The immature and adult stages of insects that undergo complete metamorphosis are usually quite different in form, often live in different habitats, and have very different habits. The early instars are often more or less wormlike, and the young in this stage are called larvae (Figures 1 and 2). The different larval instars are usually similar in form but differ in size. The wings, when they are present in the adult, develop internally during the larval stage and are not everted until the end of the last larval instar. Larvae generally have chewing mouthparts, even in those orders in which the adults have sucking mouthparts.
Fig1. Stages in the development of the sugarbeet root maggot, TeLanops myopae formis (Roder). A, Adult female; B, Adult male; C, Egg; D, Larva; E, Puparium (pupa in side). (From Knowlton 193 7, used courtesy of the Utah Agricultural Experiment Station.)
Fig2. Insect larvae. A, Maggot or vermiform larva of Hylemya platura (Meigen) (Diptera, Anthomyiidae); B, Grub or scarabaeiform larva of Phyllophaga rugosa (Melsheimer) (Coleoptera, Scarabaeidae); C, Elateriform larva of Cardiophorus sp. (Coleoptera, Elateridae); D, Elateriform larva of Alaus oculatus (L.) (Coleoptera, Elateridae); E, Campodeiform larva of Attagenus megatoma (Fabricius) (Coleoptera, Dermestidae); F, Vermiform larva of Cylas formicarius elegantulus (Summers) (Coleoptera, Brenticlae); G, Eruciform larva of Caliroa aethiops (Fabricius) (Hymenoptera, Tenthre cliniclae). a, antenna; as, anterior spiracle; L, length; ps, posterior spiracle; sp, spiracle. (A, E-G, Courtesy of USDA; B-D, From Peterson, 1948, by permission.)
Following the last larval instar, the insect trans forms into a stage called the pupa (Figure 3). The insect does not feed in this stage and is usually inactive. Pupae are often covered by a cocoon or some other protective material, and many insects pass the winter in the pupal stage. The final molt occurs at the end of the pupal stage, and the last stage is the adult.
Fig3. Insect pupae. A, Chrysalis of the sulphur butterfly, Colias eurytheme Boisduval (Lepidoptera, Pieridae); B, Fall armyworm, Spodoptera frugiperda Q. E. Smith) (Lepidoptera Noctuidae); C, Clover seed chalcid, Bruchophagus platyptera (Walker) (Hymenoptera, Eurytomidae); D, Sweetpotato weevil, Cylas formicarius elegantulus (Summers) (Coleoptera, Brentidae); E, Sawtoothed grain beetle, Oryzaephilus surinamensis (L.) (Coleoptera, Silvanidae); F, Seedcorn maggot, Hylemya platura (Meigen) (Diptera, Anthomyiidae). A and B are obtect pupae, C-E are exarate pupae, and F is a coarctate pupa. (Courtesy of USDA.)
The adult is usually pale in color when it first emerges from the pupa, and its wings are short, soft, and wrinkled. In a short time, from a few minutes to several hours or more, depending on the species, the wings expand and harden, the pigmentation develops, and the insect is ready to go on its way. This type of metamorphosis, which occurs in orders 23 to 31, is often called holometabolous, and these orders are classified together as the Holometabola.
Hypermetamorphosis is a type of complete meta morphosis in which the different larval instars are not of the same type. The first instar is active and usually campodeiform, and the subsequent larval instars are vermiform or scarabaeiform. Hypermetamorphosis occurs in parasitic insects; the first instar seeks out the host and, once in the host, molts into a less active type of larva. This type of complete metamorphosis occurs in the Meloidae and Ripiphoridae (Coleoptera), the Mantispidae (Neuroptera), the Strepsiptera, and a few Diptera and Hymenoptera.
