In the classification of insects, we must first identify the sub clsses and orders. The class Insecta (insects) is split into two sub classes and twenty – four orders. Here, a very concise outline of the main diagnostic features of each order is given together with some common examples.
Sub – Class Apterygota
The orders in this sub – class consist of wingless insects, the condition being primitive. They are the Diplura, Thysanura, Collembola and Protura. None undergo metamorphosis.
Order 1: Diplura
Classification of insects based on their order had occupied the minds of biologists for a long time. The order diplura are very small white insects with no eyes. Found in soil and under stones. Long antennae and anal cerci. Almost “living fossils”, they are survivors of a very ancient group (e.g., Campodea). All the Diplura were formerly included in the Thysanura and were known as the two pronged bristle tails.
Order 2: Thysanura
The well – known silver – fish and sugar – brats. The antennae are long and there is a median appendage on the last segment as well as the two long anal cerci. The body is covered with scales (e.g., Lepisma Saccharina, the silver – fish)
Order 3: Collembola
These are very small insects with short antennae and a forked springing organ on the fourth abdominal segment. They have no compound eyes. The abdomen is peculiar in possessing only six segments. They are extremely common in the soil, on pasture land and in decaying organic matter (e.g., the spring – tails).
Order 4: Protura
In the classification of insects,this order are minute insects with twelve segments in the abdomen. No antennae or compound eyes or cerci, very small legs. Found under the bark of trees, in turf and in soil. Size about 1mm or less.
We shall now turn to the sub class Pterygota as we take a deeper look into the classification of insects. This sub class contains 20 orders.
Sub – Class Pterygota
This sub – class contains the remaining twenty orders. All either bear wings or are secondarily wingless. There is a metamorphosis which may be complete or incomplete.
Order 5: Orthoptera
The characteristics of the order have been given. It includes cockroaches, crickets, grass – hoppers, locusts, stick insects, praying insects.
Order 6: Plecoptera
Long slender antennae and anal cerci. Mouth parts are reduced and weak. Always found near water: the nymphs are aquatic (e.g., the stone – flies).
Order 7: Dermaptera
Cerci modified to form forceps. Front – – wings reduced to small leathery tegmina. Slight metamorphosis (E.g., the earwings).
Order 8: Embioptera
Mainly tropical and sub – tropical. They spin silken tunnels in which they live. Both pairs of wings alike and oftwn absent in the females. Elongated antennae. slight metamorphosis in the male: none in the female (e.g., the web – spinners) none are British.
Order 9: Isoptera
Social insects living in large communities. Reproductive castes and sterile castes; king, queen, soldiers and workers. Fully winged, partly winged and non – winged forms. Wings both similar and can be shed. Mouth parts biting. Metamorphosis slight or incomplete. Mainly tropical and subtropical (e.g., the termites or “white ants”).
Order 10: Ephemeroptera
Short – lived imagines; aquatic nymphs. Mouth parts atrophied or almost so. Very long cerci and a median caudal filament. Long antennae. hindwings much reduced (e.g., the May – flies).
Order 11: Odonata
Long bodied, brilliant metallic colors. Mouth parts biting. Wings equal. Very large eyes. Aquatic nymphs (e.g., the dragonflies).
Order 12: Psocoptera
Very small insects. Winged with anterior wings larger or wingless. Cerci atrophied or almost so. Antennae fairly long. Biting mouth parts. Metamorphosis little or absent (e.g., book – lice and their allies).
Order 13: Anoplura
All ectoparasites of birds and animals. Wingless. Mouth parts for biting or piercing. Lattened body, short legs with claws adapted for clinging. No cerci. Biting and sucking lice. Notorious carriers of disease.
Order 14: Thysanoptera
Very small insects with short antennae. piercing and sucking mouth parts. Wings narrow and fringed with long setae. No cerci. The thrips insects do great damage to plants by sucking the sap. Pea – thrips, corn – thrips, pear thrips, etc.
Order 15: Hemiptera
Small insects with piercing and sucking mouth parts. Often wingless parthenogenetic generations. No cerci. Usually gradual metamorphosis. The plant bugs. Aphides, scale insects, mealy bugs, leaf – hoppers, the “musical” cicadas, the greenhouse white – fly. Many are carriers of virus diseases.
Order 16: Neuroptera
Medium soft – bodied insects with long antennae. being mouth parts. No cerci. Many have aquatic larvae. Alder flies, snake flies, lace – wings, ant lions.
Order 17: Mecoptera
Small insects with biting mouth parts. Long antennae. wings similar. Cerci reduced. Conspicuous bands or spots on the wings. The scorpion flies.
Order 18: Trichoptera
Medium insects of weak flight, often resembling moths. Mouth parts adapted for licking but many imagines do not feed. Antennae and cerci of medium length. Aquatic larvae, the caddis – worms, which make remarkable cases for concealment and protection
Order 19: Lepidoptera
Small to large insects with entire covering of powdery scales. Wings fastened together. Sucking proboscis formed by maxille. No cerci. Metamorphosis is complete. The butterflies and moths.
Order 20: Coleoptera
Size ranges from minute to large. Force – wings always horny elytra. Mouth parts biting. Metamorphosis complete. The beetles.
Order 21: Strepsiptera
Small insects; larvae parasites in other insects, especially Hymenoptera. Antennae bifid, mouth parts biting, but often degenerate. Fore – wings are halters, hind – wings fan – shaped. No cerci. Stylops, a British genus, has sixteen endemic species.
Order 22: Hymenoptera
Small to medium insects, many of them social. Metamorphosis complete. Antennae short. Mouth parts very midified for biting or sucking. Often castes present: queens, drones, workers, soldiers. Parthenogenesis is common. The saw – flies, ichneumon flies, chalcids, gall – wasps, ants, wasps, bees, hornets.
Order 23: Diptera
Hind – wings form halters. Antennae various but usually short. Mouth parts suctorial, often piercing also. Cerci very reduced or absent. Metamorphosis complete. Larvae are apodous. The true two – winged flies. Many are important as vectors of disease; the housefly, mosquito, teste fly: crane – flies, midges, horse – flies, hover – flies, warble – flies.
Order 24: Aphaniptera
Small insects, all ectoparasitic. Body laterally flattened. Short antennae. mouth parts for piercing and sucking. Legs adapted for jumping. No cerci. No wings.
Lets take an in-depth look at the classification of insects
Classification of insects is an essential part of entomology. Various classification schemes exist, but most are based on the structure of the wings and mouthparts or the type of metamorphosis.
A typical insect has a hard outer covering (exoskeleton) that protects soft internal parts. The mouthparts are either chewing or piercing and most have wings, but some do not.
The class Arachnida includes mites, ticks, scorpions, spiders, daddy longlegs and other arachnids. It also includes marine arthropods like horseshoe crabs and sea spiders that have chelicerae. Unlike insects, the cephalothorax and abdomen of arachnids are fused together. Arachnids have four pairs of legs and a pair of mouthparts called chelicerae or pedipalps. They are typically darkly coloured to blend in with the environment.
Arachnids are carnivorous and primarily terrestrial, though some may live in fresh water and most marine environments excluding the open ocean. They grow by molting. The body is segmented into two sections called the prosoma (a united head and thorax) and the opisthosoma (which contains organs atypical of an abdomen). Arachnids have internal breathing systems using ‘book lung’ that takes oxygen from a spiracle through fluid-filled ‘pages’ with lamellae to handle gas exchange. Male sex organs are located on the second abdominal somite. They produce spermatozoa and convey them to the female through a circum-oesophageal duct. Arachnids contain copper instead of haemoglobin, which increases their affinity for oxygen. The spiracles are adapted to aqueous respiration and they use a specialised molecule called haemocyanin for this process.
This is one of the largest orders in the classification of insects (over 40% of all insect species) with a wide range of morphological characters and habits. In general, beetles are wingless and feed by chewing with mouthparts. The front pair of wings are usually strongly horned and inserted into a broad c-shaped covering (elytra), which shields the rear pair.
The most diverse and numerous families include tenebrionids with long, straight antennae, often with a metallic sheen, lagriinae with a characteristic shape, and the cylindrical bark beetles of Colydiinae. The elaterids (click beetles) are herbivores with a peculiar ability to snap their head and abdomen against the substrate to right themselves when they are turned on their back. A few species are important pests of crop plants such as the boll weevil Anthonomus grandis.
Coleoptera also includes some species associated in varying capacities with ants, including predaceous (or parasitic) beetles such as the genera Pselaphidae, Leptinidae, and Brentidae. A large number of ant-associated beetles are scavengers and occupy habitats similar to those of ants, although a few such as the clavigerids and some cryptophagids may be predatory on ant broods or other insects in their habitat.
The butterflies and moths of this order (Lepidoptera) are very diverse in size, shape, coloration and other morphological characteristics. They are holometabolous, undergoing complete metamorphosis. They reproduce sexually and are pollinators for plants. They are often cryptic, blending into their background environment with colors and patterns such as those of leaves or bark.
They have open circulatory systems, bathing their organs in a fluid called hemolymph, which is also used as a lubricant for movement of body parts. They are susceptible to a wide variety of diseases and predation by parasitoids such as wasps in the families Chalcidoidea, Vespidae, and Sphecidae, as well as mites and spiders.
Their mouthparts are siphoning-type. During imago, they develop into long, flexible structures called proboscises. These can be extended by a lengthening protein in the brain called ecdysone hormone, secreted by neurosecretory cells at the corpora cardiaca. They are able to perceive UV light with two ocelli, simple photoreceptors that are homologous to the compound eyes in the adult. During feeding, their proboscises pierce through the host plant to extract liquids or food.
Flies (Diptera) are the most common of insects and in the classification of insects, they are classified into four classes, depending on their larval feeding habits. They have two wings, which can be fully functional or reduced to a pair of knobbed balancing organs called halteres. The abdomen is a long, segmented tube containing 11 abdominal segments called urites. Each urite has dorsal and ventral sections, known as tergite and sternite.
The digestive system of Diptera is a long, one-way tube that directs food from the mouth to the anus. In the hindgut, undigested food is joined with urea to form fecal pellets that are delivered and eliminated from the anus.
Many Diptera are important from ecological and human welfare perspectives. For example, the larval Chironomidae provide a major source of prey for fish and other invertebrates. Adult syrphid flies are important pollinators of plants. Others serve as decomposers of decaying plant and animal material, including vertebrates. They are also important predators and parasitoids of other invertebrates, including humans.
Hymenoptera includes ants, bees, wasps, sawflies, and gall wasps. They are the most numerous group of insects in the world. Most of them have two pairs of wings (if winged), chewing mouthparts, and a system of sexual reproduction where fertilized eggs become females and unfertilized eggs become males.
Some hymenopterans are true social insects, building large colonies with paper-like nests and tending the young. Others are parasitoids of other insects, laying their eggs within the living host organism, which is then slowly eaten by the larva before the host dies. There are also kleptoparasites, which take advantage of the hard work done by other species to supply their own food.
Hymenopterans have a bewildering diversity of lifestyles, intimately bound up with the way they provide food for their offspring. The majority of hymenopterans are beneficial as natural enemies of insect pests, pollinators of flowering plants, or predators of other invertebrates. Phylogenetic relationships of the entire order have been studied with both morphological and molecular data.