Guessing a person's age is relatively easy. The differences between a two-year-old and a twenty-year-old are dramatic. Even face-lifts and toupees seldom conceal our true vintage. Guessing the age of animals, though, can be tricky. Take insects, for example. A beetle looks pretty much the same at six months as it does at sixty months. Still, the ages of most North American insects in the can be deduced by applying a little knowledge of the species in question and by looking at the calendar.
The vast majority of our insects follow an annual cycle -- eggs are laid by adults, the eggs hatch, the immature or larval insects grow and transform into adults, and the adults mate, produce more eggs, and die. One of these life stages may last for months while the insect exists in a dormant state (this usually happens in the winter), but most insects pass through one generation each year.
Aphids are an exception. These miniscule creatures often go through many generations each year -- that is, individuals live for a few months at most before being replaced by a new generation. The aphids we see during the early spring often come from females that gave birth to young without ever mating (yes, these insects cloned themselves). The young mature and produce their own offspring. Later in the season, more eggs are laid that hatch into male and female aphids, which mate and leave behind the fertilized eggs that will give rise to the next year's brood of spring females.
A number of butterfly species also have more than one generation per year. The first adults of spring often produce a summer generation that then produces another generation for the remainder of the summer or the fall. The Common Buckeye is one such species.
At the opposite end of the spectrum are insects that survive for years. Some darkling beetles have been known to live for more than five years in captivity, and that number could be higher in the wild, where the beetles might pass most of the year in an inactive state. Indeed, many insects that begin life in cold-water habitats near the Arctic Circle (such as the Canada Darner dragonfly) take several years to reach adulthood.
And then there are Mayflies. It's a common misconception that Mayflies live for only a day. The truth is that the immature stage of a Mayfly can last for a year or more before the brief winged adult phase of the life cycle. The scientific name of the order -- ephemeroptera -- translates roughly to "winged but for a day."