What Is Pterogote

Hyperparasitism intrigues entomologists because of its multidisciplinary relationship to evolution, ecology, behavior, biological control, taxonomy, and mathematical models. More field studies are needed to determine whether hyperparasitoids are always detrimental to biological control programs. Perhaps, instead, they could have a beneficial influence by regulating the extreme/detrimental population oscillations of the beneficial primary parasitoids.

See Also the Following Articles

Biological Control • Parasitoids

Further Reading

Godfray, H. C. J. (1994). "Parasitoids: Behavioral and Evolutionary

Ecology." Princeton University Press, Princeton, NJ. Gordh, G. (1981). The phenomenon of insect hyperparasitism and its taxonomic occurrence in the Insecta. In "The Role of Hyperparasitism in Biological Control: A Symposium" (D. Rosen, ed.), Publication 4103, pp. 10—18. University of California Press, Berkeley. Holt, R. D., and Hochberg, M. E. (1998). The coexistence of competing parasites. II: Hyperparasitism and food chain dynamics. J. Theor. Biol.

Mackauer, M., and Völkl, W. (1993). Regulation of aphid populations by aphidiid wasps: Does parasitoid foraging behaviour or hyperparasitism limit impact? Oecologia 94, 339-350. Price, P. W., Bouton, C. E., Gross, P., McPheron, B. A., Thompson, J. N., and Weis, A. E. (1980). Interactions among three trophic levels: Influence of plants on interactions between herbivores and natural enemies. Annu. Rev. Ecol. Syst. 11, 41-65. Stiling, P., and Rossi, A. M. (1994). The window of parasitoid vulnerability to hyperparasitism: Template for parasitoid complex structure. In "Parasitoid Community Ecology" (B. Hawkins, and W. Sheehan, eds.), pp. 228-244. Oxford University Press, Oxford, U.K. Sullivan, D. J. (1987). Insect hyperparasitism. Annu.. Rev. Entomol. 32, 49-70.

Sullivan, D. J. (1988). Aphid hyperparasites. In "Aphids, Their Biology, Natural Enemies and Control" (A. K. Minks, and P. Harrewijn, eds.), Vol. 2B, pp. 189-203. Elsevier, Amsterdam. Sullivan, D. J., and Völkl, W. (1999). Hyperparasitism: Multitrophic ecology and behavior. Annu. Rev. Entomol. 44, 291-315. Whitfield, J. B. (1998). Phylogeny and evolution of the host-parasitoid relationship in the Hymenoptera. Annu. Rev. Entomol. 43, 129-151.

Seth S. Blair

University ofWisconsin-Madison

The term "imaginal disc" is used to describe structures found in the larvae of the Holometabola. Holometabolous insects can be defined as those in which the final instar metamorphoses into a radically different adult during a quiescent pupal stage; they are thought to be a monophyletic group, distinct from the Hemimetabola. During the metamorphosis of Holometabola, the epidermis must form novel structures that were lacking in the larva and, in some insects larval tissues that were lost must be replaced. The cells that give rise to the new epidermal tissues of the adult (imago) are often referred to as histoblasts. When histoblasts are organized into morphologically distinct clusters, these structures are commonly referred to as imaginal discs or imaginal buds. This article discusses this definition and briefly reviews some of the experimental studies examining the biology and, especially, the development of imaginal discs.

WHAT IS AN IMAGINAL DISC?

The terms "imaginal disc" and "histoblast" are more pragmatic than precisely defined, and their usage varies from author to author because imaginal discs vary considerably between taxa in number, morphology, and development. For example, some authors have defined imaginal discs and histoblast cells based on their undifferentiated state and early appearance in development, since in some taxa imaginal discs are formed in the embryo. These early-developing discs may sometimes secrete a thin cuticle-like substance, but they do not contribute appreciably to larval life and thus can be considered to be specialized, relatively undifferentiated structures set aside for adult development. However, in other taxa the imaginal discs cannot be detected until the final instar and are apparently derived from cuticle-secreting, differentiated larval cells. Several authors have argued that late-developing discs reflect the ancestral condition within the Holometabola.

The morphology of imaginal discs also varies. The typical imaginal disc is a pocket or sac of cells that has invaginated from the larval epidermis and is destined to form part or all of an adult appendage, compound eye, or genitalia. One portion of the sac has thickened to form the disc epithelium, whereas the rest forms a thinner peripodial membrane; the space within the sac is termed the peripodial cavity (Fig. 1). This sac evaginates ("everts") during metamorphosis and contributes to the adult cuticle (Fig. 1). However, the positions within the disc of the disc epithelial cells and peripodial cells vary, as does the disc's degree of invagination, and some authors have defined several categories of discs or quasidiscs. For example, some discs have invaginated a large distance from the larval epidermis and remain connected to it by only a thin peripodial stalk. At the other extreme, as in the wings of some Coleoptera, the regions of disc epidermis that form adult structures never invaginate from the larval epidermis. Some of these discs can still be recognized as thickenings of the larval epidermis. However, in other examples, such as the leg of the lepidopteran Pieris brassicae, the histoblasts cannot be easily recognized, and the novel portions of a single adult appendage develop from several zones of dividing cells within the larval appendage.

Thus, sometimes it is only its eventual contribution to a novel adult structure that makes a cell a histoblast, distinct from neighboring cells in the larval epidermis. Moreover, the organization of histoblasts into a morphologically identifiable imaginal disc is simply one of several ways these cells can be arranged. And though the term "imaginal disc" is used only for holometabolous insects, it is only the invaginated morphology of most discs that distinguishes them from structures such as the external wing pads of hemimetabolous

Larval epidermis

Larval epidermis

Peripodial cavity

Peripodial membrane

Peripodial cavity

Peripodial membrane

FIGURE 1 Eversion of D. melanogaster leg disc, shown in cross section. [After Blair, S. S. (1999). Drosophila imaginal disc development: Patterning the adult fly. In "Development-Genetics, Epigenetics, and Environmental Regulation" (V. E. A. Russo, D. Cove, L. Edgar, R. Jaenisch, and F. Salamini, eds.), p. 348. © Springer-Verlag GmbH & Co. KG, Heidelberg.]

FIGURE 1 Eversion of D. melanogaster leg disc, shown in cross section. [After Blair, S. S. (1999). Drosophila imaginal disc development: Patterning the adult fly. In "Development-Genetics, Epigenetics, and Environmental Regulation" (V. E. A. Russo, D. Cove, L. Edgar, R. Jaenisch, and F. Salamini, eds.), p. 348. © Springer-Verlag GmbH & Co. KG, Heidelberg.]

pterogotes, which also are morphologically distinct and form adult structures. Thus, the evolution of invaginated wing imaginal discs from the evaginated wing pad may have required only a morphological change, rather than the evolution of a radically different cell type or organ. In fact, a similar evolution has apparently occurred in some Hemimetabola, such as thrips, which have invaginated wing precursors instead of external wing pads.

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