If you come across a fly lying on its back, chances are it's dead, right? Well, maybe not. It may just be passing through a very predictable, but significant stage of decline that eventually will help scientists better understand aging and degenerative diseases in humans.

A team of researchers studying longevity in more than 200 male Mediterranean fruit flies found that nearly all of the flies in their study went through such an upside-down period, usually late in life. During this phase, the flies spent increasingly more time resting on their backs, even though they were still capable of walking, eating and even fanning their wings. Whether this behavior began at a young or advanced age, it always progressed toward death.

"It appears to be something like the progression humans make from using a cane, then a walker, then a wheelchair and then finally becoming bedridden," said James Carey, an insect demographer at the University of California, Davis. Carey is the principal investigator and co-author on the study, which will be published in the Aug. 22 issue of the British journal Proceedings of the Royal Society: Biological Sciences.

"Virtually nothing is known about chronic, progressive illness in insects," said Carey. "It's possible that this upside-down phase may serve as a biomarker that will allow us to use Medflies as a model system for studying the dynamics of morbidity -- the relationship between the onset and duration of irreversible conditions and death -- in humans; which is a multibillion-dollar issue in health care."

Carey and colleagues found that their male Medflies usually went upside down about 10 or 15 days before the end of their roughly 60-day lives. Once this began, the flies spent progressively more time on their backs as they grew older. The researchers coined the term "supine behavior" to describe this phenomenon.

"This suggests that almost all male Medflies experience a period of declining health prior to death, and morbidity is a natural stage of the aging process," said Nikos Papadopoulos, a post-graduate researcher and lead author on the study. "If it turns out that the cause of this decline is neurological and central in origin, rather than musculoskeletal, then perhaps insects also can serve as models for studying the onset of progressive neurological illnesses and dementia in humans.

The study was funded by the Center for the Economics and Demography of Aging at the University of California, Berkeley, and the National Institute on Aging.