Kingdom: Animalia, Phylum: Arthropoda
Subphylum: Hexapoda, Class: Insecta
CLICK on underlined file names and for greater detail:
Hexapoda: Class Insecta
Some authorities have suggested that the Insecta contain more species than all the rest of the Animalia. Insects are primarily terrestrial and they are spread out to almost every conceivable ecological niche, except the marine environment, which is practically devoid of insect life. Because of their extreme importance as pests of food crops and as pests and vectors of disease in public health the special disciplines of Entomology and Biological Control have been established. For greater detail on the various groups of insects please See <Entomology>
General Characteristics of Insects
Typical characters are a head with one pair of antennae, one pair of mandibles, two pairs of maxillae (the 2nd pair fused into a labium), a labrum, which does not represent a pair of appendages, and usually one pair of compound eyes accompanied by several simple eyes or ocelli.
The abdomen has a variable number of segments, with a maximum of 11. There are few signs of abdominal appendages, with the possible exceptions of (1) ovipositors and copulatory structures, (2) abdominal structures found on primitive insects and (3) prolegs that occur in the larvae of some groups.
Details of the various morphological parts of representative groups of insects may be found at
Respiration. -- Blood does not play a major role in oxygen transfer. The tracheae perform this function primarily.
Life Cycles. -- An indirect metamorphosis is involved in the majority of insects. The sexes are separate and copulation occurs and the eggs are fertilized internally.
Taxonomic Characters. -- The mouthparts, wings and metamorphosis are used more than any other characters in classifying insects.
Mouthparts: include chewing, which the most common and considered to be the most primitive. Included are the beetles, caterpillars and grasshoppers. Sucking mouthparts are characteristic of butterflies and moths. Chewing-sucking mouthparts are found in honeybees. Piercing-sucking mouthparts are found in mosquitoes, horseflies, deerflies, fleas, sucking lice and the true bugs (Hemiptera/Homoptera).
Wings: Insects are either wingless (apterous), winged with two pairs or winged with one pair either fore or aft. The apterous condition is found in very primitive forms, such as the silverfish or in highly evolved forms such as fleas, chewing and biting lice and bedbugs.
The wings are used to designate two subclasses: (1) Apteragota (wingless) and (2) Pteragota (winged or those that have lost wings).
In the primitive condition of the Pteragota the wing pairs are alike, membranous and possess many cross and longitudinal veins (e.g., dragonflies). In the more advanced forms there may be two equal and transparent wings, or the first pair have developed into a cover, or there is the presence of scales, or one or the other pair of wings is lost or there is a complete loss of wings, which is found mostly among parasitic insects.
Metamorphosis. -- Several kinds of metamorphoses may be found among the Insecta as follows:
Hemimetabolous (= incomplete metamorphosis). As in the dragonfly, immatgure stages are aquatic and called naiads. At this stage there is little resemblance to the adults. Then the change to the adult form is abrupt ( = gradual development up to a certain point after which the adult stage is rapidly formed.)
Holometabolous (= complete metamorphosis). As in the caterpillar and cocoon there is a progression from the egg to the larva, pupa and finally the adult. The larvae in this group are often referred to as grubs, caterpillars, worms or maggots. The pupa is referred to as the resting or dormant stage, which is non-feeding and enclosed in a heavily sclerotized cuticle. The adult is the sexually mature stage.
Type Animal. -- The Grasshopper because of its size and representative body parts is a good subject for studying the various parts of an insect.
Body Form. -- The head, thorax and abdomen are clearly defined.
Mouthparts. -- The chewing mouthparts consist of a labrum, or upper lip, mandibles that are heavily chitinized, the first pair of maxillae that assist in chewing, a tongue-like hypopharynx and a labium, which is the fused 2nd maxillae.
Digestive Tract. -- There are three sections: (1) the fore gut, (2) midgut and (3) hindgut.
The fore gut consists of a mouth, pharynx, esophagus, crop, gizzard and salivary glands that open into the pharynx.
The mid gut includes the stomach and gastric caecae. The latter are outpocketings of the stomach, which secrete digestive juices.
The hindgut includes the intestine, rectum, anus, rectal glands that serve to conserve water and Malpighian tubules. The latter end blindly into the haemocoel, and empty into the digestive tract at the junction of the mid and hindguts.
Circulatory System. -- The heart is usually confined to the abdomen and is provided with paired ostia. The blood is usually pumped forward through the thorax in the dorsal aorta, which opens near the brain. Blood then slowly percolates through the haemocoel but does not play a significant role in distributing oxygen.
Respiratory System. -- Tracheae lead in from openings in the body wall, which are called spiracles. Smaller Tracheoles then lead off the tracheae to supply all internal parts of the body with oxygen. The system also removes carbon dioxide. Ventilation is accomplished in part by muscular pulsations of the abdomen. The blood plays little or no role in carrying oxygen.
Excretion. -- The Malpighian tubules serve for excretion.
Locomotion. -- Legs and wings accomplish locomotion. Immature stages of some groups move about in a worm-like fashion through contractions of the body.
Sense Organs. -- The compound eyes are composed of ommatidia, each of which is an independent visual unit. Therefore, the vision obtained is a mosaic. Ocelli respond to rapid changes in the intensity of illumination, but their function is not well understood.
The antennae and palps serve a tactile and olfactory function. Sensory hairs are tactile and auditory organs are located on the sides of the thorax.
Sound Production. -- Some members have a stridulating apparatus whereby the tibia of the hind leg is rubbed against a vein on the hind wing. This serves for sexual recognition and is pronounced in crickets.
Nervous System. -- The dorsal brain consists of the protocerebrum, deutocerebrum and tritocerebrum, and they may represent the paired ganglia of the original segment of the head. There is also a ventral, double and solid nerve cord.
Reproductive System. -- Female insects have one pair of ovaries that consist of ovarioles. They have a paired oviduct, a medium oviduct, a copulatory bursa, a seminal receptacle and an ovipositor.
Male insects have one pair of testes that consist of follicles. There are two vas deferens, a median vas efferens, a seminal vesicle and a copulatory organ.
Development. -- Deveopment in the grasshopper is paurometabolous or gradual.
The Insect Orders (Increasing Complexity Arrangement →)
For greater detail and illustrations of the following please CLICK on the Order Name:
Thysanura. -- bristle tails. They have a long caudal filament plus two cerci. They are primitively wingless (apterous) and ametabolous. Food consists of starchy products and thus they are household pests, e.g., silverfish and firebrats. [Illustrations: <THYSU2>]
Ephemeroptera. -- The name meaning "short-lived" is derived from the fact that adults live for a very short time. The immature stages are called naiads, which are predaceous and long-lived. A sub imago stage occurs between the naiad and the adult. [Illustrations: <EPHEM2>]
Odonata. -- Included are the dragonflies, damsal flies, snake doctors and darning needles. The immature stage naiads are predaceous and long-lived. Adults are longer-lived than Ephemeroptera and possess a primitive wing venation. They are also predaceous and able to cause severe bites if annoyed. The Odonata is the only order of insects in which there are no wingless forms (all species are winged in the adult stage). [Illustrations: <ODONATA2>]
Orthoptera. -- The name means, "straight-winged" and the order includes grasshoppers, katydids, locusts, preying mantids, walking sticks and crickets. They have a gradual or paurometabolous metamorphosis. They have a generalized body plan for insects and thus are widely adapted for laboratory study. Many species are pests of agricultural crops. [Illustrations: <ORTHOP2>]
Isoptera. -- The name means "similar wings" and includes termites. Their food is wood and wood products, and a close relationship has developed with symbiotic flagellates termed obligate symbiosis. The symbionts digest the cellulose in the wood for the termites. They are notorious destructive pests of wooden structures, especially in the tropics.
Termites are social insects with casts. The reproductive cast includes a king and a queen. There are also secondary of accessory kings and queens, workers and soldiers. Winged termites appear only during the breeding season and only among the mature males and females. [Illustrations: <ISOPTERA>]
Embioptera. -- Embioptera are small cylindrical insects with elongated and flattened bodies; two pairs of similar wings with reduced venation. The females are apterous, while their cerci are 2-jointed; in males the cerci are asymmetrical. Metamorphosis is not present in females but occurs only slightly in males.
Plecoptera. -- The Plecoptera (stone flies) are mandibulate insects with a hetero-metabolous metamorphosis. Although they possess two pairs of well-developed wings, they are frail fliers, and do not move far from their aquatic breeding grounds. They have prominent elongated antennae and. There are 3-jointed tarsi. The wing venation may represent a primitive type. There is considerable variation in venation in the order. [Illustrations: <PLECOP2>]
Zoraptera. -- Zorapterans are minute insects, winged or wingless, with 9-jointed moniliform antennae. They have biting mouthparts. The wings, of which there are two pairs, have a reduced venation, and are capable of being shed by basal fractures as in the Isoptera. They have a wide distribution in the warmer parts of world. They live in colonies and some caste differentiation exists. [Illustrations: <ZORAPT>]
Psocoptera. -- Psocids are small insects, that
have both winged and wingless members.
They have biting mouthparts and their thoracic segments are
distinct. The wings have a reduced
venation and cross-veins are largely absent.
Metamorphosis is slight.[Illustrations:<PSOCOP2>]
These insects occur on bark and leaves of trees. They feed on lichens and dry vegetable matter. The eggs are laid on plant bark or leaves and covered by a protective sheath of silk by the female.
Phthiraptera (= Anoplura) -- The sucking lice are parasites primarily of mammals. They are vectors of European Typhus. They are secondarily wingless and metamorphosis is almost absent. [Illustrations: <Anoplura>]
Mallophaga. -- The bird lice hve chewing mouthparts. They are primarily parasites of birds, but some species also occur on mammals. They have a flattened shape and appendages that are adapted for crawling abount on bires. They are secondarily wingless and metamorphosis is almost completely absent. [Illustrations: <MALLOPHA>]
Thysanoptera. -- The thrips are tiny insects with asymmetrical
piercing mouth parts with a short labial proboscis; prothorax large and free;
tarsus 2- or 3-jointed with terminal protrusible vesicle; two pairs of similar
wings, provided with a fringe of prominent long hairs, veins few or absent;
metamorphosis slight, including an incipient pupal instar.
Hemiptera. (Usually assigned to Heteroptera) = The "True Bugs" include squash bugs, chinch bugs, boxelder bugs, bedbugs, water striders, backswimmers and lightening bugs. They have piercing-sucking mouthparts. Their first wings, or hemelytra, are partially membranous and particularly leathery. Some species are predaceous and others are parasitic on plants and animals. The "kissing bug" is a vector of Chagas Disease. [Illustrations: <HEMIPT>]
Homoptera. (Usually assigned to Heteroptera). All species are plant parasites and include the aphids, leafhoppers, mealybugs and spittlebugs. They are of considerable economic importance because of the attacks on crop plants. Some members such as the leafhoppers are vectors of plant pathogens.
The 17-year locust has a long life cycle. Populations emerge in tremendous numbers all at one time in localized areas. Eggs are laid in twigs and nymphs drop to the ground, burrow into the soil where they feed for 17 years on plant roots. Damage occurs to plants at the site where the eggs are laid. [Illustrations: <HOMOPT>]
Neuroptera. -- The alderflies, lacewings and ant lions have two pairs of large, broad wings, the posterior ones having a large posterior or anal field. The longitudinal veins branch freely and cross-veins are common, particularly behind the anterior border. A pterostigma or pigmented area placed laterally on the anterior border is either absent or poorly defined. At rest the wings are held over the back in a roof-like manner.
Coleoptera. -- The name means, "shield wing" and includes the beetles and weevils. The first pair of wings has developed into a very hardy sclerotized structure or elytra, which covers the 2nd membranous wing pair.
They are holometabolous. The larva, or grub, frequently lives in the ground. The pupa is often undetected. The adult is recognized as the beetle. Some species are serious pests of crop plants. [Illustrations: <COLEOPT2 >]
Strepsiptera. -- The twisted-wing parasites are characterized by some remarkable biological phenomena. Their parasitic activity is limited to the growth stages and the adult female. The adult males are free living. There is marked sexual dimorphism where the females are prothetelous, i.e. they have a larval body form while they are sexually mature. [Illustrations: <STREPS2>]
Mecoptera. -- This is small order of insects recognized by their vertically directed and elongated head capsule that carries the biting mouthparts at its end. They have two pairs of similar wings with simple venation in which a number of cross veins divide the whole area into a number of almost equal rhomboidal cells. Metamorphosis is complete.
Trichoptera. -- Caddis flies are medium-sized insects with bodies and wings well adorned with hairs. Their mandibles are vestigial or absent; the maxillary and labial palps well developed, and there are two pairs of nearly similar membranous wings with few cross- veins. These are held in a roof-like manner when at rest. Metamorphosis incomplete.
These obscurely colored insects have considerable powers of flight and at sexual maturity may producing mating swarms. Oviposition may occur directly into the water or the eggs may be laid on plants above the water where they will await immersion with the winter floods. [Illustrations: <TRICHO2>]
Lepidoptera. -- The butterflies and moths are holometabolous. The larva is a caterpillar with chewing mouthparts. The pupa is a chrysalis that is sometimes enclosed in a cocoon. The adults have wings with scales and sucking mouthparts. The group is of considerable economic importance from the larvae, which have chewing mouthparts. Silkworms are one of the valuable species as well as butterflies that are cherished for their beauty and relative tameness. [Illustrations: <LEPID2>]
Diptera. -- The flies, mosquitoes and midges include many pests of public health importance. Some species are vectors of malaria, elephantiasis and yellow fever. The genus Drosophila has been an important species in the study of genetics.
Hymenoptera. -- The name suggests, "clear wing," and the group includes bees, wasps, ants and sawflies. They are of great economic importance both as pests, as in the sawflies, and beneficial as plant pollinators and in biological control.
Some bees live in social communities and have casts such as female queens, male drones and degenerate female workers. [Illustrations: <HYMEN2>]
These are wingless insects, called jumping bristletails. They are among the least evolutionarily changed insects that arose in the Devonian period along with the arachnids. The name Archaeognatha comes from the Greek Archaeos or"ancient" and gnatha meaning "jaw". This refers to the articulation of the mandibles, which has a single condyle, where all higher insects have two. An alternate name, Microcoryphia comes from the Greek micro "small" and coryphia "head".
The Order Archaeognatha has previously been combined with the Order Thysanura, or bristletails, both of which groups possess three-pronged tails comprising two cerci . [Illustrations -- MICROCOR]
Please see following plates for Example Structures of the Insecta:
Plate 80 = Phylum: Arthropoda, Class: Insecta -- Section of cuticle and hypodermis
Plate 81 = Phylum: Arthropoda, Class: Insecta -- Petrobius maritimus mouthparts
Plate 82 = Phylum: Arthropoda, Class: Insecta -- Histology of the insect gut
Plate 83 = Phylum: Arthropoda, Class: Insecta -- Section through gut of Rhodnius prolixus
Plate 84 = Phylum: Arthropoda, Class: Insecta -- Trachea & tracheole structure
Plate 85 = Phylum: Arthropoda, Class: Insecta -- Closing of spiracles
Plate 86 = Phylum: Arthropoda, Class: Insecta -- Regulation of tracheal respiration
Plate 87 = Phylum: Arthropoda, Class: Insecta -- Sensillae of insects
Plate 88 = Phylum: Arthropoda, Class: Entognatha -- Basic morphology of Collembola
Plate 89 = Phylum: Arthropoda, Class: Insecta, Order: Thysanura -- Lepisma saccharina & Petrobius maritimus
Plate 90 = Phylum: Arthropoda, Class: Insecta, Order: Ephemeroptera -- Morphology
Plate 91 = Phylum: Arthropoda, Class: Insecta, Order: Dermaptera -- Forficula auricularia male
Plate 92 = Phylum: Arthropoda, Class: Insecta, Orders: Mecoptera & Neuroptera-Panorpa, Sialis, Mymeleon spp.
Plate 93 = Phylum: Arthropoda, Class: Insecta, Order: Neuroptera -- Sialis lutaria
Plate 94 = Phylum: Arthropoda, Class: Insecta, Order: Trichoptera -- Hydroptila, Odontocerum, Hydropsyche,
guttatipennis & Phryganea spp.
Plate 95 = Phylum: Arthropoda, Class: Insecta, Order: Diptera -- Mouthparts of a mosquito
Plate 96 = Phylum: Arthropoda, Class: Insecta, Order: Diptera -- Calliphora sp. mouthparts
Plate 97 = Phylum: Arthropoda, Class: Insecta -- Pseudotracheal membrane structure
Plate 98 = Phylum: Arthropoda, Class: Insecta, Order: Diptera -- Calliphora sp. labellar lobes
Plate 99 = Phylum: Arthropoda, Class: Insecta, Order: Siphonaptera -- Ctenocephalus mouthparts
Plate 100 = Phylum: Arthropoda, Class: Insecta, Order: Hymenoptera -- Apis sp. mouthparts
Plate 101 = Phylum: Arthropoda, Class: Insecta, Order: Strepsiptera -- Elenchinus sp. male & Polistes sp. host
Plate 102 = Phylum: Arthropoda, Class: Insecta, Order: Anoplura -- Pediculus humanus
Plate 103 = Phylum: Arthropoda, Class: Insecta, Orders: Hemiptera & Homoptera -- Aradus sp. & Aphanus sp.
Plate 104 = Phylum: Arthropoda, Class: Insecta, Order: Homoptera --Psylla sp. stylet penetration into plant tissue