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.
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.
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.
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.
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.
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, 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 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.
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.
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.
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.
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.
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.
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 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.
Among some insects there exists a type of asexual reproduction -- parthenogenesis -- in which an unfertilized egg will develop into an adult. This is particularly common among social bees and wasps, where the division of labor is drawn strictly along sexual lines. Workers, who perform all of the tasks (apart from reproduction) necessary to keep the hive operating, are all sterile females that develop from fertilized eggs, while males, whose sole function is to fertilize the queen, develop from unfertilized eggs.
We humans tend to imagine that all creatures perceive the world in the same way we do, but such is not the case. Many animals see no colors, while others can see colors we cannot, and vice versa. Some can only detect degrees of light but no images, and still others are totally blind. There are sounds well above or below our range of hearing that are perfectly audible to other organisms, and our sense of smell is notoriously poor compared to that of the so-called "lower" animals. There is much more going on than we realize, and insects know this.
Take color, for instance. Did you know that ultra-violet is a color? We hear about it, and we know that it exists, yet we cannot see it; insects can, however, and they respond to it -- a fact not lost on those flowering plants that depend on insects for pollination. Red, on the other hand, a warm and attractive color to us, is invisible to insects.
Even the colors mutually visible both to insects and ourselves do not translate into similar images in our respective brains. The head of many mature insects has from one to three simple eyes, or ocelli, that serve only to detect various degrees of light. The two compound eyes are composed of anything from a few to several thousand individual units called ommatidia, each one evidenced by a separate lens, also known as a facet. Beneath each facet is a second, conical lens, and the two work together to focus light down a light-sensitive structure, the rhabdome; this is connected to the optic nerve, which leads to the brain. The quality of the images they generate is not known, but they are supremely adapted to detect motion. Since each unit of a compound eye is stimulated separately, every motion is multiplied many times, and the resulting effect must be rather like watching the same channel on hundreds of television sets at once.
Taste, smell, and touch
The world of insects consists more of patterns of smells and tastes than of light and sound. Taste, smell, touch, and sometimes hearing, are all functions of minute, hairlike bristles called setae, which may occur all over the body but are usually concentrated on the antennae, mouthparts, and legs. Among the chemosensory setae, those detecting airborne chemicals account for smell, while those that perceive chemicals through direct contact with solids or liquids allow an insect to taste. Tactile setae, which detect touch, are connected to the cuticle by a ball-s is connected to the optic nerve, which leads to the brain. The quality of the images they generate is not known, but they are supremely adapted to detect motion. Since each unit of a compound eye is stimulated separately, every motion is multiplied many times, and the resulting effect must be rather like watching the same channel on hundreds of television sets at once.
Taste, smell, and touch
The world of insects consists more of patterns of smells and tastes than of light and sound. Taste, smell, touch, and sometimes hearing, are all functions of minute, hairlike bristles called setae, which may occur all over the body but are usually concentrated on the antennae, mouthparts, and legs. Among the chemosensory setae, those detecting airborne chemicals account for smell, while those that perceive chemicals through direct contact with solids or liquids allow an insect to taste. Tactile setae, which detect touch, are connected to the cuticle by a ball-and-socket joint, the slightest movement of which stimulates nerve endings on the underside of the joint.
Although insects have no noses, their olfactory sense is keener than anything we can imagine. They smell primarily with their antennae, paired appendages located between the compound eyes and above the mouthparts. Segmented, flexible, and densely covered with microscopic setae, antennae have many configurations throughout the insect world. Among their numerous functions, they serve to detect pheromones (chemical messages) emitted to communicate with other members of the species, and to locate food, water, and suitable sites for laying eggs. Depending upon the insect, antennae may also be used to perceive touch, taste, and/or sounds. Insects also taste through the chemosensory setae on their mouthparts and legs.
Sensitivity to sound varies widely among insects, as do the anatomy and location of their auditory organs. Phonoreceptors, as they are called, may simply be modified setae on the body, appendages, or antennae that can detect air vibrations, or they may be more complex structures called tympanic organs, located on the abdomen, thorax, or the forelegs. A tympanic organ consists of an exposed tympanic membrane, covering an underlying air sac. Auditory nerves are connected either to the air sac or directly to the membrane, which vibrates in response to sound waves, stimulating the nerves in a fashion very similar to the human ear. Some insects can differentiate pitch, or various frequencies of sound, while others are tone deaf. As a group, they can detect sound over a much broader range of frequencies than can people.
Behavior is the response of an animal to stimuli from its surroundings. A stimulus is detected through one or more senses and interpreted by the brain, which then instructs the body to react, either on the basis of past experience (learned behavior) or in a genetically pre-determined course of action (instinctive behavior).
Understanding an animal's behavior begins with understanding the motive of its genes, those molecular blueprints of all life forms. Genes are very selfish. They care nothing for the rest of their species, only that their host remains healthy and strong and survives to produce as many offspring as possible, thereby passing on the maximum number of its genes to future generations and ensuring the genes' survival. Even behavior seemingly unrelated to reproduction is ultimately oriented toward keeping the animal fit and helping it survive to sexual maturity.
Insect behavior is largely instinctive. Moreover, the behavior pattern of an insect is practically identical to that of every other member of its species, varying only through random jumblings of a portion of the genetic code, better known as mutations. Mutations occur regularly in all populations, resulting in variations in physical appearance or behavior. Some are beneficial, some detrimental, but most have little effect on an individual's fitness. They are the mechanism of evolution through natural selection.
The tremendous success of insects is due to both their stereotyped behavior patterns and relatively frequent mutations. Fixed behavior patterns, guiding insects to react in the ways that best promote their survival and reproductive success, enable them to flourish in their preferred niche, that portion of an ecosystem that fulfills their needs. However, the world changes, and the inability to change with it is the chief cause of extinction. Because of the fairly short life cycles and great reproductivity of insects, it is likely that when change occurs there will be mutated individuals floating around within a species that are able to cope better than others, meaning that more of their "altered" genes are passed on. With a high rate of reproduction, and decreased competition from those that could not cope, it is not long before the beneficial mutant gene displaces its predecessor and the species has evolved to meet the new challenge.
It would be impossible to list all of the specific behavior patterns unique to individual insect species. As a practical entomologist, you are best advised to learn the basic behavior patterns of the animal kingdom and to try to recognize these among the insects you observe. Significant behaviors will be discussed later in individual chapters on insect orders. The following are some common categories of behavior.
Courtship is ritualized behavior, employed to attract a mate, and is usually actively engaged in by males. The female response, also highly stereotyped, indicates acceptance or rejection.
Copulation, the physical joining of members of the opposite sex, culminates with the transfer of sperm from the male to the female.
Egg-laying is an important clue to the life cycle of an insect, because individual species are quite stringent in their site requirements. Eggs are nearly always laid either on or with easy access to a food source suitable for the larvae or nymphs.
Feeding may encompass predation, grazing, or scavenging. Social insects actively gather food and return with it to their colony to store it or to share with others.
Defense may be either active, as in insects that sting, bite, or emit noxious chemicals, or it may be passive, employed by those that hide in crevices and under stones, or it can take the form of camouflage in the case of species that "hide" out in the open on a matching background.
Communication, the deliberate transfer of information, occurs essentially between members of the same species. Insects may communicate via all five senses known to us, and we must not discount the possibility of their communicating through unknown methods as well.
Grooming is just as important to insects as to the furry animals to which it is normally attributed. Insects' antennae, eyes, wings, legs, mouthparts, and the hairlike sensory setae covering their bodies must all be kept clean in order to function efficiently.
By and large, most insects are solitary, indifferent to the company of others of their species except during mating and chance encounters. There are quite a few exceptions, however, and these have been lumped into one group called social insects. Termites, ants, most bees, and some wasps are true social insects. Many others will congregate for one reason or another at some point in their lives, but they do not stay together in organized societies throughout their life cycle.
Social insects live in cooperative, interdependent colonies of a single species, usually with a sharp division of labor among social castes. Such insects regularly care for the eggs, larvae, and pupae in the colony, a characteristic rare in the rest of the Class Insecta. They build nests of various degrees of complexity, and usually they have all descended from one female, the queen. They also practice trophallaxis, the mutual exchange of food and other desirable substances between members of the colony.
The evolution of social life among insects can be divided into several stages. Most insects do not live long enough to see their offspring, making parental care difficult. Probably the first significant step toward a social life occurred when solitary wasps and bees began to build nests, which they provisioned with food and in which they laid eggs. This practice, which still occurs among many solitary wasps and bees, represents a basic parent-offspring relationship, even though the two generations never see each other.
As their lifespans grew longer and began to overlap those of their offspring, the next step, also evident among some modern species, became possible. The mother lays her eggs in a nest with few or no provisions, but returns with prey periodically and feeds the larvae directly. Trophallaxis probably co-evolved as an inducement to this behavior, since the larvae of many social insects, when fed, are stimulated to secrete substances relished by the adults, providing the latter with a powerful incentive to provide food.
The beginning of a social structure is illustrated by certain solitary bees, which we call sociable because of their tendency to build their single- or multi-celled nests adjoining those of others of the same species. From this point, it is an easy transition to a division of labor. Individual bees may adopt specialized tasks, such as guarding the entrance to the nest. While such action obviously benefits the insect's own offspring, those of the other bees also profit.
Caring for the offspring of another seems to violate the theory of the selfish gene discussed under "Insect Behavior," which asserts that the ultimate goal of (genetically programmed) behavior must be the survival of one's own genes. However, since all members of the colony descended from one queen, her offspring are basically siblings to all others in that society. Caring for them ensures the survival of any individual's genes, even if that individual did not contribute those genes directly. Of course, the bees themselves are not conscious of all this. They simply respond as their genes direct them.
There are really only two major castes among social insects: the reproductive caste, composed of males and queens, which has the sole responsibility of making more offspring, and the non-reproductive caste, consisting of only workers or workers and soldiers, which undertakes all of the work in maintaining the colony, including building and repairing the nest, collecting food, and caring for the eggs, larvae, and pupae.
Social bees and wasps have evolved an effective means of controlling the numbers in each caste to ensure that the colony operates efficiently. Only a limited number of males are needed to promote competition for the privilege of mating with the single queen. Extra queens leave with some males to start their own colonies. The queen has control over whether a given egg gets fertilized; unfertilized eggs develop into males, and fertilized ones become females. The queen, in turn, is stimulated to either fertilize eggs from sperm stored in her abdomen or withhold sperm based on the sizes of the cells built by workers. Into smaller cells she deposits fertilized eggs, while larger cells receive unfertilized eggs. Nutrition determines if a female grows into a worker, all of which are sterile females, or a queen. Among wasps, underfed or improperly fed females become workers, while well-nourished females grow into queens. Honeybee workers secrete a white paste, called royal jelly, from glands connected to their mouthparts and feed this to all larvae for at least their first three days. Those females that receive royal jelly throughout their larval stage develop into queens, and the rest become workers.
OFFENSE AND DEFENSE
The variety of the insect world has given rise to a great number of protection mechanisms. First and foremost is the insect's durable external skeleton, a veritable suit of armor for many species. Not only does it offer protection against physical forces, but it also prevents undesirable water loss and is resistant to a great many chemicals. In addition, the physics of having muscles inside the skeleton gives insects tremendous strength in relation to their small size.
Size itself is a great defensive asset of insects. Being small enables them to go about their lives unnoticed by many larger animals that would eat them. In fact, their diminutive stature is a major reason why entomologists estimate that there may actually be twice as many insect species or more on earth as those already classified -- we may have simply overlooked the rest so far. Many just take refuge in a crack or under leaves or stones until danger has passed, and quite a few spend their entire lives out of sight of most vertebrates.
Of those that avoid danger by hiding, a great many rely upon some form of camouflage to enable them to hide out in the open. Some incorporate colors and patterns on their exoskeleton or wings to match the background material upon which they normally rest. Others have evolved fantastically modified body shapes and appendages that perfectly mimic plant parts such as twigs or leaves, right down to the swaying motion of a leaf in the breeze. Notable among these are the stick insects (Order Phasmatodea), leaflike katydids (Order Orthoptera), and treehoppers and leafhoppers (Order Homoptera).
Many of those insects that cannot hide so easily have another option -- flight. As a group, winged insects have complete mastery of the air and can execute maneuvers beyond the capability of any other creature or machine. Of course, some are better fliers than others, but even the worst are better off than they would be if they could not fly at all.
Self-advertisers and mimics
In contrast to those which remain inconspicuous, a great number of insects boldly advertise their presence with bright colors or contrasting patterns. Most of those so marked have an unpleasant experience to offer potential predators, such as a venomous sting or spines, irritating hairs, repellent glands, or toxic or distasteful compounds in their tissues. Once so educated, an animal is unlikely to make the same mistake twice, and usually gives that species a wide berth. One might think that traits inviting attack would be quickly eliminated from the population, but apparently many more such insects are spared from repeat attacks than are lost to ignorant predators.
Riding on the coattails of these conspicuous insects are mimics, whose chance resemblance to a species habitually avoided by predators has encouraged their evolution to the point where it requires very close scrutiny to tell the harmless species from the dangerous one. Classic examples of this are the monarch and viceroy butterflies of North America. As a larva, the monarch, whose hazardous migration of thousands of miles each autumn has endeared it to millions of Americans, feeds exclusively on milkweed plants. Milkweeds produce a milky latex containing cardiac glycosides, toxic substances that cause nausea and vomiting in small doses and death to vertebrates in large doses. These toxins deter nearly all animals from grazing upon milkweeds, but monarchs have adapted to not only feed on them but to incorporate the cardiac glycosides throughout their tissues, making them extremely distasteful to predators, especially to birds that might otherwise pick them out of the air. Orange-and-black viceroy butterflies are a product of the same natural selection processes that govern monarchs; those that, through mutations, more closely resemble monarchs will more successfully pass on those traits to future generations.
Defense by offense
Social insects have adopted the axiom that the best defense is a good offense. While many insects are capable of delivering a painful bite or a venomous sting, the true power of this defense is unleashed when it is multiplied by hundreds or thousands of members in a colony. Social insects communicate extensively via pheromones, chemical messengers that elicit immediate instinctive responses. Alarm pheromones are potent and travel rapidly through a colony, rallying its occupants to repel any intruders.
ENEMIES AND ALLIES
Insects have not had a fair deal. They are not popular subjects of nature study, in part because we tend to associate them with nuisance and with the transmission of diseases. As the most numerous and diverse class of organisms on earth, they are inextricably woven into the web of life, and so there are inevitably some adversarial relationships with humans. However, we also have a good many insect allies.
For one thing, insects are responsible for perpetuating many of the green plants that feed our planet and oxygenate its atmosphere. With the evolution of flying insects, plants were quick to take advantage of these reliable pollinators. Insect-pollinated plants have incorporated colors, fragrances, and nectar attractive to various insect species into their flowers, and many have even adapted to open their flowers only during the times of day when their primary pollinators are most active. Bees, butterflies, moths, and flies are the major pollinators. Hives of honeybees, perhaps the world's premier insect pollinators, are therefore frequently kept in orchards or nurseries, and on farms. As a side benefit to their pollinating activities, the bees supply us with large quantities of honey and beeswax, which is used in lubricants, salves, ointments, furniture polish, candles, and other products.
Other insects also benefit plants and, indirectly, us. The activity of burrowing insects, such as ants, loosens, aerates, and fertilizes the soil, encouraging the development of healthy plant roots. Some species feed upon noxious weeds, helping to control them, and predatory insects will readily take any insect within their power, including those damaging to plants. The value of insect predators, such as ladybird beetles, mantids, and lacewings, is legendary.
Insects have supplied us with a good many other products besides honey and beeswax. Most of the world's silk, for instance, is produced from the cocoons of the silkworm moth, Bombyx mori. Sometimes the insects themselves become the product; among certain cultures, insects, such as grasshoppers, flying ants and honey ants, are popular fare.
Despite the fact that most insects have no direct interaction with humans at all, there are some that are unquestionably harmful. Mosquitoes can transmit such diseases as malaria, yellow fever, encephalitis, and elephantiasis. Fleas are the vectors of bubonic plague, typhus, and tapeworms. Houseflies have been implicated in the spread of tuberculosis, typhoid, cholera, amoebic dysentery, anthrax, and other illnesses. Human lice, though they are basically parasites that usually cause only discomfort, also may transmit typhus and relapsing fever. One of the best known disease vectors is the tsetse fly, which transmits African sleeping sickness.
Ever since the dawn of agriculture, people have done battle with insects over the crops that both desire; so far, the conflict seems to be a draw. The use of chemical pesticides during the 20th century has given humans the edge, but their use must be carefully regulated to avoid worldwide environmental disaster.
Insects may destroy crops in one of two ways. Most obviously, their consumption of the plant means there is less for us. Grasshoppers, true bugs, aphids, and beetles are the most serious offenders in this category. Infestation of our stored grains and cereals is also a serious problem, but most of this damage is caused by only a few species of beetle. Perhaps more serious are the many plant diseases introduced through the wounds where insects have been feeding, boring, or laying eggs.
Adding insult to injury, certain insects inflict considerable damage on our possessions as well. Termites are a worldwide threat to any wooden structure. Carpenter ants and wood-boring beetles pose a similar but much lesser problem. Silverfish (Thysanura) and book lice (Psocoptera) can destroy books and papers as they feed on paste, glue, and the sizing used to create glossy pages. Clothes moths were once significant destructive pests of natural fabrics, but their significance diminished with the increased use of synthetic fibers. Carpet beetles (Family Dermestidae) are a much more serious problem today, as they attack both natural and synthetic fibers.
Despite the serious nature of insect pests, it behooves the practical entomologist to dispe