Pterygota: Evolution of Winged Insects - PDF

PTERYGOTA: Evolution of Winged Insects The Pterygota, which refers to winged insects, is divided into two major groups: Endopterygota and Exopterygota. The division is primarily based on the developmental patterns of the wings and metamorphosis.  Endopterygota: These insects develop their wi...

PTERYGOTA: Evolution of Winged Insects The Pterygota, which refers to winged insects, is divided into two major groups: Endopterygota and Exopterygota. The division is primarily based on the developmental patterns of the wings and metamorphosis.  Endopterygota: These insects develop their wings internally as part of their complete (holometabolous) metamorphosis. Holometabolous development involves four life stages: egg, larva, pupa, and adult, where each stage is morphologically distinct.  Exopterygota: In contrast, the Exopterygota have wings that develop externally as part of their incomplete (hemimetabolous) metamorphosis. These insects undergo gradual transformations in which the immature forms, known as nymphs or naiads, resemble the adults but lack fully developed wings. Within the Hemimetabolous insects, there is a distinction between Paleoptera and Neoptera. Paleoptera, such as mayflies and stoneflies, cannot fold their wings over their bodies, and their wings are either held upright or spread outward when at rest. Neoptera, on the other hand, have evolved the ability to fold their wings back over their bodies when not in flight, a critical adaptation for many species in terms of mobility and protection. Order Ephemeroptera (Mayflies) The name Ephemeroptera comes from the Greek words ephemeros, meaning "living for a day," and pteron, meaning "wing," referencing their short adult lifespan. Mayflies are unique among insects because they undergo a subimago stage, a winged but sexually immature stage that molts into the adult (imago). This order exhibits both incomplete metamorphosis and the peculiar ability to molt after acquiring full wings. The life cycle progresses through four stages: egg, naiad (aquatic nymph), subimago, and imago (adult). Mayflies are important in aquatic ecosystems, as their larvae form a key part of the diet for numerous aquatic predators. Mayflies are characterized by triangular heads and strongly developed eyes, particularly in males, which possess large compound eyes placed laterally. Their multi-segmented antennae are uniquely shorter than the head, and their degenerate mouthparts reflect their limited role in feeding as adults. The forewings are typically larger than the hindwings, and their membranous wings feature a complex network of cross-veins. The reduced prothorax and synthorax (fused meso- and meta-thoracic segments) support the functionality of these wings. Order Odonata (Dragonflies and Damselflies) The name Odonata derives from the Greek odous (tooth) and gnathos (jaw), referring to the modification of their labium. Odonates are distinctive for their large, prominent compound eyes, which are often color-patterned and provide a nearly 360-degree field of vision. Their antennae are small and inconspicuous, and their mouthparts are specialized for capturing prey, with strongly toothed mandibles and unsegmented maxillary palps. Adult dragonflies and damselflies are characterized by their large, membranous wings, which are structurally complex with numerous secondary reticulations. The synthorax, formed by the fusion of the meso- and meta-thoracic segments, supports a rigid thorax, crucial for the powerful flight muscles. Abdomen is cylindrical and elongate with visible segments. The life cycle, like that of mayflies, involves an aquatic nymphal stage (naiad) followed by the emergence of winged adults. However, unlike mayflies, odonates undergo a true metamorphosis, molting to the adult stage after the subimago. The suborders of Odonata are: 1. Zygoptera (damselflies) 2. Anisoptera (dragonflies) 3. Anisozygoptera (a small and less-diverse suborder) Orthopteroid Group of Insects The Orthopteroid group includes several orders with mandibulate mouthparts, such as the Orthoptera (grasshoppers, locusts, crickets, and katydids) and Blattodea (cockroaches). They share several features, including tegmina (thickened forewings), hindwings provided with large anal area and jugal lobe, exopterygote wing development, and paurometabolous metamorphosis (gradual development), as well as the presence of cerci at the tip of the abdomen. Order Blattodea (Cockroaches) Blattodea, or cockroaches, are characterized by a strongly dorsoventrally compressed body (that is, flattened body) and hypognathous head that is inflexed and not freely movable. The prothorax is highly enlarged and movable, forming a shield-like pronotum referred to as the protergal shield that often overlaps the head and covers part of the wing bases posteriorly. Cockroaches have sclerotized forewings and membranous hindwings, which have the radial sector of the hindwings having numerous pectinate branches. The hindwings are typically folded fanlike when not in use. Eggs are laid in oothecae, hard, (dark brown) sclerotized cases that provide protection to the developing embryos. Some species exhibit ovoviviparity, with the embryos developing within the female before being born. Cockroaches are primarily omnivorous and are nocturnal, feeding on organic matter. Some species have become pests in dwelling places, markets and storehouses such as Periplaneta americana; some are vectors of human diseases, including Salmonella and Polio. Order Mantodea (Praying Mantids) The name Mantodea refers to the praying posture of the insects, where the forelegs are raised in a raptorial position. Mantids exhibit freely movable heads, aided by the slender neck which is barely inserted into the pronotum, and this allows them to have a wider view when scanning for food. They are known to feed on other insects and spiders. The prothorax is elongate – with the pronotum elongate, narrow and freely movable which is an adaptation of the generalized prothorax. The forelegs are modified into raptorial legs used for seizing prey. The femur and tibia are heavily spined on opposite sides so as to form a femora-tibial region which is held in apposition so that the 2 can be carried forward to catch their preys. These spines (shape, arrangement and number) are characteristic in distinguishing various groups in Mantodea e.g., discoidal spines can help to determine families within Mantodea. Their wings consist of thickened forewings (tegmina) and membranous hindwings, which are fanlike when folded. Costa vein is strongly developed, subcosta vein is short and unbranched. The first radial (R1) vein and radial sector near base of wing runs as parallel unbranched veins to the base of the wings. This is used to differentiate between Mantodea and Blattodea. Mantids are predatory insects and are important as biological control agents due to their ability to capture and feed on pest insects. Their eggs are laid in oothecae, which are produced by the female from a frothy secretion that hardens into a protective capsule. Order Phasmida (Stick and Leaf Insects) Phasmida, also known as stick insects and leaf insects, have evolved remarkable camouflage abilities that allow them to blend seamlessly into their surroundings. Their sluggish movements and vegetarian diet contribute to their cryptic lifestyle. Most species lack wings, but when wings are present, the forewings developed as opaque and tough tegmina, often acting as shields. The body is either cylindrical (stick-like) or flattened (leaf-like), with the prothorax much shorter than the meso- and meta-thoracic segments. All 3 pairs of legs are similar – slender, long, and well-adapted for walking slowly across vegetation. The eggs are usually laid singly although at times in groups but not in any form of ootheca. Many species exhibit parthenogenesis, where females can reproduce without mating, a feature that contributes to rapid population growth. Order Orthoptera (Grasshoppers, Locusts, Crickets, and Katydids) The name is derived from 2 Greek words: ortho (straight) and ptera (wing). Orthopterans and are commonly referred to as grasshoppers, locusts, crickets, and katydids. Their head is hypognathous and frequently, the anterior part of the head projects beyond the eyes to form a structure called fastigium. The prothorax is typically large, often with lateral lobes, and the forewings are leathery (tegmina). The upper surface of the pronotum can be flat or saddle shaped and in some cases, may be crested depending on the family. The hindwings are membranous and broader. Orthopterans produce stridulation sounds, which are used for communication during mating or territorial disputes. The pro- and meso- thoracic legs are generally similar. Typically, they are characterized by strong, saltatorial hind (metathoracic) legs with the femur thickened to accommodate the powerful leg muscles that enable powerful jumping. However, in some groups, they may be modified for fossorial functions e.g., Gryllotalpidae. This order contains two suborders: 1. Ensifera (long-horned grasshoppers, crickets, and katydids, with more than 30 segments in the antennae) 2. Caelifera (short-horned grasshoppers, with fewer than 30 segments in the antennae) In Orthoptera, the hindwings are much broader and provided with large anal area. Unlike in mantids and blattids, the costa vein is sub-marginal in the forewings. In the hindwings the costa is marginal, the cubitus bifurcates near the wing base to give an anterior portion that is branched and the posterior portion that is unbranched. The hindwing also has numerous veins. Generally, the females are recognized by the presence of ovipositor whereas the males typically have enlarged 9th abdominal sternum that contains the genitalia and it may be rounded on the end (grasshoppers) or bear a pair of styles (crickets and katydids). Orthopterans are mostly phytophagous, few may be omnivorous and predacious. Eggs are laid singly or in batches. Eggs are held together by proteinous materials which are foamlike and they are used to cement soil particles together to form egg pods. This is different from the form of Ootheca found in Blattodea. Most pests found in Orthoptera belong to the superfamily Acridoidea of the suborder Caelifera. This includes Locusta migratoria, Schistocerca gregaria, Zonocerus variegatus, and Oedaleus senegalensis. Orthoptera exhibit phase polymorphism, where individuals may exist as solitary forms or gregarious forms, which form swarms under high population density. In between the solitary and gregarious forms, there is the transients. These grasshopper species with gregarious forms are referred to as locusts. Order Isoptera (Termites) These are the termites, iso – equal and ptera – wing. This implies equal wings in the fore and hindwings. The two wings are membranous and equal in size, shape, consistence and texture. The wings are devoid of any regular cross veins. Mouthparts are of the mandibulate type with a characteristic large basal plate on the labium called the gulamentum. The external genittalia are rudimentary or absent unlike other orders aforementioned. They have a high level of social organization with the development of caste. Within a colony, there are 3 distinct castes, 1. Reproductives – The reproductive is divided into 2 – the primary and secondary reproductive. The primary reproductives are macropterous (fully winged) and usually recognized by presence of scale or wing stubs. They lose the macropterous wings  after their nuptial flight. These stubs persist throughout their lives. They are usually dark-brown with fully developed compound eyes. The secondary reproductives are neotenics (retain features of immature insects). They are less pigmented than the primary reproductives and usually have reduced wings, thus referred to as being brachypterous; or may be completely apterous. There is no aerial flight, and as a result, the compound eyes are much reduced. 2. Soldiers – in the soldier caste, head is massively developed and, in some cases, head are larger than the rest of the body. Soldiers are of two types – the mandibulates and the nasutes. Mandibulates have strongly developed mandibles with which they defend the colony against intruders, usually ants, by cutting and slashing with their mandibles. The nasutes do not have well developed mandibles, rather, they are vestigial. They have long snouts through which they spray a sticky, toxic liquid that effectively repels or entangles invaders. 3. Workers – The worker castes are completely apterous and they lack eyes. In the colony, they have the highest number of individuals maintaining the nest and care of the royal pair with their young and the soldiers. They generally feed on wood and materials containing cellulose. Cellulose is digested by the action of bacteria living in their guts with their own enzymes to digest cellulose. Termites are responsible for considerable damage to construction timber. They are important in soil biology because they move and mix organic matter in different levels of horizons. They also help to decompose large levels of organic matter especially cellulose. Examples include the foraging termite, Amitermus euuncifer; wood harvester and fungus grower, Macrotermes spp.; and grass harvester, Irinervitermes germinates. Order Dermaptera (Earwigs) These are the earwigs, usually confused with Japygidae of Diplura because of the forcep-like cerci at the end of their abdomen, which are used in defense and mating behaviour. This feature also makes them confused with staphylinidae (Coleoptera). In Dermaptera, the head is prognathous, broad and flattened, vertex usually bears ecdysial suture. The best way to distinguish between Dermaptera and Diplura is the conspicuous ecdysial suture because it is absent in Diplura. Also, the labium in Dermaptera has a distinct gula covering most of the ventral surface of the head. There are both winged and wingless forms. When wings are present, they are characteristically shaped. The forewings are short and thickened to form the tegmina without defined veins. The hindwings are semi-circular and characteristically they have remigium reduced and sclerotized. The vanus is enlarged and has a unique pattern of venation. The anal area has 3 veins much reduced and confined to the remigium. The first anal vein branches and gives off 10 radiating branches that support most of the plane of the veins that constitute most of the membranous wings. In between are a number of cross veins and intercallaries. This wing structure is unique to Dermaptera. Each abdominal segment is elongated to about 1/3 of its length. The cerci are usually modified into forceps-like structures. In terms of habit, they are nocturnal, thigmotactic (they tend to cluster when touched). Also, the female shows maternal care of young ones until they reach early developmental stages. In Africa, Hemimerridae and in Asia, Arixeniidae are known ectoparasites of banana rats and bats respectively. Order Grylloblattodea (Ice Crawlers) Grylloblattodea is a small order with only a single family, Grylloblattidae. Their head is flattened and prognathous, mandibles are sickle shaped and the galea are 2 segmented as found in Phasmida. These insects are apterous, elongate, and cryptozoic, typically inhabiting cold environments such as caves or icefields. The first abdominal segment bears an eversible sac which is located ventrally. The 3rd segment bears a pair of long sensory cerci. Their diet consists of dead insects and mosses, and they are known for their slow movement and sensitive cerci. Order Plecoptera (Stoneflies) These are the stoneflies. They are soft bodied and fragile insects and usually have very long antennae. The head is prognathous and broadly sessile on the prothorax. Compound eyes are well developed and 2/3 the prothorax is quadrangular in shape. Both fore- and hindwings are membranous. The hindwing anal area is expanded. In the hindwing, the radial sector and the median sector are fused. Radius and cubitus shows unilateral pectinate branching. The adult frequent streams and nymphs are aquatic. This order exhibits hemimetabolous development and is an important part of freshwater food webs, with nymphs serving as prey for fish. There is development of tracheal gills for respiration. Order Embioptera (Webspinners) Embioptera, or webspinners, are characterized by silk-producing glands on their forelegs, used to create silk tunnels. They are typically encountered in the tropics. Head is prognathous, tarsi is 3-segmented while basitarsus of the foreleg is modified into a globular glandular structure containing a series of silk glands. They are gregarious and typically sub- social, and the silken structures they produce serve as both shelter and a mechanism for foraging. The female shows maternal care of young ones as found in Dermaptera.