Insect Body Plans And Developmental Programs

Three distinct patterns of growth are observed in insects, distinguished by body form and type of metamorphosis: ametabolous, hemimetabolous, and holometabolous. Insect orders exhibiting ametabolous development belong to the Apterygota, or wingless insects. Included in this group are the primitive orders Protura, Collembola, and Diplura, whose earliest stages are miniature adults in form, except for the absence of external genitalia. They grow continuously and lack metamorphosis. Attainment of reproductive competence occurs at an indefinite time in development, and they continue to molt even as adults. Little is known about the hormonal control of development in the Ametabola.

The majority of insects are winged and have either partial or complete metamorphosis. The Hemimetabola, such as grasshoppers and crickets, emerge from the egg formed as small, immature versions of the adult and are called nymphs. They lack wings and functional reproductive organs. After a series of molts, the number usually constant from one generation to another, nymphs pass directly to the winged, reproductive adult stage in a single step. This mode of development is referred to as incomplete metamorphosis. The more advanced insect orders, including moths, beetles, flies, and wasps, develop as vermiform (wormlike) larvae during the immature stages. The complete metamorphosis of these groups is a two-step process in which a sessile, nonfeeding pupal stage is intermediate between larva and adult. The pupal stage allows for a complete change in body form from larva to winged, hexapod adult. This total transformation of body form during complete metamorphosis requires a high degree of postembryonic cellular programming under the control of hormones. Three types of cellular processes are dictated, mostly by hormonal signaling involving ecdysteroids and juvenile hormones. First, many larval-specific structures such as body wall muscles and neurons must be eliminated through a programmed cell death known as apoptosis. Second, some cells persist to the adult stage, but are extensively remodeled to serve adult functions. Finally, new cells derived from imaginal discs are born.

The diversity of insect groups places limits on generalizations about hormonal control of development. The effects of hormones in one group may not be the same for other groups, because of different patterns of growth and cellular specification. In the Lepidoptera, for example, larval epidermal cells change their cuticle secretory program at metamorphosis and switch to production of pupal and subsequently adult cuticle. Exogenous juvenile hormone (JH) application at this time maintains the larval secretory program, resulting in supernumerary (extra) instars. Similarly, properly timed application of JH in the early pupal stage of moths causes a second pupal stage. However, the fruit fly Drosophila and other higher flies are largely resistant to such effects of JH. This may relate to differences in the developmental program of fly epidermal cells. The entire epidermis in the head and thorax is programmed for secretion of larval cuticle only and dies at metamorphosis. It is replaced by imaginal disc tissue, which remains undifferentiated throughout larval life in the presence of JH. Abdominal epidermal cells also die after pupation and are replaced by abdominal histoblasts. Thus the development program of epidermis in higher flies is entirely distinct from the Lepidoptera, and its lack of response to JH in the early stages may be a consequence of this very different design.

These differences in responses to hormones complicate interpretation of many findings, especially because it has become fashionable to test overall hypotheses using the moth, Manduca, for physiology and endocrinology experiments, and Drosophila for genetic manipulations. Generalizing about common mechanisms for these two evolutionarily distant groups should be done cautiously.

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