Lifehistory Patterns And Phases

Growth is an important part of an individual's ontogeny, the developmental history of that organism from egg to adult. Equally significant are the changes, both subtle and dramatic, that take place in body form as insects molt and grow larger. Changes in form (morphology) during ontogeny affect both external structures and internal organs, but only the external changes are apparent at each molt. We recognize three broad patterns of developmental morphological change during ontogeny, based on the degree of external alteration that occurs in the postembryonic phases of development.

The primitive developmental pattern, ametaboly, is for the hatchling to emerge from the egg in a form essentially resembling a miniature adult, lacking only genitalia. This pattern is retained by the primitively wingless orders, the silverfish (Zygentoma) and bristle-tails (Archaeognatha) (Box 9.3), whose adults continue to molt after sexual maturity. In contrast, all pterygote insects undergo a more or less marked change in form, a metamorphosis, between the immature phase of development and the winged or secondarily wingless (apterous) adult or imaginal phase. These insects can be subdivided according to two broad patterns of development, hemimetaboly (partial or incomplete metamorphosis; Fig. 6.2) and holomet-aboly (complete metamorphosis; Fig. 6.3 and the vignette for this chapter, which shows the life cycle of the monarch butterfly).

Developing wings are visible in external sheaths on the dorsal surface of nymphs of hemimetabolous insects except in the youngest immature instars. The term exopterygote has been applied to this type of "external" wing growth. In the past, insect orders with hemimetabolous and exopterygote development were grouped into "Hemimetabola" (also called Exoptery-gota), but this group is recognized now as applying to a grade of organization rather than to a monophyletic phylogenetic unit (Chapter 7). In contrast, pterygote orders displaying holometabolous development share the unique evolutionary innovation of a resting stage or pupal instar in which development of the major structural differences between immature (larval) and adult stages is concentrated. The orders that share this unique, derived pattern of development represent a clade called the Endopterygota or Holometabola. In the early branching Holometabola, expression of all adult features is retarded until the pupal stage; however, in more derived taxa including Drosophila, uniquely adult structures including wings may be present internally in larvae as groups of undifferentiated

2nd-instar nymph nymph

Fig. 6.2 The life cycle of a hemimetabolous insect, the southern green stink bug or green vegetable bug, Nezara viridula (Hemiptera: Pentatomidae), showing the eggs, nymphs of the five instars, and the adult bug on a tomato plant. This cosmopolitan and polyphagous bug is an important world pest of food and fiber crops. (After Hely et al. 1982.)

2nd-instar nymph nymph

Fig. 6.2 The life cycle of a hemimetabolous insect, the southern green stink bug or green vegetable bug, Nezara viridula (Hemiptera: Pentatomidae), showing the eggs, nymphs of the five instars, and the adult bug on a tomato plant. This cosmopolitan and polyphagous bug is an important world pest of food and fiber crops. (After Hely et al. 1982.)

cells called imaginal discs (or buds) (Fig. 6.4), although they are scarcely visible until the pupal instar. Such wing development is called endopterygote because the wings develop from primordia in invaginated pockets of the integument and are everted only at the larval-pupal molt.

The evolution of holometaboly allows the immature and adult stages of an insect to specialize in different

Immature Stage Insect
Fig. 6.3 Life cycle of a holometabolous insect, a bark beetle, Ips grandicollis, showing the egg, the three larval instars, the pupa, and the adult beetle. (After Johnson & Lyon 1991.)

resources, contributing to the successful radiation of the group (see section 8.5).

6.2.1 Embryonic phase

The egg stage begins as soon as the female deposits the mature egg. For practical reasons, the age of an egg is estimated from the time of its deposition even though the egg existed before oviposition. The beginning of the egg stage, however, need not mark the commencement of an individual insect's ontogeny, which actually begins when embryonic development within the egg is triggered by activation. This trigger usually results from fertilization in sexually reproducing insects, but in parthenogenetic species appears to be induced by various events at oviposition, including the entry of oxygen to the egg or mechanical distortion.

Following activation of the insect egg cell, the zygote nucleus subdivides by mitotic division to produce many daughter nuclei, giving rise to a syncytium. These nuclei and their surrounding cytoplasm, called cleavage energids, migrate to the egg periphery where the membrane infolds leading to cellularization of the superficial layer to form the one-cell thick blastoderm. This distinctive superficial cleavage during early embryogenesis in insects is the result of the large amount of yolk in the egg. The blastoderm usually gives rise to all the cells of the larval body, whereas the central yolky part of the egg provides the nutrition for the developing embryo and will be used up by the time of eclosion, or emergence from the egg.

Regional differentiation of the blastoderm leads to the formation of the germ anlage or germ disc (Fig. 6.5a), which is the first sign of the developing embryo, whereas the remainder of the blastoderm becomes a thin membrane, the serosa, or embryonic cover. Next, the germ anlage develops an infolding in a process called gastrulation (Fig. 6.5b) and sinks into the yolk, forming a two-layered embryo containing the amniotic cavity (Fig. 6.5c). After gastrulation, the germ anlage becomes the germ band, which externally is characterized by segmental organization (commencing in Fig. 6.5d with the formation of the protocephalon). The germ band essentially forms the ventral regions of the future body, which progressively differentiates with the head, body segments, and appendages becoming increasingly well defined (Fig. 6.5e-g). At this time the embryo undergoes movement called katatrepsis which brings it into its final position in the egg. Later, near the end of embryogenesis (Fig. 6.5h,i), the edges of the germ band grow over the remaining yolk and fuse mid-dorsally to form the lateral and dorsal parts of the insect - a process called dorsal closure.

In the well-studied Drosophila, the complete embryo is large, and becomes segmented at the cellularization stage, termed "long germ" (as in all studied Diptera, Coleoptera, and Hymenoptera). In contrast, in "short-germ" insects (phylogenetically earlier branching taxa, including locusts) the embryo derives from only a small region of the blastoderm and the posterior segments are added post-cellularization, during subsequent growth. In the developing long-germ embryo, the syncytial phase is followed by cell membrane intrusion to form the blastoderm phase.

Functional specialization of cells and tissues occurs during the latter period of embryonic development, so that by the time of hatching (Fig. 6.5j) the embryo is a tiny proto-insect crammed into an eggshell. In ametabolous and hemimetabolous insects, this stage may be recognized as a pronymph - a special hatching stage (section 8.5). Molecular developmental processes involved in organizing the polarity and differentiation of areas of the body, including segmentation, are reviewed in Box 6.1.

6.2.2 Larval or nymphal phase

Hatching from the egg may be by a pronymph, nymph,

Immature Stages Larva

Fig. 6.4 Stages in the development of the wings of the cabbage white or cabbage butterfly, Pieris rapae (Lepidoptera: Pieridae). A wing imaginai disc in an (a) first-instar larva, (b) second-instar larva, (c) third-instar larva, and (d) fourth-instar larva; (e) the wing bud as it appears if dissected out of the wing pocket or (f ) cut in cross-section in a fifth-instar larva. ((a-e) After Mercer 1900.)

Fig. 6.4 Stages in the development of the wings of the cabbage white or cabbage butterfly, Pieris rapae (Lepidoptera: Pieridae). A wing imaginai disc in an (a) first-instar larva, (b) second-instar larva, (c) third-instar larva, and (d) fourth-instar larva; (e) the wing bud as it appears if dissected out of the wing pocket or (f ) cut in cross-section in a fifth-instar larva. ((a-e) After Mercer 1900.)

or larva: eclosion conventionally marks the beginning of the first stadium, when the young insect is said to be in its first instar (Fig. 6.1). This stage ends at the first ecdysis when the old cuticle is cast to reveal the insect in its second instar. Third and often subsequent instars generally follow. Thus, the development of the immature insect is characterized by repeated molts separated by periods of feeding, with hemimetabolous insects generally undergoing more molts to reach adulthood than holometabolous insects.

All immature holometabolous insects are called larvae. Immature terrestrial insects with hemimeta-

bolous development such as cockroaches (Blattodea), grasshoppers (Orthoptera), mantids (Mantodea), and bugs (Hemiptera) always are called nymphs. However, immature individuals of aquatic hemimetabolous insects (Odonata, Ephemeroptera, and Plecoptera), although possessing external wing pads at least in later instars, also are frequently, but incorrectly, referred to as larvae (or sometimes naiads). True larvae look very different from the final adult form in every instar, whereas nymphs more closely approach the adult appearance at each successive molt. Larval diets and lifestyles are very different from those of their adults. In

Immature Stages Larva

Fig. 6.5 Embryonic development of the scorpionfly Panorpodesparadoxa (Mecoptera: Panorpodidae): (a-c) schematic drawings of egg halves from which yolk has been removed to show position of embryo; (d-j) gross morphology of developing embryos at various ages. Age from oviposition: (a) 32 h; (b) 2 days; (c) 7 days; (d) 12 days; (e) 16 days; (f) 19 days; (g) 23 days; (h) 25 days; (i) 25-26 days; (j) full grown at 32 days. (After Suzuki 1985.)

Fig. 6.5 Embryonic development of the scorpionfly Panorpodesparadoxa (Mecoptera: Panorpodidae): (a-c) schematic drawings of egg halves from which yolk has been removed to show position of embryo; (d-j) gross morphology of developing embryos at various ages. Age from oviposition: (a) 32 h; (b) 2 days; (c) 7 days; (d) 12 days; (e) 16 days; (f) 19 days; (g) 23 days; (h) 25 days; (i) 25-26 days; (j) full grown at 32 days. (After Suzuki 1985.)

contrast, nymphs often eat the same food and coexist with the adults of their species. Competition thus is rare between larvae and their adults but is likely to be prevalent between nymphs and their adults.

The great variety of endopterygote larvae can be classified into a few functional rather than phylogen-etic types. Often the same larval type occurs conver-gently in unrelated orders. The three commonest forms are the polypod, oligopod, and apod larvae (Fig. 6.6). Lepidopteran caterpillars (Fig. 6.6a,b) are characteristic polypod larvae with cylindrical bodies with short thoracic legs and abdominal prolegs (pseudopods). Symphytan Hymenoptera (sawflies; Fig. 6.6c) and most Mecoptera also have polypod larvae. Such larvae are rather inactive and are mostly phytophagous. Oligopod larvae (Fig. 6.6d-f) lack abdominal prolegs but have functional thoracic legs and frequently prognathous mouthparts. Many are active predators but others are slow-moving detritivores living in soil or are phytophages. This larval type occurs in at least some members of most orders of insects but not in the Lepidoptera, Mecoptera, Diptera, Siphonaptera, or

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