This booklet introduces high school and junior high school students to the major groups of aquatic insects and to population sampling techniques. Chapter 1 consists of a short field guide which can be used to identify five separate orders of aquatic insects: odonata (dragonflies and damselflies); ephemeroptera (mayflies); diptera (true flies); coleoptera (beetles); and hemiptera (true bugs). In addition, members of the coleoptera and hemiptera can be keyed to family. Pertinent illustrations are included. Chapter 2 includes a discussion of and procedures for the mark-capture technique, a method that biologists use to estimate the size of a wild population. Although other methods can be used, the mark-capture works particularly well in estimating the population of aquatic insects in a single, small body of water on a given day. In addition, the concepts of randomness, ratio, and proportion are stressed in the chapter. Team and class data sheets for insect population sampling are included. (JN)