The young of insects with this type of metamorphosis are called nymphs4 and are usually very similar to the adults. If compound eyes are present in the adult, they are present in the nymph. If the adults are winged, the wings appear as budlike outgrowths in the early instars (Figure 1) and increase in size only slightly up to the last molt. After the last molt, the wings expand to their full adult size. Simple metamorphosis occurs in the hexapod orders l to 22.
Fig1. Stages in the development of the grass bug, Arhyssus sidae (Fabricius). A, Egg; B, First instar; C, Second instar; D, Third instar; E, Fourth instar; F, Fifth instar; G, Adult female. (Courtesy of Readio 1928 and the Entomological Society of America.)
Differences in the kind and amount of change occur in the insects with simple metamorphosis, and some entomologists recognize three types of metamorphosis in these insects: ametabolous, paurometabolous, and hemimetabolous. Ametabolous insects (with "no" metamorphosis) are wingless as adults, and the only obvious difference between nymphs and adults is size. This type of development occurs in the apterygote orders (Protura, Collembola, Diplura, Microcoryphia, and Thysanura) and in most wingless members of the other orders with simple metamorphosis. In hemimetabolous metamorphosis (with "incomplete" metamorphosis), the nymphs are aquatic and gill-breathing and differ considerably from the adults in appearance. This type of development occurs in the Ephemeroptera, Odonata, and Plecoptera, and the young of these insects are sometimes called naiads. Paurometabolous insects (with "gradual" metamorphosis) include the remaining insects with simple metamorphosis. The adults are winged; the nymphs and adults live in the same habitat; and the principal changes during growth are in size, body proportions, the development of the ocelli, and occasionally the form of other structures.
