Insects Terrestrial Descendants of Marine Crustaceans

During the Devonian, more than 400 million years ago, arthropods made the leap from the marine environment onto land. Of the several groups who successfully colonized the

Abdomen

Carapace covering head and thorax

Abdomen

Carapace covering head and thorax

Terestrial Crustrations

terrestrial habitat, none is more prominent today than the six-legged individuals of the phylum Hexapoda—the insects.

Insects are found in most terrestrial and freshwater habitats, and they utilize nearly all species of plants and many species of animals as food. Some are internal parasites of plants and animals; others suck their host's blood or consume surface body tissues. The 1.4 million species of insects that have been described are believed to be only a small fraction of the total number of species living today.

Median compound eye

Median compound eye

Nauplius Larva

Antenna

Mandible

Antennule

Antenna

Mandible

33.10 Crustacean Structure

(a) The bodies of crustaceans are divided into three regions, each of which bears appendages. (b) A nauplius larva has one compound eye and three pairs of appendages.

Very few insect species live in the ocean. In freshwater environments, on the other hand, they are sometimes the dominant animals, burrowing through the substratum, extracting suspended prey from the water, and actively pursuing other animals. Insects were the first animals to achieve the ability to fly, and they are important pollinators of flowering plants.

Insects, like crustaceans, have three basic body regions: head, thorax, and abdomen. They have a single pair of antennae on the head and three pairs of legs attached to the thorax (Figure 33.11). Unlike the other arthropods, insects have no appendages growing from their abdominal segments (see Figure 21.5).

An insect exchanges gases by means of air sacs and tubular channels called tracheae (singular, trachea) that extend from external openings inward to tissues throughout the body. The adults of most flying insects have two pairs of stiff, membranous wings attached to the thorax. However, flies have only one pair of wings, and in beetles the forewings form heavy, hardened wing covers.

Wingless insects include springtails and silverfish (Figure 33.12). Of the modern insects, they are probably the most similar in form to insect ancestors. Apterygote insects have a simple life cycle, hatching from eggs as miniature adults.

Development in the winged insects (Figure 33.13) is complex. The hatchlings do not look like adults, and they undergo substantial changes at each molt. The immature stages of insects between molts are called instars. A substantial change that occurs between one developmental stage and another is called metamorphosis. If the changes between its instars are gradual, an insect is said to have incomplete metamorphosis.

Head Thorax Abdomen

Head Thorax Abdomen

Diagram Bug And The Thorax
33.11 Structure of an Insect This diagram of a generalized insect illustrates its three-part body plan. The middle region, the thorax, bears three pairs of legs and, in most groups, two pairs of wings.

33.12 Wingless Insects

The wingless insects have a simple life cycle.They hatch looking like miniature adults, then grow by successive moltings of the cuticles as these springtails are doing.

In some insect groups, the larval and adult forms appear to be completely different animals. The most familiar example of such complete metamorphosis occurs in members of the order Lepidoptera, in which the larval caterpillar transforms itself into the adult butterfly (see Figure 1.1). During complete metamorphosis, the wormlike larva transforms itself during a specialized phase, called the pupa, in which many larval tissues are broken down and the adult form develops. In many of these groups, the different life stages are specialized for living in different environments and using different food sources. In many species, the larvae are adapted for feeding and growing, and the adults are specialized for reproduction and dispersal.

Entomologists divide the winged insects into about 29 different orders. We can make sense of this bewildering variety by recognizing three major lineages:

► Winged insects that cannot fold their wings against the body

► Winged insects that can fold their wings and that undergo incomplete metamorphosis

► Winged insects that can fold their wings and that undergo complete metamorphosis

Because they can fold their wings over their backs, flying insects belonging to the second and third lineages can tuck their wings out of the way upon landing and crawl into crevices and other tight places.

33.13 The Diversity of Insects (a) Unlike most flying insects, ^ this dragonfly cannot fold its wings over its back. (b) The Mexican bush katydid represents the order Orthoptera. (c) Harlequin bugs are "true" bugs (order Hemiptera); (d) These mating mantophasma-todeans represent a recently discovered Hemipteran lineage found only in the Cape region of South Africa. (e) A predatory diving beetle (order Coleoptera). (f The California dogface butterfly is a member of the Lepidoptera. (g) The flies, including this Mediterranean fruit fly, comprise the order Diptera. (h) Many genera in the order Hymenoptera,such as honeybees, are social insects.

Wingless Bugs Hemiptera

33.12 Wingless Insects

The wingless insects have a simple life cycle.They hatch looking like miniature adults, then grow by successive moltings of the cuticles as these springtails are doing.

Harlequin Bug Life Cycle

The only surviving members of the lineage whose members cannot fold their wings against the body are the orders Odonata (dragonflies and damselflies, Figure 33.13a) and Ephemeroptera (mayflies). All members of these two orders have aquatic larvae that transform themselves into flying adults after they crawl out of the water. Although many of these insects are excellent flyers, they require a great deal of open space in which to maneuver. Dragonflies and dam-selflies are active predators as adults, but adult mayflies lack functional digestive tracts and live only long enough to mate and lay eggs.

The second lineage, whose members can fold their wings and have incomplete metamorphosis, includes the orders Or-thoptera (grasshoppers, crickets, roaches, mantids, and walking sticks; Figure 33.13b), Isoptera (termites), Plecoptera (stone flies), Dermaptera (earwigs), Thysanoptera (thrips), Hemiptera (true bugs; Figure 33.13c), and Homoptera (aphids, cicadas, and leafhoppers). In these groups, hatch-lings are sufficiently similar in form to adults to be recognizable. They acquire adult organ systems, such as wings and compound eyes, gradually through several juvenile instars. Remarkably, a new insect order in this lineage, the Man-tophasmatodea, was first described in 2002 (Figure 33.13d). These small insects are common in the Cape Region of southern Africa, an area of exceptional species richness and en-demism for many animal and plant groups.

Insects belonging to the third lineage undergo complete metamorphosis. About 85 percent of all species of winged insects belong to this lineage. Familiar examples are the orders Neuroptera (lacewings and their relatives), Coleoptera (beetles; Figure 33.13e), Trichoptera (caddisflies), Lepidoptera (butterflies and moths; Figure 33.13/), Diptera (flies; Figure 33.13g), and Hymenoptera (sawflies, bees, wasps, and ants; Figure 33.13h).

Members of several orders of winged insects, including the Phthiraptera (lice) and Siphonaptera (fleas), are parasitic. Although descended from flying ancestors, these insects have lost the ability to fly.

Molecular data suggest that the lineage leading to the insects separated from the lineage leading to modern crustaceans about 450 million years ago, about the time of the appearance of the first land plants. These ancestral forms penetrated a terrestrial environment that was ecologically empty, which in part accounts for their remarkable success. But this success of the insects is also due to their wings, which arose only once early during insect evolution. Homologous genes control the development of insect wings and crustacean appendages, suggesting that that the insect wing evolved from a dorsal branch of a crustacean limb (Figure 33.14). The dorsal limb branch of crustaceans is used for respiration and osmoregulation. This finding suggests that the insect wing evolved from a gill-like structure that had a respiratory function.

Development of appendages in the crayfish is governed by the pdm gene.

Development of the insect wing is governed by the expression of the same gene.

(a) Ancestral multibranched appendage

(a) Ancestral multibranched appendage

Dorsal branches

Dorsal branches

Development of the insect wing is governed by the expression of the same gene.

Dorsal Gene Insect

pdm gene product

33.14 Origin of Insect Wings The insect wing may have evolved from an ancestral appendage similar to that of modern crustaceans. (a) A diagram of the ancestral, multibranched arthropod limb. (b,c) The pdm gene, a Hox gene, is expressed throughout the dorsal limb branch and walking leg of the thoracic limb of a crayfish (a) and in the wings and legs of Drosophila (b).

pdm gene product

33.14 Origin of Insect Wings The insect wing may have evolved from an ancestral appendage similar to that of modern crustaceans. (a) A diagram of the ancestral, multibranched arthropod limb. (b,c) The pdm gene, a Hox gene, is expressed throughout the dorsal limb branch and walking leg of the thoracic limb of a crayfish (a) and in the wings and legs of Drosophila (b).

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Responses

  • Gerry
    How many pairs of legs with thorax in hexapoda?
    8 years ago
  • caio
    When molting occur in insects?
    8 years ago

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