The lifestyle of early insects was immutably linked to the rise and diversification of vascular plants in the Late Paleozoic. Although many groups of living insects feed on plants, there is little evidence that their Paleozoic ancestors did the same. One group, the palaeodictyopteroids, had mouthparts designed for tearing apart loosely packed spore pods or cones or piercing such spore-bearing packets and sucking out the contents. Evidence for this is found in fossils that show the gut of these insects jammed with spores. Some insects also had piercing mouthparts that conceivably could have punctured the skin of a plant to consume its cell juices. Reinforcing the idea that some insect groups had grown dependent on plants for nutrition is the fact that some taxa, including the palaeodictyopter-oids, were becoming extinct by the end of the Carboniferous Period, when the once-dominant flora of seed ferns, lycopods, and cordaites diminished substantially in favor of gymnosperms. Those insects became extinct because of their inability to adapt biologically to eating the new kinds of plants.
The world of Paleozoic insects, therefore, was not yet a world of widespread herbivory. Plants, however, played an important role in the food chain. Most insects either fed on dead organic matter— such as dead plants, fallen spores, and shed plant parts—or were predatory, feeding on smaller varieties of insects. The ecosystem of the Late Paleozoic created zones of opportunity for plants and animals alike. Crawling insects worked the floor of the tropical forest or climbed the limbs of plants in search of detritus, spores, and sometimes other insects on which to feed. Flying insects sailed among the tallest trees to pick one another off out of the air or to pluck crawling insects from the branches and leaves to which they clung. The largest insects, such as dragonflies with wings measuring up to 29 inches (74 cm) wide, were the largest flying predators of their time and not likely to get eaten by the slow-moving vertebrates that crept along the swamp base. Dragonflies kept their distance from the ground so as not to get tangled in low-growing vegetation.
Insects were the first flying creatures—a most remarkable adaptation that eventually was repeated through the independent evolution of flight in three kinds of vertebrates: pterosaurs, birds, and bats. Just how flight evolved in insects remains a great debate, and two hypotheses have emerged. The first idea, proposed in 1935, suggested that wings derived from an advantageous outgrowth of the body wall and did so independently of any preexisting limbs. A more recent hypothesis, first proposed in 1973, is that insect wings were an adaptation of an existing, ancestral appendage, such as gills. Recent work by Michalis Averof and Stephen Cohen of the European Molecular Biology Laboratory in Heidelberg, Germany, makes a compelling case for the gill-origin theory. Averof and Cohen's research identified two genes related to wing-specific functions in insects that are also found in crustaceans. Their theory is that wings evolved from gill-like appendages of a common ancestor of both crustaceans and insects. The evolution of insect flight also benefited from the higher levels of atmospheric oxygen during the Carboniferous Period. Higher atmospheric oxygen levels created denser air, which provided more lift for flying insects.
The subject of the marine origins of insect ancestors and the mention of gills are reminders of yet another anatomical innovation of insects. Insects do not have lungs like fish and other vertebrates. Nor do they have external gills like their marine arthropod ancestors. Insects breathe by a process of gas exchange with the atmosphere. The outside surface of an insect contains several tiny openings that are part of a remarkable tracheal system for consuming oxygen. There are two pairs of openings on the insect's thorax and additional openings on each of its other body segments. Each opening or trachea branches into smaller tubes that then branch again to connect directly with an insect's tissues. In this way, energy-providing oxygen is fed directly into the tissues rather than being transported to the tissues by blood as it is in vertebrates. Furthermore, these openings to the outside air can be closed to prevent water loss through evaporation and can be contracted and expanded by the insect to increase or slow down the rate of respiration.
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