File: <entomology -- an introduction

The present arrangement of insect orders is based on ever increasing complexity and presumed evolution of the most primitive [Thysanura] to the more advanced [Hymenoptera]. Previous and synonymous names of groups are included in parentheses. The Entognatha (Collembola) are included herein as a primitive insect precursor group.

Emphasis has been placed on morphological and behavioral characteristics that are easily discernable, and a simple diagrammatic style suitable for lecturing is used for most of the illustrations. Insect families that are generally more abundant or which have the greatest economic or public health importance are treated in greatest detail. A hand lens and a binocular microscope with 20X magnification are advisable for those wishing to view living and preserved specimens. Greater detail on a particular group or species may be found by referring to publications listed in the References or through Internet searches.

This is a self-contained database with a minimum of links outside its limits. Independent Internet searches are encouraged for greater detail on a particular insect group. All information contained herein is for general public use according to the rules set by the Creative Common Deed.

General Characteristics of Insects

The term "Entomology" is derived from the Greek "Entoma" meaning "cutting-into." Entomology then refers to the fact that insects have a resting stage and there is segmentation of the body. Therefore, a complete translation might be, "Study of the individual divided into sections that forms in cysts." Entomological instruction also has traditionally included the study and control of phytophagous mites, which cause extensive damage to food plants (See Arachnida).

With regard to the position of Insecta in the Animal Kingdom, it is estimated that there are at least 1,400,000 animal species in the Kingdom. This may be broken down into Chordata = ca. 41,000; Mollusca = ca. 81,000; Protozoa = ca. 32,000 and Arthropoda other than Insecta = ca. 73,500. The Arthropoda including Insecta have over 900,500 species. In North America there have been over 100,000 described insect species. Most specialists will agree that many more exist that have not been described or discovered. Taxonomic separation is so difficult for some groups that only laboratory tests of compatibility can determine with certainty if there is reproductive isolation. The Muscidifurax genus of parasitic Hymenoptera is a prime example. A graphic representation of the known relative abundance as of 2010 may be viewed in <ent1>.

History of Entomology. -- The Bible mentions locusts and flies, and there are references to insects by the early Egyptians and in Chinese writings. Aristotle in 350 BC. made an attempt to classify insects.

In 1758 Linnaeus began the system of binomial nomenclature and described seven orders of insects. Between 1795 and 1819 Peck, a professor of Biology at Harvard University, stimulated the development of Economic Entomology. In 1841 Harris, a student of Peck's, produced the first book on Economic Entomology. This was the first case of Tax Funds expended for such a work.

The year 1854 marks the birth of Entomology as a profession. Townend Glover, of the U. S. Department of Agriculture and Asa Fitch an employee of the State of New York were hired as entomologists.

From 1867 to 1890 there was a rapid growth of Entomology in the United States. In 1807 Michigan State College offered the first course in Entomology and in 1888 Michigan established the first Agricultural Experiment Station. By 1950 there were over 4,600 professional entomologists in the United States alone.

In the latter 1800's C. V. Riley, Chief Entomologist of the U. S. Dept. of Agriculture obtained the Rodolia cardinalis predator from Australia to combat the invaded cottony-cushion scale in California. Then L. O. Howard organized the Bureau of Entomology in the U. S. Dept. of Agriculture. J. H. Comstock, professor at Cornell University, followed these by writing textbooks on Entomology.

The year 1942 marked the beginning of synthetic insecticide development which has persisted into the 21st Century.

Fossil Records. -- The first records of insect-like invertebrates are found in the rocks of the Devonion period. These take the form of fragmentary remains of invertebrates similar to the Entognatha, and there is little doubt that wingless insects of this kind were abundant in that period when there was a great abundance of vegetation of pterydophytes and other plants under very wet conditions. It is probable that thysanuran insects similar our contemporary silver fish, Lepisma, lived then.

During the Carboniferous there were rapidly increasing numbers of winged insects, and in this period and the Permian that followed there existed insects, which have been classified into orders that were precursors of what exist today. Some of these orders became extinct early, and therefore the Palaeodictyoptera, huge insects with wings up to 50 cm., appeared in the Lower Carboniferous only to disappear in the lower Permian. They certainly were able to fly as their wings had those longitudinal veins that indicated use. The preservation of immobile flap-like extensions on the sides of the non-wing-bearing segments characterized some of these insects, a trait that points to the origin of wings.

Following them there appeared a number of orders that possessed primitive mouthparts and wing venation. Such orders as Protoephemeroptera, Protodonata, Protohemiptera and Protohymenoptera are known to have existed. These names do not necessarily suggest a direct ancestry to the modem orders whose names they resemble. However, they do show that iri the Permian there occurred a stable use of types bestowing considerable variation on the subphylum. The fact that they were all had mandibles and, seemed to be exopterygote in wing growth supports the belief that from these general characters were evolved later the more specialized features of many of our insects of today. Probably the strongest support of the view that insects with mandibulate mouthparts, external wing growth and with wings with mostly parallel longitudinal veins with a rich network of cross veins, are the primitive ones, rests iri the persistence, with little change, of cockroaches, from the Carboniferous till today.

Insects with internal wing growth and complex metamorphosis appeared later. Therefore, the Trichoptera, Lepidoptera, Neuroptera, Mecoptera, Diptera and Hymenoptera appeared in the Mesozoic and became abundant in the Tertiary period. The Coleoptera are much older than these as they existed back to the Upper Permian. In the Tertiary there had already occurred among them habits and form leading to water beetles, weevils and the leaf-eating chrysomelids. This earlier establishment of the Coleoptera is of interest since the present day Coleoptera have particularly in their mouthparts, a number of characteristics that place them in the generalized category.

In conclusion it seems obvious that the complex orders of Lepidoptera, Diptera, Hymenoptera, etc., may have existed in the Permian, an age, together with the Carboniferous, where only generalized insects occurred that were not yet equipped with feeding mechanisms for utilizing flowering plants. Nevertheless, it is significant that the main evolution of specialized bees, flies and butterflies happened together with the evolution of the flowering plants to which by their manner of feeding they are now wholly dependent.

Insect Numbers. -- Periodic outbreaks of insects occur. Some familiar ones are grasshopper swarms, migrating butterflies, wireworm invasions, mayfly swarms, forest tent caterpillar invasions, lake fly swarms and Mormon cricket invasions. The abundance of very tiny parasitic species, for example, is almost impossible to determine.

The honeybee is an example of extraordinary numbers. In summer a healthy honeybee colony contains about 55,000 individuals and produces about 200,000 individuals per year.

The persistence of insects in the environments may be realized when considering that no single species has ever been deliberately eradicated.

Insect Distribution. -- Insects are found almost everywhere except in the sea where they may occur in rare exceptions. Insects also parasitize most other animals (e.g., even sea lions have lice).

Useful Attributes of Insects. -- Insects do twice as much good as harm. They pollinate plants, serve as natural predators and parasites of noxious insects, are scavengers and produce useful products such as honey, wax, silk and shellac. Their great fecundity and rapid development continue to make them valuable for studies in animal behavior and genetics.

Insects also are valuable scavengers and soil insects improve soil texture. Many insects destroy weeds. They are important food sources for fish, birds and mammals and in some parts of the world for humans.

The aesthetic value of insects for collections, designs and in zoological parks cannot be ignored.

Harmful Aspects of Insects. -- There are more than 11,000 pest species of insects in the world. They can cause over 12 percent loss to agronomic crops and they are capable of infesting almost every crop grown. Their high reproductive potential makes them especially important as pests. Such groups as mosquitoes, tsetse flies, wasps, etc., not only cause great annoyance but some are able to transmit serious pathogens to humans and other animals or they may kill directly. Some species have figured prominently as carriers of typhoid, dysentary and malaria.

Economic Significance. -- The Petrochemical Industry has figured actively in the control of pest insects. It costs about 1.8 million U.S. dollars to marked an insecticide. Entomology is also a fascinating science due to the diversity of forms, habitats, response to stimuli, endurance, etc. There continues to be a great demand for entomologists in the control of insect pests.

Orders of Insects (Also see entorder.htm)

Subphylum: Hexapoda

Class: Entognatha -- Diplura, Protura, Collembola

Class: Insecta – Insects

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Thysanura -- Bristletails

Ephemeroptera -- Mayflies

Odonata -- Dragonflies, Damselflies

Orthoptera -- Locusts, Grasshoppers, Crickets,

Cockroaches, Walking sticks, Mantids

Dermaptera -- Earwigs

Isoptera -- Termites, White ants

Embioptera -- Webspinners

Plecoptera -- Stone flies

Zoraptera -- Zorapterans

Psocoptera -- Psocids

Mallophaga -- Chewing lice

Phthiraptera (= Anoplura)-- Sucking lice

Thysanoptera -- Thrips

Hemiptera -- Plant bugs, Stink bugs, Chinch bugs,

Ambush bugs, Water striders, Toad bugs

Homoptera -- Cicadas, Psyllids, Aphids, Scale insects

Neuroptera -- Alderflies, Snakeflies, Lacewings, Antlions

Coleoptera -- Beetles

Strepsiptera -- Twisted-winged parasites

Mecoptera -- Scorpion flies

Trichoptera -- Caddis flies

Lepidoptera -- Butterflies, Moths

Diptera -- Flies

Siphonaptera -- Fleas

Hymenoptera -- Bees, Wasps, Ants

External Morphology of Insects

[For greater detail see <Morphology>]

The insect's body is composed of metameres. This is a series of metameres that together comprises the exoskeleton.

Exoskeleton Function. -- The exoskeleton gives the body form and shape. It contains the body fluids and affords protection from desiccation and predators. It consists of chitin arranged in plates called sclerites joined by a membrane.

Body Regions. -- Every region of the insect has been designated for specific functions. The head is a composite of five metameres and serves for sensory reception and food ingestion. The thorax is a composite of three metameres and serves for locomotion. The abdomen is a composite of ten or more metameres.

Function of The Body Regions. -- The head is sensory and serves for the ingestion of food. The thorax contains structural parts that are used for locomotion. The abdomen houses the visceral structures. A large portion is involved in respiration, and all of the excretion and reproduction activities are confined there.

Division of a Metamere. -- The following diagram <ent2> shows the various divisions:

The various plates are called sclerites. Between the sclerites there exit membranous invaginations, which permit articulation of one plate on another. The exoskeleton stretches like cellophane, which retains its stretched shape (unlike rubber).

The termite queen has on her body brown specks that are the remnants of dorsal sclerites (= tergites). Her abdomen will never resume its original smaller size.

Metameres telescope one upon the other:

The legs attach to the body by membranes:

A suture is any point on an insect's body where there is an invagination.

Subdivisions of Thoracic Metamere.-- The following diagram shows the possible subdivisions of a thoracic metamere.

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The Insect Integument

Gross Morphology. -- The hypodermis secretes the exoskeleton. The epicuticle is a waxy, water repellent substance. The exocuticle is hard and heavily pigmented. The endocuticle is soft and not as deeply pigmented.

Chemical Composition. -- Chitin is a soft, pliable and not pigmented nitrogenous polysaccharide. Polymerized proteins may become impregnated into the chitin making it hard (= sclerotin). Various degrees of sclerotization exist. KOH dissolves out the pigments and is useful in clearing specimens in preparation for laboratory examination.

Apodemes. -- These are invaginations of the body wall. They serve to brace various parts of the body. They are called a tentorium in the head.

Sense Perception. -- The integument also is sensitive to touch (tactile), chemical (smell) and sound (hearing).

Outgrowths of The Body Wall. -- Outgrowths without any modifications beneath are spines.

Setae. -- Setae have a sensory cell associated with them called a trichogen.

Poison glands may occur where the seta is hollow and associated with glands. Urticating hairs of poisoneous caterpillars are examples.

Setae are also used as a method of classification: their pattern is constant per species.

Johnston's Organ located on the second antennal segment may function in sound perception.

The Insect Head

The position of the head is variable and may be projected forward (hypognathous), downward (prognathous) or backward (opistognathous).

Compound Eyes. -- These vary in size in different insects. They consist of ommatidia, which are hexagonal pieces placed together. The numbers of ommatidia in a compound eye varies and may vary from several hundred to 28,000 in a dragonfly. The insect's vision is a mosaic pattern, which is very efficient for detecting motion.

Ocelli. -- There are usually three ocelli but some species may have only one, each of which consists of a single lens. Ocelli detect motion and changes in light intensity.

Other Regions. -- Various other portions of the head are the Vertex, Gena, Tentorial Pits and Frons (see diagram <ent11>).

Antennae. -- There is one pair of antennae, and the various types that are found in insects may be viewed in the following diagram (ent13):

The Insect Mouthparts

The mandibles, maxillae and labrum may be remnants of appendages. There is also a clypeus, labium and hypopharynx. These are discussed as follows: The mouthparts are important in insect classification.

The Labrum holds food in the mouth.

Mandibles are tooth-like structures that articulate with the head, and there are two of them. Large muscles are attached to them and they hare modified to perform many tasks. They are also segmented but this is usually not obvious. Maxillae consist of several parts as shown in Figure Ent14. They push food into the mouth.

The Hypopharynx is a single bulbous tongue-like salivary gland that is attached to the roof of the mouth. It is highly sensory.

The Labium is believed to be a fusion of two primitive structures (Fig. Ent16). It consists of palpi, which function as a food pushing structure and also holds food in the mouth.

Occiput. -- This is the side of the head as shown in Figure Ent15:

Types of Mouthparts

The Chewing type is the basic primitive kind found in the grasshopper.

The Cutting-sponging type is found in the horsefly and deerfly. The mandibles are long and lance-like for cutting or stabbing. The maxillae are long and slender and help the mandibles. The labium is sponge-like, while the hypopharyns and epipharynx are a sucking tube.

The Sponging type is found in the housefly. The mandibles and maxillae have disappeared. The fly sponges up with the labium and sucks up with a tube.

The Chewing-lapping type is found in the honeybee. The mandibles and labrum are like that in the Chewing type, but the bee uses them for wax molding. The maxillae and labium have combined to form a tube-like structure for lapping and sucking.

Sucking Mouthparts -- These are specially modified mouthparts found in Diptera, Hymenoptera and Homoptera. They are homologized with chewing mouthparts and are located underneath the clypeus (Fig. Ent17)

Piercing-sucking mouthparts are found in the mosquito, leafhoppers and fleas. The hypopharynx and epipharynx are extended into stylets of which there are four or six. The labium forms a protective sheath around the stylets. Stylets form two tubes. Saliva goes down one tube and food up the other.

Siphoning mouthparts are found in butterflies and moths. The maxillae are the only part remaining and have fused into a long tube. Only liquids are fed upon.

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The Insect Thorax

The thorax is composed of three metameres: (1) Prothorax, (2) Mesothorax and (3) Metathorax. The pleural regions of these are associated with well-developed legs and wings. There are usually no spiracles on the prothorax, and wings may occur only on the meso and metathorax. A pronotum is formed as an extension of the prothorax, which gives strength. A sulcus or invaginated line is present that aids in wing movement. Coxal sclerites are located in a membrane around the coxa that assists in moving the leg. The spiracles are complex structures, the opening and closing of which may be controlled by the insect.

The Insect Leg. -- There are five parts to the insect leg: the coxa, trochanter, femur, tibia, and tarsus. The tibia may bear sharp, fixed spines. There may also be a movable spine or spur located at the distal end, which can articulate. On the distal of the tarsus there is a pair of claws. There is also a median pad-like structure called the empodium and lateral pad-like structures called pulvillae.

The Insect Wings. -- Insects are the only invertebrates with wings and wings are important in insect classification. They are added to the body and not derived from modified appendages as in the bird or bat. They are possibly expansions of the body wall that were originally used as gliding structures. The pleural regions are formed out of the necessity for lateral bracing. Movements of the body wall caused by muscle contraction move the wing. Longitudinal muscles are strongest on the down stroke. The insect must also tilt its wings and they rotate on the pleural sulcus.

In many insects the wings are formed as pads externally (e.g., grasshopper). In other insects they are internally formed (e.g., caterpillar). Branches of the tracheal trunks lead into the pads. In the latter stages of development a sac collapses, cells slough away leaving two sheets of cuticula. The veins are more concentrated and more heavily sclerotized at the leading edge of the wing.

Wing Morphology. -- There is much variation in forms. Wings developed late in insect evolution. Pleural sclerites are not developed in primitive forms (e.g., Thysanura). In some highly evolved parasitic orders the wings are wanting. Structures in the thorax reveal that they were primitively winged (e.g., fleas) as pleural sclerites are present.

There are usually two pairs of wings, but Diptera have one pair with remnants of a second pair, called halteres. There appears to be a trend toward the development of one metamere, the mesothorax, for flight. Wings are borne only on the meso and metathorax but never on the prothorax.

Some insects, like beetles, have their forewings modified as covers. Many wings have scales or hairs. Scales are arranged as shingles on a roof. A scale is a modified seta as is also a hair.

The generalized insect wing and its venation may be viewed at ent20.

Venation varies with different forms. In the long and narrow wing the vanal veins are absent. In the wide wing the great width is due to expansion of the vanal region.

Sometimes many veins will coalesce together and many will completely disappear. Coalescences may be determined by close examination. Where R-4 and R-5 have come together the code given is R4+5.

Crossveins are designted by lower case letters as follows: humeral (h), radial (r), sectorial (s), radial-medial (r-m), medial-cubital (m-cu).

Cells in the wings may be either closed or open.

There may be some modified structures on the wings. Wing Uniting Mechanisms are often present. A stiff spine at the base of the hind wing is called a frenulum. In moths a single spine is characteristic of the male and a cluster of spines of the female. Hamuli are found in bees. [See ent21]

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The Insect Abdomen

The abdomen consists of relatively simple metameres: a tergum plus a sternum. There are typically 10 metameres and a maximum of 12.

Spiracles. -- In the primitive insect every metamere had an opening into the respiratory system. Presently there is a maximum of 8 pairs of spiracles on the first eight metameres.

Spiracles may be of various shapes. Some are a simple opening, some are recessed and some are fringed with setae. The setae may be covered with fatty substances that inhibit water passage.

There may be any one of several opening and closing mechanisms. There is the stationary ridge plus a movable ridge. The can be two movable ridges. There are also the pinch cock and rod meshwork mechanisms.

Abdominal Appendages. -- In the primitive insect every metamere had a pair of appendages. There are no locomotory appendages on the abdomen of modern insects, but remnants do exist. For example, the Thysanura (silverfish) bears styli, which are vestigial legs. Cerci are also caudal appendages on the 10th or 11th metameres. In the cerci of the silverfish there can be 90 or more segments. In the grasshopper the cerci are very tiny. Cerci may serve as tactile structures.

Reproductive Structures. -- A genital opening, or gonopore, always occurs on metamere #8 in the female and metamere #9 in the male.

Modifications for inserting eggs exist. There is the simple conveyor tube, or a cutting device that functions similar to a can opener, and a tamping mechanism.

In some insects the entire abdomen may be modified for oviposition. A telecoping effect may be present as in the housefly.

In the male there may be claspers, which serve as good taxonomic characters (See ent22).

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Details of Insect Taxonomic Groups

Examples of beneficial species occur in almost every insect order, and considerable information on morphology and habits has been assembled. Therefore, the principal groups of insect parasitoids and predators provide details that refer to the entire class Insecta. These details are available at <taxnames.htm>.

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References

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