Hypermetamorphosis is a form of complete insect metamorphosis or holometaboly in which at least one of the instars in the life cycle differs considerably from the others. The term heteromorphosis, preferred by some entomologists, carries a degree of ambiguity in that it also refers to the relatively minor differences characterizing consecutive instars in virtually all insects, as well as to the phenomenon of organ replacement following mutilation. Hypermetamorphosis is most common in parasitoids, where it usually is the first instar that deviates structurally and behaviorally from the others. In some groups one or more of the subsequent instars also are distinctive. In the same way that holometabolous development allows a division of function between the larva and adult insect, hypermetamorphosis can be viewed as an exaggerated form of holometaboly characterized by additional division of function within the larval stage.
GENERAL CHARACTERISTICS
AND TERMINOLOGY
Two broad categories of hypermetamorphosis can be recognized in insects. In the most widespread form, there is a decoupling of oviposition site and the larval food; in the other, the oviposition and larval feeding sites are identical. For convenience, these can be referred to as type I and type II hypermetamorphosis, respectively.
Type I adult females do not oviposit directly at the larval feeding site; instead, the first instars must find the food source. Such larvae are active, slender, and well-sclerotized (Fig. 1A); they are further characterized by their ability to exist for a considerable time without nourishment and without becoming desiccated. Depending on the group, the first instar may attain the feeding site by direct searching, or indirectly by phoresy, in which it is carried to its food usually by the host itself. Once on the larval food, the first instar begins feeding, and subsequently molts into a grublike and less mobile larva (Fig. 1B). Phoretic larvae are commonly equipped with elongate caudal cerci and a terminal suction process (or pygopod), which allows them to stand erect, thus facilitating contact with passing hosts. Although comparative studies are few, hypermetamorphic taxa seem to be more fecund than nonhypermetamorphic relatives. This apparent difference is attributed to the reduced likelihood of larvae finding suitable hosts. Two general characteristics of adult females with this form of development are the absence of a well-developed ovipositor and the laying of eggs in masses rather than one at a time.
Type I hypermetamorphosis is found in the Strepsiptera (all groups), Neuroptera (Mantispidae), Coleoptera (several families), Diptera (Nemestrinidae, Acroceridae, most Bombyliidae, some Tachinidae), Hymenoptera (Perilampidae, Eucharitidae, some Ichneumonidae), and Lepidoptera (Epipyropidae).
Although there is some inconsistency in usage, the active first instar in type I hypermetamorphic taxa has generally been referred to as a triungulin or triungulinid if it has legs (Neuroptera, Coleoptera,
FIGURE 1 Larvae of Meloe dianellus (Coleoptera: Meloidae) illustrating the four types of meloid larvae: (A) first instar (planidium with legs), (B) first grub (fifth instar), (C) coarctate (sixth instar), and (D) second grub (seventh instar). Natural lengths: (A) 2 mm, (B-D) 7-10mm. [From Pinto, J., and Selander, R. (1970).
Strepsiptera, Lepidoptera; Fig. 1A), or as a planidium if it is legless (Diptera, Hymenoptera; Fig. 2A). However, triungulin is an inappropriate term for most of the groups it is applied to. Usage stems from the fact that the first instar of Meloe (Coleoptera: Meloidae), the first hypermetamorphic group studied, has trident-shaped claws. Such claw structure does not occur in other hypermetamorphic families and is uncommon in the Meloidae itself. Thus, planidium, signifying "little wanderer," although usually restricted to the larvae of Hymenoptera and Diptera, is perhaps more appropriately applied to all type I first instars. This usage is followed here and was adopted by R. E. Snodgrass in his 1954 review of insect metamorphosis.
In type II hypermetamorphosis the oviposition and larval feeding sites do not differ. In these insects the first instar (sometimes the second as well) has a distinctive morphology (Fig. 2B-2E) compared with the relatively simple and often amorphous subsequent instars (Fig. 2F), but these modifications are associated with functions other than host finding. The adaptive significance attributed to the often bizarre larvae of type II hypermetamorphic groups includes locomotion, protection and combat, ingestion, and respiration. This type of development occurs in several families of parasitic Hymenoptera and in the dipteran family Cryptochetidae (Cryptochetum). Correlated with the greater possibilities of larval function, the morphology of type II first instars is considerably more variable than that found in type I taxa.
Reviews by Clausen and Hagen identified 12 structural forms of first instars in the parasitic Hymenoptera with type II hypermetamor-phosis. The generalized type in nonhypermetamorphic species (and in hypermetamorphic species as well after the first instar) is referred to as hymenopteriform. It is ovoid or fusiform and relatively featureless (Fig. 2F). Examples of deviations in hypermetabolic groups include the following. The caudate type has a taillike prolongation of the terminal segment and is found in many Ichneumonoidea and Chalcidoidea (Fig. 2C). It is believed to be an adaptation for locomotion within the host and/or food absorption. The mandibulate and
