Terrestrial Mandibulates

A Winning Combination

Tunis, Algeria—Treating it as an invading army, Tunisia, Algeria, and Morocco have mobilized to fight the most serious infestation of locusts in over 30 years. Billions of the insects have already caused extensive damage to crops and are threatening to inflict great harm to the delicate economies of North Africa.*

Humans suffer staggering economic losses due to insects, of which outbreaks of billions of locusts in Africa are only one example. In the western United States and Canada, an outbreak of mountain pine beetles in the 1980s and 1990s killed pines on huge acreages, and the 1973 to 1985 outbreak of spruce budworm in fir/spruce forests killed millions of conifer trees. These examples serve to remind us of our ceaseless struggle with the dominant group of animals on earth today: insects. Insects far outnumber all other species of animals in the world combined, and numbers of individuals are equally enormous. Some scientists have estimated that there are 200 million insects for every single human alive today! Insects have an unmatched ability to adapt to all land environments and to virtually all climates. Having originally evolved as land animals, insects developed wings and invaded the air 150 million years before flying reptiles, birds, or mammals. Many have exploited freshwater and saltwater (shoreline) habitats, where they are now widely prevalent; only in the seas are insects almost nonexistent.

How can we account for the enormous numbers of these creatures? In common with other arthropods, insects have a combination of valuable structural and physiological adaptations, including a versatile exoskeleton, segmentation, an efficient respiratory system, and highly developed sensory organs. In addition, insects have a waterproofed cuticle, and many have extraordinary abilities to survive adverse environmental conditions.

In this section we introduce animals commonly placed in subphylum Uniramia. Some scientists now question the validity of a taxon “Uniramia,” but we tentatively retain the grouping. We discuss the controversy further under Phylogeny. Uniramians are primarily terrestrial arthropods. Only a few have returned to aquatic life, usually in fresh water.

The term “myriapod,” meaning “many footed,” is often used for a group of four classes of uniramians that have evolved a pattern of two tagmata—head and trunk—with paired appendages on most or all trunk somites. Myriapods include Chilopoda (centipedes), Diplopoda (millipedes), Pauropoda (pauropods), and Symphyla (symphylans).

Insects have evolved a pattern of three tagmata—head, thorax, and abdomen—with appendages on the head and thorax but greatly reduced on or absent from the abdomen. The common ancestor of insects may have resembled a myriapod in general body form.

Uniramians have only one pair of antennae, and their appendages are always uniramous, never biramous like those of crustaceans. Although some insect young are aquatic and have gills, the gills are not homologous with those of crustaceans.

Insects and myriapods use tracheae to carry respiratory gases directly to and from all body cells in a manner similar to onychophorans and some arachnids.

Excretion is usually by malpighian tubules.