flow variation during the rainy season. It was this flow variation that allowed us to make our expedition down the river. Monsoon storms between June and September can deliver as much as three inches of rain per hour—this intense rain results in great increases in runoff to streams. The river we were rafting, the Rio Aros, is not much wider than a raft during the dry season, but during the monsoons it can swell to nearly the size of the Mississippi River. One morning we watched the river rise four vertical feet in less than an hour; we were therefore always sure to set up camp at least twelve vertical feet above the river’s surface. Riding the Current for the Riverine Backswimmer 129 This intense flushing of upland and riparian habitats by the monsoon rains likely provides quite a buffet of terrestrial prey for Martarega in addition to the ants that fall from the canopy. While Martarega’s rear legs are relatively low-geared and made for long-distance swimming, their front legs have “high leverage coefficients,” making them quite able to hold on to large, struggling insects. In one quiet reach of the Rio Aros, I watched through binoculars as a Martarega restrained and slowly killed a much larger grasshopper that made an ill-advised leap into the river. Like most aquatic hemipterans, Martarega inject a digestive enzyme into their captive prey and slowly digest them from the inside out. It was clearly not a pleasant end for the grasshopper. A close examination of most Martarega individuals reveals another feature that distinguishes them from the rest of their...