Bugs Rule!
An Introduction to the World of Insects
By WHITNEY CRANSHAW, RICHARD REDAKPRINCETON UNIVERSITY PRESS
Copyright © 2013 Princeton University Press
All rights reserved.
ISBN: 978-0-691-12495-7
Contents
PREFACE....................................................................ixACKNOWLEDGMENTS............................................................xi1 Introduction to the Arthropods...........................................12 What One Sees on the Outside—External Features of Insects................143 The Internal Organization................................................394 Growth and Metamorphosis.................................................585 The Arachnids—Spiders, Scorpions, Mites, and Other Eight-Legged Wonders..756 The "Other" Arthropods...................................................1197 Oldies but Goodies.......................................................1368 Insects Fly!.............................................................1449 Jumpers and Strollers—Grasshoppers, Crickets, and Walkingsticks..........15510 Variety Is the Spice of Life—Some Minor, but Interesting, Insect
Orders.....................................................................17111 Roach Cities and Assassin Cousins.......................................18212 Lousy Nitpickers........................................................20213 Life on an All-Fluid Diet...............................................21214 Insect Bruisers and Their Lacewinged Cousins............................25215 City Builders That Rule.................................................29616 Scale-Winged Beauties and Custom Homebuilders...........................33917 "Gift" Bearers of Plague—or a Plump Insect Wedding Present..............37718 Marvels of the Air—Two-Winged Wonders...................................387APPENDIX I: STATE INSECTS..................................................425APPENDIX II: LARGEST ARTHROPODS............................................431APPENDIX III: SUMMARY OF FEATURES OF THE HEXAPOD ORDERS....................435GLOSSARY...................................................................459INDEX......................................................................475
CHAPTER 1
Introduction to the Arthropods
What Is an Arthropod?
The subjects of this book are the arthropods that live among us, primarily theinsects but also some of their relatives, such as arachnids, millipedes, centipedes,and a few crustaceans. When formally classified, these animals are placed in thephylum Arthropoda, which comprises a huge number of species with a tremendousdiversity of forms and habits. Nonetheless, all arthropods share certain featuresthat together define them as a distinct form of life:
• All arthropods have a body supported by ahardened external skeleton (exoskeleton), a reversetype of engineering compared to our internalskeleton. To allow growth, this exoskeleton must beperiodically shed, and a new one rebuilt.
• The body of an arthropod is divided into segments,a feature shared by some other animal groups,such as earthworms (phylum Annelida) andvelvet worms (phylum Onychophora).
• The appendages of arthropods—their legs,antennae, and mouthparts—are jointed. This isthe feature that defines the phylum. (In Greek,arthropod means "jointed foot.")
• Internally, the nerve cord runs along the lower(ventral) part of the body and is not enclosed ina protective spinal column. These featurescontrast with those found in phylum Chordatato which we belong.
• Blood is moved by the aid of a tube-like heart,located along the back (dorsal) part of thebody.
• The overall body arrangement is bilaterallysymmetrical, so that, if the body were cutthrough the center from head to tail, the twohalves would be a mirror image of one another.
The Diversity andAbundanceof Arthropods
The arthropods are, by far, the most diverse life formon the planet. Insects alone, with approximately970,000 known species, comprise over one-half of allkinds of life known to occur on the planet. Yet despitethe impressive numbers, these reflect only "knownspecies," ones that have been suitably described in thescientific literature and accepted as distinct species.This number represents only a small fraction of thenumber of species estimated to be present on theplanet today. This number is also a tiny fraction of allthe insects that ever were on the planet. It has beensuggested that perhaps 95% of all insects that haveever existed, since their first appearance some 400million years ago (mya), are now extinct.
Today, the number of insect species thought tooccur is often estimated at about four to five millionspecies. The great majority of these, at least 80%,remain unknown to science so far. Progress is beingmade to close this gap, with over 7,000 new insectspecies being described annually, over 20 per day onaverage. At this rate of new discovery, impressive asit is, perhaps we can expect a full catalog of the fivemillion insects to be ready in about 550 years or so.
A much more difficult question to answer is"How abundant are insects and other arthropods interms of total population numbers?" One of theproblems is that the overwhelming number ofarthropods are minute and live in soil. For example,one of the first attempts at counting all of thearthropods in a sample of soil was done in an Englishpasture during November 1943. About 2.5 billionarthropods were estimated per hectare, with mitescomprising some 62% and springtails 23% of thetotal number. On the basis of surveys such as this ithas been estimated that the insects, springtails, mites,and other land-dwelling arthropods outnumberhumans by as much as 250 million to 1. Furthermore,these arthropods collectively comprise over 80% ofthe total biomass of the terrestrial animals, faroutweighing all the other land dwellers such asearthworms, reptiles, birds, and mammals.
The Many Rolesof Arthropods
If all mankind were to disappear, the worldwould regenerate back to the rich state ofequilibrium that existed ten thousand yearsago. If insects were to vanish, the environmentwould collapse into chaos. (E. O. Wilson,The Diversity of Life)
Although small in size, arthropods, in theirtremendous numbers, collectively account for themost biomass of all land animals. In the Amazon rainforest, the weight of just one family of insects, theants, is estimated to be four times more than all themammals, birds, fish, reptiles, and amphibianscombined. Furthermore, the roles of arthropods inecosystems are myriad, but central to the functioningof planet Earth:
Pollination of flowering plants. Insects are essentialto the pollination of most flowering plants, andmany of the flowering plants are the result ofcoevolution with their insect pollinators. Thetremendous variety of flower types reflectdifferent ways that plants have evolved to moreefficiently attract pollinators. In response, newspecies of insects have arisen to better exploitthese sources of nectar and pollen. In addition tonative plants, essentially all fruits, vegetables, andmany of the forage crops (e.g., clover, alfalfa) aredependent on insects to produce seed.
Recycling plant and animal matter. Many insectsdevelop by feeding on dead plant matter, deadanimal matter, or animal dung. In this role, theyfunction as macrodecomposers that are in thefirst-line "clean-up crew" essential to the recoveryand recycling of nutrients. Through insect feeding,these substances are broken down into muchsmaller particles and partially digested, whichgreatly accelerates the process of decay that freesthe nutrients to nourish later generations of plants.In the absence of insects, nutrient-recycling systemsbreak down and organic matter accumulates.
Soil formation and mixing. The great majority ofterrestrial arthropods live within the soil. Theseanimals help to turn the soil and incorporate organicmatter and nutrients. The impacts of these activitiescan be very dramatic, with some of the social insects(e.g., the ants and termites) moving and mixingtremendous amounts of soil as they tunnel. Theseprocesses are critical to soil formation and themaintenance of soil fertility. Without these insects,plant growth would be reduced and restricted.
Centrality to animal food chains. Through theirfeeding activities, plant-feeding insects (about25% of the species on the planet) convert plantbiomass to animal biomass. In turn, thesecreatures serve as the primary source of food forother insects (another 25% of the planet'sspecies) and for many birds, fish, and mammalsthat are, in turn, food for yet still more animals.Thus, plant-eating insects are the critical linkbetween plants and much of the rest of animallife on Earth (including humans).
Maintenance of plant communities. Although the effectsof large plant-feeding mammals are conspicuous, it isthe activities of insects that most often determinewhat plant life is present. Insects do this in manyways, including feeding on plants (phytophagy),feeding on seeds, pollination, and dispersing seeds.
Unfortunately, most people recognize only thosearthropods that are directly and immediately affectinghuman activities. These species are often considerednegatively, as competitors, because of their ability tocause several types of harm—destruction of crops,damage to stored products or structures, transmissionof plant and animal pathogens, and stings or bites—ormerely some degree of annoyance. Those that doaffect us in these ways are judged to be "pests," asubjective and very flexible term that is defined byhow much impact they are personally perceived tohave. It is important to keep in mind that only a tinyfraction of all arthropods are ever elevated to thisinfamous status. A listing of all insects worldwide thatare considered pests for one reason or another wouldinclude fewer than 10,000 species, approximately 1%of the total number of known insect species. A list ofspecies that are directly beneficial to humans may belarger by an order of magnitude.
All too often people try to separate the insectworld into "good bugs" and "bad bugs."Alternatively, one often hears the question "Whatgood is this insect/scorpion/spider?" These types ofcategorizations fail to recognize the tremendousimportance of the arthropods to the functioning ofthis planet, usually in ways we little understand. It isperhaps important to keep in mind the words ofpioneer conservationist/naturalist John Muir: "Whenwe try to pick out anything by itself, we find ithitched to everything else in the Universe."
Insects are neither good nor bad. They are, alongwith all other extant life forms, a representation ofthe latest expression of what has evolved on Earth.
Classification of theArthropods
In the classification of biological organisms, all lifeforms are grouped according to how related they are,usually based on physical features. Within thisorganization, all life forms are arranged in a series ofsubgroupings that become increasingly specialized.This science of classification is known as taxonomyand is conducted by specialists known as taxonomists.Closely associated with taxonomy, and often guidingthe classification arrangements, is the science ofsystematics that seeks to determine the relatedness ofdifferent life forms. Systematists make extensive useof the fossil records of extinct species along with allmanner of biological features of present (extant)species. Increasingly, genetics also guides systematics.The powerful new tools that allow sequencing ofgenes are producing a revolution in the systematicsof insects (and many other organisms) during whichwe are seeing many taxonomic arrangements beingmodified and many new species being recognized.
Using this system, all animals are classedtogether within the broadest type of grouping, akingdom—specifically the kingdom Animalia.The kingdom containing all animals is nextsubdivided into various phyla (singular, phylum),one of which is Arthropoda—the arthropods thatare the focus of this book. (Examples of someother animal phyla include Annelida, thesegmented worms; Nematoda, the round worms;Mollusca, the mollusks; and Chordata, the animalswith a hollow, ventral nerve cord, which includeshumans.) In turn, a phylum is subdivided intosections, each known as a class. Four classes ofarthropods (millipedes, centipedes, arachnids,hexapods/insects) are the primary focus of thisbook. Also discussed, in part, are a group ofarthropod classes collectively known as crustaceans(subphylum Crustacea).
The classes are subdivided into orders.For example, once you have identified something asan insect (from the class Hexapoda), the nextgrouping is the order of insects where it has beenplaced. Butterflies and moths, insects that havecharacteristic scale-covered wings, are placed bytaxonomists in the order Lepidoptera. Beetles thathave a hardened front pair of wings are in the orderColeoptera, while the flies, with their unique singlepair of wings, are classified in the order Diptera.Because of differences in how scientists classify theinsects, you may see some differences in the numberof and names for the orders among the many booksdescribing insect life and in their names. Theclassification system used for this book follows thatof the 7th edition of Borror and DeLong'sIntroduction to the Study of Insects (2005), whichlists in the class Hexapoda 28 orders of insects and3 orders of entognathous hexapods.
Orders are subdivided into families, scientificnames usually ending in "idae." For example, thebeetles (order Coleoptera) are divided into scoresof families, including lady beetles (Coccinellidae),weevils and bark beetles (Curculionidae), and leafbeetles (Chrysomelidae). Each family is dividedinto genera (singular genus), and each genus intovarious species.
Each species of insect, as well as all other lifeforms, has its own scientific name. This name isconstructed by combining the genus name and what isknown as the specific epithet. The genus name iscapitalized, the specific epithet is not, and both arewritten in italics. For example, the scientific name ofthe house fly is Musca domestica and that of the tomatohornworm is Manduca quinquemaculata. The idea ofgiving each species a scientific name that is universallyrecognized was formalized by Carolus Linnaeus(sometimes Anglicized as Carl Linnaeus), a Swedishphysician and biologist (1707–1778). The outline hedeveloped, published in the book Systema Naturae(1st edition 1735), was revolutionary and remains thefundamental framework whereby all living organismsare classified, based on shared features.
Since each scientific name has two parts, it isdescribed as binomial nomenclature. This has becomethe universally recognized standard for discussing theidentity of different organisms in a world that shares fewother common languages. In the formal naming of anorganism, the person who originally described it is alsoplaced at the end of the name. Therefore in the scientificliterature the house fly would be Musca domesticaLinnaeus and the tomato hornworm Manducaquinquemaculata (Haworth), recognizing that these twoinsects were originally described by Linnaeus andHaworth, respectively. In this book, the descriptor namesare left out for simplification, not to diminish in any waythe contributions of those who took it upon themselvesto first identify the insect as being a unique species.
Several mnemonic phrases have been developedto help reinforce learning of the basic taxonomicgroups—kingdom, phylum, class, order, family,genus, species—including the following:
King Philip cuts open five green snakes.Kings play cards on fat green stools.Kings play chess on Fridays, generally speaking.King Philip cried out—"for goodness sake!"
It must be recognized that whatever type ofclassification is used, it is a human construct and thussubject to change. Orders, families, and even classesof organisms may be rearranged following revisionsmade by taxonomists as new information becomesavailable through discoveries of new species, betterfossil records, and the use of modern moleculargenetic techniques.
As our understanding of how different organismsare related has increased, additional groupings have beenrequired. These are most often created by the prefix"sub" or "super." For example, a subclass is a divisionof a class but will still contain within it one or moreorders of the class. A superfamily will contain one or morefamilies within the same order. The taxonomicarrangement used for this book is presented in table 1-2.
Common Name orScientific Name?
Some insect orders and families, and many individualspecies of insects, have a common name. This is thefamiliar insect name in English, in contrast to themore formal scientific name. For example "beetles"is the common name for the order Coleoptera,"swallowtails" is the common name for the butterflieswithin the family Papilionidae, and "house fly" is thecommon name for the insect Musca domestica.
Scientific names are universal; they are the same inevery country. That is their utility and appeal—althoughnames of genera and even families are sometimesrearranged when new information (now usuallygenetics) leads to taxonomic revisions. However, mostpeople find it easier to learn and use the commonnames when discussing local insects. Unfortunately,such common names may be used for very differentinsects in different locations, thus leading to someconfusion. For example, an insect formally known asthe armyworm (Mythimna unipuncta) is a commonpest caterpillar of grain crops in much of the centralUnited States. However, when outbreaks of the foresttent caterpillar (Malacosoma disstria) occur in forestsof northern Minnesota and Wisconsin and thecaterpillars are seen marching across roads, this verydifferent insect is called an "armyworm" and elsewhereother caterpillars seen in bands are often referred as"armyworms." Similarly, an odd group of insects knownas Jerusalem crickets are known locally by a wide varietyof names such as "children of the earth," "old baldheadedman," and "potato bugs." (In turn, a great number ofother generally round-bodied arthropods are known as"potato bugs," including pillbugs and the Coloradopotato beetle.) Therefore the use of formally acceptedcommon names provides a means to discuss and writeabout insects in a manner that allows the identificationof the species to be consistently recognized.
The Entomological Society of America attempts tostandardize the common names of insects used in theUnited States in the publication Common Names ofInsects and Related Organisms. Common names ofarachnids are similarly codified by the AmericanArachnological Society. Around the world, similarpublications have been developed by various professionalorganizations committed to the study of arthropods.
(Continues...)Excerpted from Bugs Rule! by WHITNEY CRANSHAW, RICHARD REDAK. Copyright © 2013 Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS.
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