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, and the side (lateral) regions are the pleura (singular, pleuron ). Thus, a combination of these terms can isolate any region on the thorax, such as the pronotum, mesosternum, and so on. A triangular region on the mesonotum, the scutellum, is present on all adults, but conspicuous on true bugs (Order Hemiptera). One pair of legs is attached to each segment of the thorax near the bottom of the pleura. From the thorax outward, the segments of the leg are the coxa, trochanter, femur, tibia, tarsus, and pretarsus. In addition, adult insects may be wingless or they may have a pair of wings on the mesothorax alone or on both the mesothorax and metathorax. The abdomen The abdomen, which is softer and more flexible than the head or thorax, consists of eleven segments, although some may be reduced in size and not easily visible. The dorsal surface is called the tergum; the ventral side is the sternum. It is devoid of appendages except for terminal cerci of various sizes and shapes and genitalia, or reproductive structures. Females may bear an ovipositor for egg-laying, and male genitalia may or may not be extended. The abdomen is necessarily flexible because it houses the tracheal system -- the breathing apparatus of the insect -- and must expand and contract in order to take in and expel air through spiracles, which are openings on each side of the abdomen. There is generally one pair of spiracles per abdominal segment, and they lead to a branching network of air tubes, or tracheae, and air sacs throughout the body. From these, oxygen can flow to all organs and tissues, and waste gases can be passed out of the body. Normally, anterior spiracles inhale and posterior spiracles exhale. Circulatory and digestive systems Unlike that of vertebrates, the circulatory system of insects is completely independent of their respiratory system and is not involved in oxygen transport. The open system is therefore simple, with a tubelike heart that sucks blood in the posterior end and expels it toward the anterior end. The effect is rather like swirling water in a bathtub, and it makes a stark contrast to our own closed circulatory system in which the blood is always enclosed in vessels, no matter how small. Though inefficient by our standards, insect circulatory systems serve their purpose, since the blood transports only food and waste products. Despite a number of variations, most insect digestive systems are complete, meaning that a closed tube extends from the mouth all the way through the body to the anal opening, where waste is expelled. There are three main regions: the foregut, midgut, and hindgut, variously modified according to the food eaten by that species. The strength of insects relative to their small size is legendary; ants, for example, are known to be capable of carrying many times their own weight. These remarkable feats are made possible by the special arrangement of muscles, which are attached to the inside of their skeletons, affording tremendous leverage. Muscles are basically attached either within individual segments, enabling the insect to expand or contract, or to adjacent segments, allowing the entire body to flex or simply to curl by coordinating the muscles in a series of segments. Joints generally move only in one plane, so that a series of joints oriented in different planes are necessary to give the legs a full range of motion. MOUTHPARTS AND FEEDING An insect's mouth is composed of distinct parts, each serving a specific function. These mouthparts are a clue to the insect's feeding habits and therefore can tell us much about its life cycle and ecological relationships. Among insects, mouthparts are one means of identification, as they are diversely modified to ingest different types of food, but they all fall into one of several categories. Chewing mouthparts Chewing mouthparts are the most common type, and are also the mechanism that most closely resembles that of the human mouth. From front to back, chewing mouthparts consist of the labrum, analogous to an upper lip; a rather massive pair of toothed, jawlike mandibles, adapted for cutting, crushing, and grinding; a pair of maxillae, smaller but also jawlike for grasping, and the labium, or lower lip. Each of the maxilla is equipped with an antennalike appendage -- the maxillary palps -- which is used for touching and tasting potential food. The labium bears shorter sensory labial palps, and is used to guide food into the mouth cavity. Resting on the labium inside the mouth is a tonguelike hypopharynx. Major insect groups with chewing mouthparts include dragonflies, damsel-flies, grasshoppers, crickets, katydids, and beetles. Many bees combine chewing mouthparts with an elongated labium for lapping fluids, especially nectar. Sucking mouthparts The other major type of mouthparts are sucking mouthparts. Whereas insects with chewing mouthparts consume solid food for the most part, sucking insects ingest only liquid food, usually plant juices or body fluids. Sucking mouthparts have been modified into a proboscis, or beak, composed of an elongated tubelike labium that sheaths the slender, swordlike mandibles and maxillae, which do the actual piercing; these are called stylets, and they enclose the food and salivary channels. When you watch an insect, such as a mosquito, about to bite, you can see the labium bend back in the middle to expose the stylets. After the stylets pierce, saliva is injected through the salivary channel, which causes the subsequent irritation of a mosquito bite, then the food is sucked up through the food channel. The most common examples of sucking mouthparts are the piercing-sucking variety, which are found in the true bugs, leafhoppers, treehoppers, fleas, sucking lice, and some flies. Lacerating-sucking mouthparts, found on some flies, are similar, but instead of piercing, the stylets are modified to cut the skin minutely, and the fly sucks the blood that flows from the wound. Butterflies and moths do not pierce or cut; they have siphoning mouthparts, their proboscis being coiled like a watch spring under the head when not in use and extended to its full length to sip nectar from flowers. Sponging mouthparts Most flies have sponging mouthparts that do not quite fit into any of the above categories. A fleshy labium on the proboscis tip acts like a sponge, extending to soak up liquids and food particles. WINGS AND FLIGHT The advantages of flight undoubtedly played a large role in the success of the Class Insecta. Insects were the first creatures on earth capable of flight, which allowed them to more easily escape their enemies, to cover more territory in search of food, water, or mates, and to colonize new areas. They could cross large bodies of water, which were insurmountable barriers to most non-flying terrestrial animals. Except for flies, all flying insects have two pairs of wings, one of which is attached to the upper mesothorax and the other to the upper metathorax. It is likely that their wings originated as flaps that could be extended from the thorax, allowing wingless insects to escape danger by leaping from an elevated perch and gliding some distance away. Insect wings are unique, having evolved specifically for flight, while the wings of birds and bats are merely modifications of pre-existing limbs. The earliest insects known to be capable of true flight had two pairs of wings that remained extended and did not fold, even when the creature was at rest. Each pair flapped independently of the other pair, a contemporary parallel to this feature being found in the wings of dragonflies, which are members of a primitive but common order. Many advanced insects, such as the beetles, butterflies, and wasps, have evolved means to link their forewings and hind wings together to form two coordinated flight surfaces rather than four. Most insect wings are laced with distinct veins, the pattern of which is often critical to the identification of individual species. The spaces between the veins are cells; those extending to the wing margin are open cells, and those enclosed by veins on all sides are closed cells. Adult insects that emerge from a pupa have wings that at first look crumpled and useless. Extensions of the tracheal system run through the veins, and blood circulates in the spaces around the tracheae. As air is pumped through the veins, the wings unfurl and straighten. As they harden, veins provide both strength and a degree of flexibility, and the wings become capable of sustaining flight. Veins tend to be thicker and stronger near the body and along the forward, or "leading" edge, and thinner and more flexible near the tip and along the trailing edge. The trailing edge curls on both the upstroke and the downstroke, pushing against the air behind it and producing not only lift but forward propulsion and reduced drag. Propulsion Insect wings do not move simply by muscles pulling at the base, as one might guess. Instead, two different groups of indirect flight muscles, housed inside the thorax, work to alternately elongate and flatten the thorax in a vertical direction. The wings, wedged between the upper and lower thoracic sections, move by leverage on a pivotal point, or fulcrum. As the vertical muscles contract, the thorax flattens and the wings move up; then the horizontal muscles contract, pulling the sides in, driving the upper and lower thorax higher, and the wings move down. Smaller direct flight muscles at the base of each wing adjust the angle of the stroke and therefore the direction of flight. The frequency of wing beats varies from species to species, from one individual to another, and even in the same individual at different times. Generally speaking, insects such as butterflies, which have large, light bodies and large wings, need far fewer wing beats to stay aloft than do those with small wings and relatively heavy bodies, such as a housefly or a honeybee. Maximum air speed is also highly variable, but is generally less than 20 miles per hour. Insects, like most other animals, function more efficiently at warmer temperatures. As cold-blooded creatures, insects cannot rely on their body metabolism to generate heat and must use alternative methods to warm themselves. It is not unusual on a chilly day to see such insects as moths and bees rapidly vibrating their wings as they warm up their flight muscles for takeoff. Others will bask in a patch of warm sunlight; most notable among these are many butterflies that spread their wings and orient the surfaces toward the sun's rays, causing them to function as solar collectors. Wings have evolved to serve a number of other purposes besides flight. Male crickets and katydids, for example, have developed specialized structures on their forewings; when rubbed together on warm summer nights, these structures produce the pulsing songs by which the males seek to attract females. See pages 38-9 for more about wings. LIFE CYCLES The vast majority of insects lay eggs, and the development of the embryo progresses outside the mother's body. Most species undergo noticeable changes in form as they mature, a process known as metamorphosis. Nearly all insects display either hemimetabolous (incomplete) metamorphosis or holometabolous (complete) metamorphosis, although a few change so little, except in size, that they are said to have ametabolous metamorphosis, meaning that there is practically no change in form. Incomplete metamorphosis Hemimetabolous insects are usually distinguished by immature stages, called nymphs, that resemble adults, the main changes being an increase in size and the development of sexual organs and wings. Nymphs have mouthparts and compound eyes like their adult forms, and eat the same foods. Their wings begin as external pads on the thorax and develop with successive molts. The stage preceding each molt is known as an instar, and each succeeding instar more closely resembles the adult stage than did the previous one. Molting allows for growth, as the newer cuticle is more elastic than the old one. Among certain hemimetabolous insects, specifically dragonflies, damselflies, mayflies, and stoneflies, the immatures, known as naiads, are aquatic and do not resemble their terrestrial adult forms. Complete metamorphosis The life cycle of holometabolous insects consists of four distinct stages. From the egg hatches a larva, whose primary functions are to eat and grow. Wormlike in appearance, larvae do not resemble adults; in fact, they scarcely resemble insects. A larva usually possesses a series of simple eyes on its head, though these may be difficult to distinguish. It will also have either chewing or chewing-sucking mouthparts, a pair of very short antennae, and sometimes three pairs of true legs, although there may be other appendages that resemble legs, or they may have no legs at all. Wings, though developing, are hidden under the cuticle. Larvae molt several times to accommodate growth, and each stage preceding a molt is known as an instar. At the end of the larval stage, a final molt may occur with a pupa emerging, or the last larval "skin" may harden into puparium. Pupa do not eat and their movement is usually restricted to no more than a wiggle. In this stage, a great transformation is occurring. Some tissues differentiate, others break down and are reabsorbed and reorganized to form new structures. Through the hardened pupal case, the developing wings can often be seen, as can the compound eyes, antennae, mouthparts, and legs. Inside, reproductive organs develop and the digestive system undergoes modifications. The pupal stage can last from four days to several months, depending upon the species. At eclosion, the adult emerges, its wings crumpled and its body soft. Within hours, the wings unfurl, becoming stronger as the veins dry and stiffen, and the exoskeleton also dries, hardens, and gains pigment. In most species, adults have a few weeks to accomplish their primary mission of mating and egg-laying, but their tenure in this stage may last either less than two hours, in the case of certain mayfly species, to several years. Sexual reproduction Sexual reproduction among insects is the norm; the male of a species transfers sperm to a female, and the sperm are then stored in a special sac in her abdomen. Here the relationship ends, with each of the pair going their separate ways; in some cases, the male dies soon after mating. Egg-laying, or oviparous, females are equipped with abdominal appendages called ovipositors, which are variously modified to deposit eggs in a site suitable for their development, always close to an appropriate food source. As the eggs are laid, they meet sperm on the way out of the female. Fertilization occurs through a small opening, the micropyle, usually shortly after the eggs are deposited. Among some insects, the eggs remain inside the mother until they hatch. In that case, if the embryo feeds only on material stored inside the egg, it is ovoviviparous. In rare instances an embryo can be viviparous, being nourished by the mother's tissues prior to hatching. Once the eggs are laid, they are abandoned by the mother, who usually dies shortly afterward. As compensation for this and for the often vulnerable nature of the newly-hatched offspring, most species lay a large number of eggs. Asexual reproduction Among some insects there exists a type of asexual reproduction -- parthenog Excerpted from The Practical Entomologist by Rick Imes All rights reserved by the original copyright owners. Excerpts are provided for display purposes only and may not be reproduced, reprinted or distributed without the written permission of the publisher.
