One character that divides insects into distinct groups is their mode of development. Some of the more diverse groups of insects are holometabolous. This means their life cycle includes four stages: egg, larvae, pupae, adult. Each stage usually occupies a very different habitat. For example, the life cycle of the holometabolous black fly (pictured at right) includes a larval stage that is aquatic. The adults are not aquatic - they feed on mammal blood. Effectively, this separation of the two main feeding stages reduces the competition between larvae and adults within the same species. This may be one explanation why holometabolous insects are so successful. At the other extreme, insects like the silverfish do not undergo metamorphosis, and are called ametabolous.

A holometabolous life cycle, showing egg, larval, pupal, and adult stages. Drawing modified from R. W. Crosskey (1990), The Natural History of Blackflies, John Wiley & Sons. A closeup of living black fly larvae. These aquatic insects filter food from flowing water with the net-like projections protruding from their head. Photo courtesy of Bill Ballard.