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Dr. E. F. Legner, University of California, Riverside





Insects have lived on this planet for more than 350 million years.  During this time they have adapted to almost every type of habitat.  They are in the soil beneath your feet, the air above your head, even in such strange places as the nasal membranes of livestock, beneath the skin of humans, and in dung.  You will find insects in the frozen extremes of the Arctic and in the hot, dry climates of deserts.


Most orders of insects are represented on every continent.  However, because of a continent=s isolation from other land masses, each continent tends to have its own unique insect fauna.  North America has a whole host of groups and species particular to itself, such as the periodical cicadas and dobsonflies.  Within a single continent, deserts, mountain ranges, and climate further limit insect distribution.  While some insect species are widely distributed, most have restricted dispersion.  Most of the so-called common species are common only in certain areas.  Detailed studies often startle the investigators such as the recent finding that about 1/3rd of the entire animal biomass of the Amazon rain forest is composed of ants.


In coastal southern California and northern Baja California, Mexico, are the chaparral and thorn scrub regions.  This is a unique, dry area that supports flora characterized by dense, spiny shrubs, grasses and a variety of oaks, and many insect species can only be found in this region.

The lowland tropical rain forests of southern Florida and parts of Mexico are characterized by high rainfall, a vast number of large-leaved plants, and tall trees.  These regions boast by far the greatest number of insect species with their bright colors and weird shapes.


Did you know that some insects live in caves?  Many cave-dwelling insects appear to be remnants of the Ice Age.  Initially moving to caves to escape the harsh conditions outside, they now have become adapted to the conditions below ground and are unable to exist outside.


          Approximately 5% of insects spend all or part of their life cycles in water.  The majority of these insects are found in freshwater, but a few species live in brackish water and intertidal zones.  Species of four orders of insects all have aquatic stages.  They are the mayflies, dragonflies and damselflies, stoneflies and caddisflies.  In adapting to the aquatic environment, these insects underwent anatomical changes such as developing abdominal gills, terminal breathing tubes, and in some, internal gills in order to use the oxygen dissolved in the water.  The bodies of aquatic insects are usually streamlined to cut down resistance, and their legs are often modified to form swimming paddles.  One of these groups, the back swimmers, has 34 species in North America.


The oceans contain few insect species.  There is one oceanic water strider that lives on the surface of the water hundreds of miles from shore.  These insects reportedly lay their eggs on floating sea bird feathers and other debris and may never set foot on land.  But why insects don=t generally choose to live in the ocean is not fully understood.  It is doubtful that living in salt water is the difficulty.  After all, some insects live in equally harsh environments, such as pools of crude oil and the brine of the Great Salt Lake.  The answer may line in insect evolution.  It is thought that before the evolution of insects, the ocean was very heavily inhabited by precursors to the insects-- other arthropods such as crabs, shrimp and lobsters.  Insects evolved from these earlier arthropods and hence, they found a new niche on land, became a success, and have never had a Aneed@ to return to the ocean.  If prehistoric insects were ever oceanic, they clearly didn=t remain so


Soil is an environment that supports a large number of insects.  It provides insects with protection and food.  Many insects spend part or all of their lives within soil.  Soil is composed of minerals and decomposing organic matter in various proportions.  Some soils make better habitats than others.


Insects, Weather, and Plants


Many insects are closely associated with plants, either directly as plant feeders or indirectly as predators or parasites of plant pests.  The distribution of insects is determined by the distribution of plants, and thereby is associated with weather conditions as well.


The far north regions of North America are referred to as the tundra, characterized by quick-growing plants and insects with short life cycles.  Tundra insects include a few hardy butterflies belonging to the genera Parnassius (apollos) and Erebia (arctics and alpines) and primitive, wingless groups such as springtails and parasites of mammals and birds.


South of the tundra in Canada and extending into the Pacific Northwest and the Rocky Mountains are expansive stands of closely growing coniferous trees.  The number of insect species is limited in this region.  However, populations of the few species represented are usually abundant.  Great swarms of mosquitoes and black flies emerge during the brief spring and summer.


Mixed deciduous forest dominates much of the eastern United States.  The insects of this region are well known because they have been intensely studied.  The great variety of plant life makes possible an equally varied insect fauna.  Almost all known families of insects are represented here.

The central portion of North America is grassland covered with natural stands of grasses or cultivated grain crops.  The western portion does not receive enough rainfall to support cultivated grains and is generally used as grazing land.  Grasshoppers, true bugs, and many kinds of moths are abundant in the great plains.  Although the insect species are limited, those present are abundant.  A few of the grasshoppers are major pests of grasslands and grass crops, feeding on all parts of the plant above the ground.


As you continue west through the central plains, rainfall decreases until sparse vegetation surrenders into desert.  However, the variety of insect life does not dwindle.  Rather, the number of species found in the desert increases.  Desert habitats support a specialized and interesting insect fauna that disappears during the long dry periods but emerges in a variety of colorful and strange species as soon as the brief rainy season begins.  Darkling beetles in particular can be found throughout arid regions of the U.S.  These beetles take over the ecological niche that is occupied by ground beetles in the more lush areas, as both families of beetles are nocturnal and are commonly found under stones, logs, leaves, bark or debris.