Introduction to Insects
by Gary A. Dunn, M.S., F.R.E.S., Director of Education
AMAZING INSECTS
Insects are truly amazing. They are all around us. There is almost nowhere you can go on this planet to avoid them. There are insects that can endure the frigid conditions of the polar regions and the highest mountains; others inhabit steamy jungles and parched deserts. Some species inhabit freshwater ponds and streams, while others swim on the surface of the ocean or live in brine (extremely salty water). A few even live in hot springs where the water temperature reaches 140oF. There are even insects that live in caves and never see the light of day.
Insects take advantage of all types of habitats - large and small. They associate themselves with plants (as leaf feeders, borers, miners, gall makers and decomposers) and animals (as predators, parasites, blood suckers, scavengers and decomposers).
And, insects have been doing their thing for a very long time. Discoveries of insect fossils show us that insects have been around for at least 350 million years. For a period of 150 million years (up until 200 million years ago) they were alone in ruling the skies. That's right, the ability of insects to fly existed 150 million years before that of the reptiles, birds and mammals. In fact, other animals probably in great part took to the air so that they could eat the flying insects.
The spread of insect ancestors into so many of our planet's environments has resulted in an amazing diversity and abundance of insect species. There are well over 3 million species of insects, and more are being discovered every day. When compared to the numbers of other animals, insect far outnumber them. Insects make up 90% of the animal kingdom; to say it another way, 9 out of every 10 animals is some type of insect!
Look around you and you will have no problem finding members of the insect world: dragonflies, grasshoppers, cockroaches, bugs, beetles, moths, butterflies, ants, wasps, bees and flies. The fantastic world of insects is just waiting to be discovered by you.
INSECT CLASSIFICATION
All living organisms can be placed into groups for purposes of classification and identification. The creation of artificial categories (based upon similarities and differences) is made in an attempt to get a better understanding of the relationships among the various living organisms. The system of hierarchical classification, where each "higher" (larger) category encompasses all of the "lower" (smaller) categories, was first proposed by Carl Linne in the mid 18th Century. The system works quite well and remains in widespread use even today.
The basic hierarchical classification system includes seven principle categories (arranged in the following order from highest to lowest):
The species is the basic unit of the classification system. A species is a group of morphologically similar organisms which can and do reproduce with one another. Closely related species (those sharing one or more significant morphological features) are grouped together into the next highest category, the genus. Related genera make up a family; related families are grouped into an order; related orders are grouped into a class; and, related classes are grouped into a phylum.
All of the members of the Class INSECTA belong to the Phylum ARTHROPODA (commonly referred to as the arthropods). The arthropod species comprise about 95% of all known animal species. In fact, all but 5% of the arthropods are insects, so the insects outnumber all other animal species by 9 to 1! No wonder insect classification and identification is so challenging.
The Class INSECTA is distinguished from other arthropod classes (and most other animals) by the following characteristics:
WHEN IS A "BUG" REALLY A BUG?
You probably hear the term BUG all the time. Just what is a "bug"? To many people the term "bug " can be applied to any number of small, creepy crawlies - even the flu! It can be even used as a word of contempt: "You bug me!", or "You`re buggy (crazy)!". Those of the scientific mind are quick to tell people that "bugs are members of the insect order Hemiptera, characterized by piercing-sucking mouthparts, half leathery and half membranous forewings and gradual metamorphosis". We call them true bugs; that way they're not confused with other kinds of "bugs".
So, when you're talking to an entomologist and you hear the term waterBUG, ladyBUG, lightningBUG, billBUG or greenBUG, then he/she must be talking about true bugs, right? Wrong! The waterbug is actually a cockroach, the ladybug, lightningbug and billbug are actually beetles, and the greenbug is actually an aphid. Even the professionals seem to be all mixed up.
Names like lightningbug and ladybug are firmly rooted in our language, and despite their scientific incorrectness, are probably here to stay. Fear not, however, for there is a way to distinguish between true bugs and other "bugs" when communicating with other insect enthusiasts. If the word "bug" is directly attached to the end of an insect name (for example, ladybug), then it is NOT a true bug. When referring to true bugs the word "bug" stands alone (for example, giant water bug). Now isn't that simple.
In parting we can say that all bugs are insects, but not all insects are bugs!
GROWTH, DEVELOPMENT, AND LIFE CYCLES
Growth. All insects hatch from eggs. The eggs may be deposited when immature (showing very little embryonic development), or when very nearly mature (advanced embryonic development), or they may be retained within the body of the female until they hatch. Most female insects simply abandon their eggs at the time of oviposition, but a few insects will care for their eggs. Some eggs are covered with protective materials for insulation, water loss prevention or camouflage. Insect eggs, like insects themselves, are variable in size, shape and color. When the insect is ready to hatch from the egg it must force its way through the thick outer shell.
Molting. As internal tissue growth occurs (both an increase in the size of individual cells as well as the number of cells), a point is reached where the exoskeleton cannot stretch any further to accommodate additional growth. Since insects are encapsulated within a non-living, more-or-less rigid exoskeleton, there needs to be some provision for continued growth. In order for insects to grow past the point where the exoskeleton will stretch no further, the exoskeleton must be shed and replaced with a larger one. The process by which the exoskeleton is shed as called molting. The insect brain regulates the molting process by controlling the release of special hormones within the body. Most insects molt 4 or 5 times during their life, but some may molt as many as 40 times. The length between molts is determined by the quality and quantity of food the insects receives, as well as the environmental temperature (insects are cold blooded). Once an insect becomes an adult it lacks the molting hormones and no further growth is possible.
Metamorphosis. At the same time that insects are shedding their skeleton their bodies may change shape. This process is known as metamorphosis. Their are four more-or-less distinct types of metamorphosis and they are (in order of the degree of complexity): (1) simple metamorphosis (egg-young-adult), (2) gradual metamorphosis (egg-nymph-adult), (3) incomplete metamorphosis (egg-naiad-adult), and (4) complete metamorphosis (egg-larva-pupa-adult). Refer to the diagrams for additional details. Notice that they all have the egg and adult stage in common, but the active intermediate immature stage (young, nymph, naiad, and larva) is different. Complete metamorphosis occurs in about 80% of the insect orders, whereas gradual metamorphosis occurs in about 10% of the orders. Simple and incomplete metamorphosis are less common, each occurring in about 5% of the orders.
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