Practical Entomologist Imes, Rick. Cet article n’est pas disponible.Practical Entomologist Imes, Rick

Book by Imes Rick

Chapter 1

WHAT ARE INSECTS

Just what are insects, anyway? Often, any small creature with more than four legs is indiscriminately labeled a "bug," but true bugs represent only one of many different groups of insects. What's more, many of these creepy, crawling critters are not insects at all, but may belong to one of several related but very different groups.

Insects, as it turns out, are characterized by several easily recognized traits that set them apart from any other group of organisms. Like other members of the Phylum Arthropoda (which, literally translated, means "jointed foot"), and unlike mammals, for example, insects possess an external skeleton, or exoskeleton, which encases their internal organs, supporting them as our skeleton supports us and protecting them as would a suit of armor on a medieval knight. Unlike other arthropods, their body is divided into three distinct regions -- the head, thorax, and abdomen. Insects are the only animals that have three pairs of jointed legs, no more or less, and these six legs are attached to the thorax, the middle region of the body.

Most insects possess two pairs of wings, which are also attached to the thorax; the major exception to this rule are the flies, whose second pair of wings is reduced to tiny vestigial appendages that function as stabilizers in flight. Wings, when present, are a sure indicator that an arthropod belongs to the insect class. However, most ants and a number of more primitive insect groups are normally wingless, so the absence of wings does not by itself mean that the creature in question is not an insect.

Adaptability

It was proposed in the foreword that insects could be considered the dominant form of life on earth, Insects have discovered the basic premise that there is strength in numbers. Their life cycles are quite short, less than one year in most cases, and many have a much shorter span, either by design or through predation. They compensate for this by producing astronomical numbers of offspring: so many, in fact, that were it not for the world's insect-eating animals we would surely be overrun within a very short time.

Short lifespans and high reproductivity arm insects with their greatest advantage -- adaptability. It works like this: mutations, those genetic variations resulting in physical, biological, or behavioral changes, occur randomly in every population of organisms. When large numbers of offspring are produced, mutations are therefore relatively frequent, and some invariably enhance an individual's ability to compete for its needs or to adjust to changes in its surroundings. Beneficial mutations afford better odds of reaching sexual maturity and passing on the advantageous trait to future generations. Thus equipped, such "improved" individuals can rapidly replace large segments of their species' population that have been decimated by some disturbance in their surroundings.

CLOSE RELATIVES

There are several groups of animals that could possibly be confused with insects, and all of these are members of the group known as arthropods. Arthropods compose most of the known animal species, and about 800,000 of the 900,000 or so species of arthropods are insects. The others include crustaceans, spiders, centipedes, and millipedes.

Exoskeletons

All have exoskeletons containing varying amounts of chitin, a durable organic compound. It was once thought that the amount of chitin present determined the rigidity of the exoskeleton, but more recent research showed that its hardness is proportional to the protein content of the outer layer, or cuticle, and that more chitin is found in the soft inner cuticle. In addition to providing protection against injury, the exoskeleton is very water resistant, which inhibits water loss through evaporation. This major evolutionary adaptation allowed arthropods to colonize dry land while other invertebrates were restricted to aquatic habitats.

For all of its advantages, the exoskeleton of an arthropod is also a hindrance. Its weight limits the maximum size that any arthropod may attain, so none becomes very big and the largest are invariably aquatic, where buoyancy helps offset the greater burden. The non-elastic nature of the exoskelton's outer cuticle is an obstacle to growth, for in order to attain a larger size, hard-shelled arthropods must first shed, or molt, their outer layer, which splits open along a genetically-determined seam. Through this opening emerges the now soft-bodied animal, whose elastic inner cuticle can accommodate growth. Those arthropods that rely upon a very hard exoskeleton for defense are particularly vulnerable at this time and often hide until their growth period is over and their armor has again hardened. Most arthropods molt from four to seven times throughout their life.

Also common to all arthropods are bodies that are segmented to varying degrees, jointed appendages (some of which have differentiated to perform specialized functions), and relatively large and well-developed sensory organs and nervous systems, which enable the animals to respond rapidly to stimuli.

Crustaceans

Named for the Latin term crusta, meaning "hard shell," nearly all crustaceans are aquatic, and most live in marine environments, although a few of the most familiar, such as crayfish and water fleas, inhabit freshwater, while others, such as certain species of crab, are to be found in brackish water. Lobsters, fairy shrimp, and barnacles are well-known marine crustaceans; sowbugs, those small armored creatures one finds under rocks or in soil, are among the few terrestrial crustaceans.

The head and thorax of crustaceans are combined into one structure, the cephalothorax, which may be covered by a shieldlike carapace. Their number of paired appendages is variable, but they have at most only one pair per body segment. Only some of these are "legs," attached to the cephalothorax and used for walking. In some species, the first pair of legs are equipped with large pincers modified for grasping offensively or defensively. Other appendages are variously adapted for different functions, such as equilibrium, touch, and taste, chewing, food handling, mating, egg-carrying, swimming, and circulating water over the gills.

Some crustaceans are so unusual that their membership in the Class Crustacea can only be determined in their larval stages by zoologists. The barnacles that tend to encrust any marine surface and the water fleas commonly used in high school biology lab experiments are two such oddballs.

Horseshoe crabs

Though unlikely to be mistaken for any type of insect, these "living fossils" are nonetheless arthropods, and the two groups share some very basic features. Horseshoe crabs, named for the shape of their brown, domed carapace, are marine animals. There are two prominent compound eyes, located atop the carapace, as well as two inconspicuous simple eyes. They have a dorsal abdominal shield edged with short spines, and a bayonetlike tail that, despite its formidable appearance, functions mainly to turn the beast over after it has been flipped upside-down by the surf, lest it remain stranded out of water or succumb to ravenous gulls. Horseshoe crabs have six pairs of jointed appendages on the cephalothorax.

Spiders and their kin

Members of the Class Arachnida (from the Greek term for spider, arachne) include spiders, scorpions, ticks, mites, and others. It is this group more than any other that is usually confused with insects. Like crustaceans, the body of an arachnid is divided into a cephalothorax and an abdomen. Arachnids have four pairs of jointed legs, all attached to the cephalothorax, although some, like scorpions, possess a pair of large pedipalpi, appendages armed with formidable pincers that may resemble legs but are actually modified mouthparts. They also have one pair of chelicerae, mouthparts that, among spiders, each terminate with a fang, at the tip of which is a duct connected to poison glands. Unlike either insects or crustaceans, arachnids have no antennae.

Centipedes and millipedes

The name centipede means "one hundred feet," and centipedes are characterized by having one pair of legs per segment; while few centipedes have exactly one hundred legs, the number is a fair estimate. Their long, flattened, multi-segmented bodies comprise between 15 and 181 segments. The head bears a pair of long antennae, a pair of mandibles for chewing, and two pairs of maxillae for handling food. A pair of poison claws on the first segment behind the head enables a centipede to deliver a painful bite if handled carelessly. Most species live under stones or logs, emerging at night to prey upon earthworms and insects, which they kill with their venomous bite.

The prefix "milli-" means thousand, so does a millipede have one thousand feet? Not really, but one might think so to watch this wormlike creature walk. Each of the 9 to 100 or more abdominal segments sports two pairs of legs, this being the chief difference between millipedes and centipedes. The undulating movement of all these legs as the millipede slowly travels is nothing short of mesmerizing. They avoid light, and live for the most part beneath rocks and rotten logs, scavenging dead plant and animal matter. When threatened, they may roll into a tight ball or a spiral to protect their more vulnerable undersides.

TAXONOMY: ORDER FROM CHAOS

Taxonomy is the scientific discipline which puts order into an immensely diverse world and allows scientists to discuss any organism and know with certainty that they are talking about the same species. There are two important divisions of taxonomy. Classification is the arrangement of organisms into orderly groups. Nomenclature is the process of naming organisms.

Common names are generally used in everyday conversation, but they alone do not positively identify a particular species. Many plants and animals have more than one common name, and are often known by different names in different geographical areas, while the same common name may be assigned to two or more totally different species. Clearly, the potential for confusion is great, with well over 800,000 insect species identified and many more still undiscovered.

Contemporary scientists around me worm categorize organisms by means of a classification hierarchy, a system of groupings arranged in order from general to specific relationships. They are, in order of increasing specificity: kingdom, phylum (or division, in the plant kingdom), class, order, family, genus, and species. Each of these is a collective unit composed of one or more groups from the next, and more specific, category. Taking them in reverse order, a genus is a closely-related group of species; a family is an assembly of associated genera; an order is a set of similar families, related orders are combined to form a class, similar classes make up a phylum, and all related phylums constitute a kingdom. The complete classification of a honeybee, for instance, is Kingdom Animalia (animals), Phylum Arthropoda (joint-footed animals), Class Insecta (insects), Order Hymenoptera (bees, ants, and wasps), Family Apidae (bumblebees and honeybees), Apis mellifera.

All of the above categories are strictly human concepts, and as such they are subject to differences in interpretation throughout the scientific community, even with such a clear-cut system in place. Among taxonomists, there are the "splitters" and the "lumpers." Splitters are inclined to create many subdivisions among organisms, basing these upon more minute criteria, while lumpers tend to generalize and recognize fewer categories in the same group of organisms. Taxonomy is an active science, and there are occasional changes among accepted classifications that may confuse anyone who does not keep up with scientific literature. In such a case, a glance at the date of the publications containing the questionable terms will indicate which is likely to be the more recent interpretation.

The binomial system

While classification has always been a fairly simple affair, nomenclature has not. By the beginning of the 18th century, the use of Latin in schools and universities was widespread, and it had become customary to use descriptive Latin phrases to name plants and animals. Later, when books began to be printed in different languages, Latin was retained for the technical descriptions and names of organisms. Since all organisms were grouped into genera, the descriptive phrase began with the name of the genus to which the organism belonged. All mints known at that time, for example, belonged to the genus Mentha. The complete name for peppermint was Mentha floribus capitatus, foliis lanceolatis serratis subpetiolatis, or "Mint with flowers in a head; leaves lance-shaped, saw-toothed, and with very short petioles." The closely related spearmint was named Mentha floribus spicatis, foliis oblongis serratis, which meant "Mint with flowers in a spike; leaves oblong and saw-toothed." Though quite specific, this system was much too cumbersome to be used efficiently.

In 1753, Swedish naturalist Carolus Linnaeus introduced a two-word system of naming organisms. This system quickly replaced the older, clumsier method, and came to be known as the Binomial System of Nomenclature (binomial -- two names). According to this, individual species are identified by linking the generic name with another word, frequently an adjective. Occasionally, however, the splitters will create two or more subspecies out of what had been a single species, in which case the subspecies name is tacked on after the genus and species, creating a trinomial (three names). All scientific names are Latin, although some have descriptive Greek roots. The first name is always capitalized, but never the second, and both are always either underlined or italicized. When more than one member of the same genus is being discussed, the first name may be abbreviated, as in D. melanogaster for Drosophila melanogaster.

ANATOMY AND MORPHOLOGY

Morphology is the study of external form and structures, the criteria that result in insects being classified as insects and not as something else. Variations on these features define different orders, families, and genera of insects. Related to morphology is anatomy, the internal arrangement of organs and muscles. Learning the basics of both will help you to understand insect lives.

As we mentioned in the beginning of this chapter, the bodies of insects are sheathed in a tough exoskeleton, the hardness of which varies from one species to the next. Because they have no backbone, the support of the exoskeleton is absolutely essential to their mobility on land. The bodies of all insects are divided into three obvious regions -- the head, the thorax, and the abdomen.

An insect's head is composed of numerous plates, or sclerites, fused together to form a solid capsule that bears one to three simple eyes, two compound eyes, one pair of antennae, and mouthparts. It houses the brain, a fairly simple bundle of nerves from which the nerve cord extends and runs the length of the body along its ventral surface.

The thorax of an insect is divided into three distinct segments. From the head backward, they are the prothorax, mesothorax, and metathorax, each of which is rather box-shaped and composed of four hardened sclerites. The upper (dorsal) sclerites of the thorax are called the notum, the lower (ventral) surface is the sternum,