Insects

Overview:

Class Insecta
Most successful of the arthropods
More spp. than in all other classes of animals
Very adaptable and demonstrate a wide variety of feeding behaviors, anatomical adaptations, distribution, physiological processes, etc.
1. Feeding behaviors - parasitic (blood-suckers), herbivorous (sap-suckers and leaf (foliage) eaters, and even predators. Lots of specialization results in less competition, therefore wider distribution and greater adaptability. Other specific examples include:
  
  Phytophagous
  feed on plants; most insects are of this type.
  Saprophagus
  feed on live or dead animals; beetles and many larval forms are of this type.
2. Anatomical adaptations - well-developed, hard exoskeleton which protects body from damage, dessication, etc.; composed of:
  
  Sclerites
  hard, plates of exoskeleton connected by flexible hinge joints and muscles, allowing for movement.
  Scleroprotein
  light-weight, yet strong; makes flight possible.
  Exoskeleton does not grow with the body, therefore, must be shed periodically by molting to alow for growth; does limit the size of most insects.
  Internally, digestive tract arranged in specialized, functional units of:
  
  Foregut
  anterior portion of digestive tract containing the mouth of labrum (anterior lip) and labium (posterior lip), esophagus, crop, and gizzard.
  Midgut
  stomach for gastric digestion and gastric ceca for some adsorption nutrients and of readsorption of water from the solid wastes.
  Hindgut
  posterior portion of digestive tract containing the intestine, colon, rectum and anus; water and ion reabsorption also occur here as well as solid and soluble waste excretion.

Classification:

Class Insecta is divided into approx. 20 distinct orders, some of which include:

Order Coleoptera - Gr. sheath and wing; beetles; largest of any classification order; wings: two pairs, the front pair thick and hard, meeting in the midline when closed, and forming a protective sheath-like cover for the body. The hind pair is membranous, folded beneath the front pair at rest, and employed for flying; mouthparts: biting and chewing; metamorphosis: complete; distribution: worldwide in every habitat; numbers: approx. 275,000 spp.; economic significance: fairly minor; most spp. are neutral; some are pollenators; many are predaceous; some are crop and stored-food pests.
Order Diptera - Gr. two and wing; true flies and mosquitoes; wings: one pair; mouthparts: piercing and sucking, or lapping and sponging; metamorphosis: complete; distribution: worldwide in most habitats; numbers: approx. 75,000 spp.; economic significance: flies and mosquitoes are capable of transmitting many diseases to man and other animals; the dipterans are the most serious insect pests from the viewpoint of public health.
Order Hemiptera - Gr. half and wing; true bugs; wings: two pairs, the first pair thickened at the base to form a protective cover, the outer half of the first pair of wings is membranous and overlapping; mouthparts: piercing and sucking; metamorphosis: gradual (incomplete); distribution: worldwide in most habitats, including aquatic environments; numbers: approx. 50,000 spp.; economic significance: some serious plant pests - important transfitters of plant deseases; also some animal diseases are transmitted by bugs (e.g. bedbugs); some spp. are predacious and eat econimically and environmentally beneficial insects.
Order Homoptera - Gr. like and wing; aphids (plant lice), leafhoppers, and cicadas; wings: two pairs (sometimes absent), both of the same texture, and held over the body in a roof-like position at rest; mouthparts: piercing and sucking; metamorphosis: gradual (larva are called nymphs); distribution: worldwide, terrestrial habitats; numbers: approx. 40,000 spp.; economic significance: almost all are plant feeders, large numbers of aphids can seriously weaken a plant, and some spp. transmit plant diseases.
Order Hymenoptera - Gr. membrane and wing; ants wasps, and bees; wings: two pairs of membranous wings, the hind pair smaller, sometimes lacking wings (e.g. most ants except for the winged, reproductive form); mouthparts: chewing, or chewing and sucking; metamorphosis: complete; large, diverse order demonstrating significant "industrial" and social organization; stinger of bees and wasps is a modified ovipositor (egg-laying appendage) and therefore is present only on females distribution: worldwide in most habitats, diurnal (active during the day); numbers: approx. 120,000 spp.; economic significance: generally, beneficial, but mostly neutral, valuable as pollenators and producers of honey, many predacious and some parasitic forms help control insect and other arthropod pests.
Order Isoptera - Gr. equal and wing; termites; wings: two pairs, long and narrow, nearly equal in size, most often wingless; mouhtparts: biting and chewing; metamorphosis: gradual; description: social, with two or more distinct forms, reproductive forms (males and females) and non-reproductive forms (workers and soldiers), only reproductive forms have wings, live in colonies with a social system called a caste - males and females serving to reproduce the sp. while workers carry out the activities of the colony and soldiers defend the colony, their diet of wood is actually digested by protozoan, mutualistic symbiosis, sometimes erroneously referred to as white ants; distribution: worldwide in tripical and warmer temperate regions; numbers: approx. 2,000 spp.; economic significance: seriously destructive to wood structures in many regions.
Order Lepidoptera - Gr. scale and wing; butterflies and moths; wings: two pairs, usually large, often brightly colored, covered with tiny overlapping scales that are easily rubbed off; mouthparts: chewing in larvae, sucking in adults; metamorphosis: complete larvae are called caterpillars; distribution:worldwide in most habitats; numbers: approx. 105,000 spp.; economic significance: important pollenators, but some larvae are destructive crop and forest pests.
Order Mallophaga - Gr. wool and eat; biting lice; wings: absent; mouthparts: biting and chewing; metamorphosis: gradual (incomplete); numbers: approx. 2,000 spp.; economic significance: serious, parasitic pest, parasitic on birds and mammals.
Order Odonta - Gr. toothed; dragonflies and damselflies; wings: two pairs, large, net-veined; mouthparts: biting and chewing; metamorphosis: gradual (incomplete), larvae are aquatic and called nyads; description: dragonflies are unable to fold their wings and hold them stiffly out at the sides when at rest, damselflies are able to fold their wings vertically over their bodies when resting; distribution: worldwide, moist habitat required for development, adults are mainly diurnal or crepuscular (active in the evening); numbers: approx 5,000 spp.; economic significance: gernerally beneficial predators.
Order Orthoptera - Gr. straight and wing; cockroaches, grasshoppers, crickets, and true locusts; wings: sometimes absent or vestigial, when present, front pair is usually stiff, and straight, paper-like, hind pair membranous and folded fan-like when at rest; mouthparts: biting and chewing; metamorphosis: gradual; description: very diverse group, mostly plant-eaters, some predators such as the praying mantis, also included in this order are katydids and walking sticks; distribution: worldwide, nearly all habitats, both diurnal and nocturnal spp.; numbers: approx. 25,000 spp.; economic significance: some are serious crop pests (locusts and grasshoppers), some are household pests (cocroaches), preying mantids are beneficial predators.
Order Siphonaptera - Gr. tube and wingless; fleas; wings: absent; mouthparts: piercing and sucking; metamorphosis: complete; numbers: approx. 1,000 spp.; economic significance: parasitic adults, long-lived (up to 3 years) disease vectors.
Invertebrate chordates