Insects are short-lived creatures, whose lives can be measured by their voltinism — the numbers of generations per year. Most insects take a year or less to develop, with either one generation per year (univoltine insects), or two (bivoltine insects), or more than two (multivoltine, or polyvoltine, insects). Generation times in excess of one year (semivoltine insects) are found, for example, amongst some inhabitants of the polar extremes, where suitable conditions for development may exist for only a few weeks in each year. Large insects that rely upon nutritionally poor diets also develop slowly over many years. For example, periodic cicadas feeding on sap from tree roots may take either 13 or 17 years to mature, and beetles that develop within dead wood have been known to emerge after more than 20 years’ development.
Most insects do not develop continuously throughout the year, but arrest their development during unfavorable times by quiescence or diapause (section 6.5). Many univoltine and some bivoltine species enter diapause at some stage, awaiting suitable conditions before completing their life cycle. For some univoltine insects, many social insects, and others that take longer than a year to develop, adult longevity may extend to several years. In contrast, the adult life of multivoltine insects may be as little as a few hours at low tide for marine midges such as Clunio (Diptera: Chironomidae), or a single evening for many Ephemeroptera.
Multivoltine insects tend to be small and fast-developing, using resources that are more evenly available throughout the year. Univoltinism is common amongst temperate insects, particularly those that use resources that are seasonally restricted. These might include insects whose aquatic immature stages rely on spring algal bloom, or phytophagous insects using short-lived annual plants. Bivoltine insects include those that develop slowly on evenly spread resources, and those that track a bimodally distributed factor, such as spring and fall temperature. Some species have fixed voltinism patterns, whereas others may vary with geography, particularly in insects with broad latitudinal or elevational ranges.