Counting Calories: 
Fat Storage and Energy Needs in 
Migratory Dragonflies

Despite their alien appearance, beneath their glossy exoskeletons insects have a lot in common with humans, and nutrition is no exception. Dragonflies may not binge on snacks (unless perhaps mosquitoes are the Doritos of the insect world), but there's no doubt that a proper food base is critically important throughout these high-powered predators' lives. In nymphs, food availability and energy reserves affect the size of each stage (instar), development rate, and emergence success; in adults, it affects ability to forage for food, patrol and protect territory, reproduce, and sustain long-distance flight.

Like humans, insects use sugars (carbohydrates) and fats (lipids) as energy sources. Much of our daily metabolic activity takes place in the liver, and fat is stored in specialized adipose cells; in insects, these functions are all carried out by the fat body, an organ comprised of loose tissue distributed throughout the body space and around the gut and reproductive organs. Fat body is active in metabolism and storage of most of an insect's fats, proteins, and sugars, and like the vertebrate liver, is regulated by a variety of hormones.

Fat body may be even more important in migratory insects. Flight creates a tremendous energetic demand, with flight muscles' need for fuel increasing 50 to 100-fold. Fat is the main fuel for flight, as it provides more energy and generates more metabolic water when burned compared to stored sugars. As flight is initiated, the muscles in the thorax use sugar as an energy source, but stored lipids are mobilized from the fat body and shuttled to the muscles to sustain longer flights.

Energy storage is important to our understanding of dragonfly migration. Though migrants can feed along the way, and may conserve energy at times by gliding instead of flapping, the distances they cover require an enormous energy input. Newly-emerged (teneral) adults probably lack large fat reserves, and since most migrants begin their long-distance flights soon after emerging and complete their maturation en route, they must have a way to replenish their energy reserves along the way. But how much fat can an individual dragonfly store? Does it differ between males and females, residents and migrants of the same species, or members of different migratory species? Is there a relationship between time of year, latitude, and fat content?
Some researchers are investigating fat content in migratory dragonflies. Dr. Mike May, an MDP steering committee member and odonate expert at Rutgers University, examined Common Green Darner (Anax junius) on the Atlantic seaboard and found that fat content was lowest in spring migrants and residents and highest in fall migrants. There is much more work to be done, and you can help in Dr. May's research if you live in Canada, the USA, or Mexico, and are willing to collect adults of any of the five main migratory species (Common Green Darner, Black Saddlebags, Wandering Glider, Spot-Winged Glider, or Variegated Meadowhawk). If you are interested in contributing to this study, please click below and check out "Is there more I can do at my pond" for more details. 

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