Module 3 Module 3 Insect Growth and Development Learning Outcomes Learning Outcomes • • • • Explain the molting process Illustrate the life cycle of an insect Discuss the different types of metamorphosis Discuss the various stages of holometabolous insects • Discuss the insect seasonal cycle and its types and explain the basis of survival under extreme condition Insect Exoskeleton Insect Exoskeleton • The insect exoskeleton performs to protect the body of insects from external pressure, such as extreme environmental conditions and natural enemies. • The exoskeleton is composed of non-cellular cuticles, epidermal cells, and the basement membrane • The presence of this structure on insects and other arthropods is the main reason why they are small. Insect Exoskeleton • The insect exoskeleton performs to protect the body of insects from external pressure, such as extreme environmental conditions and natural enemies. • The exoskeleton is composed of non-cellular cuticles, epidermal cells, and the basement membrane • The presence of this structure on insects and other arthropods is the main reason why they are small. Insect Exoskeleton The Cuticle The Cuticle FIGURE 1 The cuticle is a non-living structure formed from the secretion of epidermal cells containing chitin and proteins. The cuticle is composed of 3 primary layers - the epicuticle, exocuticle, and endocuticle, as shown in Figure 1. The Cuticle FIGURE 1 The cuticle is a non-living structure formed from the secretion of epidermal cells containing chitin and proteins. The cuticle is composed of 3 primary layers - the epicuticle, exocuticle, and endocuticle, as shown in Figure 1. The Cuticle FIGURE 1 The cuticle is a non-living structure formed from the secretion of epidermal cells containing chitin and proteins. The cuticle is composed of 3 primary layers - the epicuticle, exocuticle, and endocuticle, as shown in Figure 1. The Cuticle FIGURE 1 The cuticle is a non-living structure formed from the secretion of epidermal cells containing chitin and proteins. The cuticle is composed of 3 primary layers - the epicuticle, exocuticle, and endocuticle, as shown in Figure 1. The Cuticle FIGURE 1 The cuticle is a non-living structure formed from the secretion of epidermal cells containing chitin and proteins. The cuticle is composed of 3 primary layers - the epicuticle, exocuticle, and endocuticle, as shown in Figure 1. The Cuticle The cuticle can also be traced from the internal organs such as the air tube lining (tracheae), salivary glands, and some parts of reproductive and digestive organs. FIGURE 1 Layers of Cuticle The Epicuticle Layers of Cuticle The Epicuticle The epicuticle is the outermost thinnest layer of the cuticle composed of wax, cuticulin, and cement. The epicuticle provides body protection from water loss and disease-causing microorganisms. Layers of Cuticle The Exocuticle The exocuticle is the second layer below the epicuticle or the middle part, which gives the cuticle its characteristics strength and resiliency. This is possible due to the formation of chitin in this layer. Layers of Cuticle The epidermal cells It is noteworthy that the primary function of the exoskeleton is for protection. However, exoskeleton still needs to be flexible and mobile. Hence, certain body parts of insects need to be flexible such as in joints so that they can still be mobile (walking, swimming, flying, digging, grasping, clasping, etc.). Here, the cuticle is thin and soft to make them more mobile and flexible. Layers of Cuticle The basement membrane basement membrane The is composed of a collagen fibers and an amorphous mucopolysaccharides (basal lamina), which effectively separates the epidermal cells from the cuticle. The Molting Process The Molting Process Figure 2. The various organs that produce hormones responsible for molting in insects. The molting process of insects starts with the cessation of feeding and, consequently, clearing of the guts contents. The Molting Process Brain hormone Figure 2. The various organs that produce hormones responsible for molting in insects. (also known as activation hormone or AH and prothoracicotrophic hormone or PTTH) The Molting Process Ecdysone Figure 2. The various organs that produce hormones responsible for molting in insects. or molting hormone or prothoracic gland hormone The Molting Process Juvenile hormone Figure 2. The various organs that produce hormones responsible for molting in insects. (maintains larval or juvenile form after molting) The Molting Process The Molting Process • Brain hormone produced by the neurosecretory cells located in the brain's protocerebrum region will be dispensed in the hemolymph (insect blood) through the brain accessory structure. • Once in the blood, the brain hormone circulates in the prothorax where the prothoracic gland is located, activated, and then produces ecdysone or molting hormone, which is responsible for molting insects. Figure 3. The molting process. The Molting Process • The epidermal cells will divide and become tightly packed in preparation for the separation of epidermal cells to the cuticle; the process is known as apolysis. • During apolysis, space created known as ecdysial space will be filled with a molting fluid (molting fluid contains enzymes such as proteinase and chitinase that digest and metabolize up to 90% of the old cuticle to form new cuticle). Figure 3. The molting process. The Molting Process • As the new cuticle is produced, formed various layers. The outer layer of the newly formed cuticle is resistant to the chitinase and proteinase enzymes, thus surviving the enzymes' digestive process. • Once the new cuticle was formed, the next process known as ecdysis is to remove the old cuticle from the insect body. Here, the insect muscles will pump blood in the thorax to expand it. Figure 3. The molting process. The Molting Process • The expansion of the thorax causes the rupture of the old cuticle. The insect will then exit through this rupture, head, and thorax first; then, abdomen and appendages follow. • After ecdysis, the newly formed cuticle is still very soft, pliant, and unpigmented. • To harden the cuticle and pigmented, the process known as sclerotization happens. This is done by swallowing air or water to increase blood pressure in order to expand the body, one part at a time. Figure 3. The molting process. The Molting Process • • • Once expanded, the new cuticle hardens and is permeated with pigment. Recently, hormone bursicon plays a role in the sclerotization process (Pedigo, 1999). The over-all process of molting is depicted in Figure 3 Figure 3. The molting process. Figure 3. The molting process. Insect Development Insect Development Stages of insect development are divided into: a. early embryonic development b. post-embryonic development Insect Development Please take note that insects are the most diverse organisms in the animal world! An estimated 1 million species have been explored and identified by scientists. Based on the statistics, an estimated 700 new species of insects were added to the number per year. Scientists still believe more have been waiting for exploration by them. So it is not surprising that we encountered new species in the days to come. Who knows one of you could have one species, and it will be named after you if you study entomology with specialization in taxonomy! Metamorphosis Metamorphosis is the process by which an insect undergoes changes in form and structure throughout its entire life. According to Pedigo (1999), metamorphosis is an orderly, genetically programmed changes in insect form during the completion of the insect life cycle. Ametabolous type Ametabolous type no metamorphosis; no changes in the form of the young known as juvenile with the adult except the size and genitalia development. Figure 4. The life cycle of a silverfish is an example of an ametabolous type of development wherein there are no relative changes in the structure of the immature from the adult except their size. Common examples of this type are the primitively wingless insects in the order Thysanura collembolan, Diplura. Ametabolous type Figure 4. The life cycle of a silverfish is an example of an ametabolous type of development wherein there are no relative changes in the structure of the immature from the adult except their size. The stages are egg, juvenile, and adult. All stages can be found in the same habitat and share the same food source. Unlike most insects, molting continues in the adult stage and is inseminated several times because the spermathecal lining is losing every molt, thus unable to store sperm cells. Paurometabolous type has gradual metamorphosis with three major stages: egg, nymph, and adult. Figure 5. The life cycle of grasshopper. Typical examples are from the order Orthoptera, Hemiptera, Blattodea, Phasmatodea. The nymph resembles the adult except for the wings, size, and genitalia. They share the same food and habitat. Hemimetabolous type has incomplete metamorphosis, which the immature known as naiad is entirely different from an adult. Figure 6. The life cycle of odonatans. Eggs and naiads are in water while adult are terrestrial. Typical examples are from the order Odonata, Ephemeroptera, and Plecoptera. The stages are egg, naiad, and adult. The naiads are aquatic while the adult is terrestrial, which means that they live in different habitat and have various food sources. Holometabolous type has a complete metamorphosis life cycle. Have four distinct stages: egg, larva, pupa, and adult. The larval stage is the feeding stage of most insects from this type, while the pupal stage is the quiescent stage. The adult may or may not feed anymore. Figure 7. The life cycle of odonatans. Eggs and naiads are in water while adult are terrestrial. Common examples under this type are Coleoptera, Lepidoptera, Hymenoptera, Diptera. Stages in the Life Cycle of Holometabolous Insect Egg stage Stages in the Life Cycle of Holometabolous Insect Egg stage Egg hatching • The hatching process often begins when the embryo in the egg swallows fluid or air. • This action gives the embryo more bulk and turgidity • Then, the embryo must rupture the egg covering to escape • Ruptures may be caused when the insect produces rhythmic muscular activity and presses or strikes against the covering with its head. • In grasshopper, the eggshell rupture irregularly along the egg surface Stages in the Life Cycle of Holometabolous Insect Egg stage Egg hatching • In green stink bug, rupture along preformed lines of weaknesses; its eggs • have easily ruptured "caps" that the insects open like a lid (Figure__). • In dragonflies, a T- or Y-shaped central "tooth" (egg burster) forces the rupture • In many butterflies and moths, simply chew their way out of the eggshell. • The act of the larva leaving the egg is called eclosion. Figure 8. A newly-hatched hemipteran. Stages in the Life Cycle of Holometabolous Insect Larval Stage Types of larva 1. Vermiform Type – maggot-like; legless and with or without a well developed head. An example of this type is from the Order Dipte Figure 9. A housefly maggot. Stages in the Life Cycle of Holometabolous Insect Larval Stage Types of larva 2. Eruciform type – caterpillar-like; the body is cylindrical with a welldeveloped head with a short antenna, thoracic legs, and abdominal prolegs. Examples are from the order Lepidoptera (moths and butterfly larvae). Figure 10. An eruciform larva Stages in the Life Cycle of Holometabolous Insect Larval Stage Types of larva 4. Scarabaeiform type – grub-like; curved body shaped or C shaped; sluggish larva; with thoracic legs and no abdominal prolegs. Examples are from the order Coleoptera of the Family Scarabaeidae. Figure 12. A stag beetle grub with characteristics C-shaped, sluggish, and feeding on dead/decomposing wood materials. Stages in the Life Cycle of Holometabolous Insect Larval Stage Types of larva 5. Elateriform type – wireworm like; elongated and hard-shelled body; short legs. Examples of this type are from the Order Coleoptera of the Family Elateridae. Figure 13. A wireworm larva typical example of a elateriform larvae. Stages in the Life Cycle of Holometabolous Insect Pupal Stage Types of Pupa 1. Obtect type – the appendages are more or less glued to the body, and are sometimes covered with cocoon. Examples of this type are from the Order Lepidoptera. Figure 14. An obtect type pupa of the sphingid moth. Stages in the Life Cycle of Holometabolous Insect Pupal Stage Types of Pupa 2. Exarate type – this type of pupae looks like a mummified adult where the appendages are not glued to the body. They are not covered with a cocoon. Common examples of this type are Coleoptera, Hymenoptera Figure 15. An exarate pupa from a particular beetle where appendages are free is not glued to the body, unlike the obtect type. Adult Stage Adult Stage It is also known as imago and considered the dispersal and reproductive stage in the life cycle of insects. It is the dispersal stage in the sense that wings are now fully developed (if winged) and are now capable of flight to look for a mate or to evade harsh conditions or escape predation or parasitism. This stage is also the reproductive stage because the reproductive structures are now fully developed and capable of producing offspring. Insect seasonal cycle Is the progression of one or more life cycles occurring during one year. This phenomenon is crucial for insects’ survival under harsh conditions such as the winter season in temperate regions. These seasonal events are in the form of dormancy, migration, development, and reproduction. Knowledge of insect seasonal cycle and timing of seasonal events among insects is crucial in attempting to manage the pest. Types of Seasonal Cycle 1. Univoltine cycle – refers to a single generation per year. One generation would mean the completion of one life cycle. Common examples of this type are most grasshoppers, rootworms. This type, when observed in the field, most of the population would be in the same growth stage. Types of Seasonal Cycle 2. Multivoltine cycle – refer to more than one generation per year. Examples of this type are houseflies and most insects such as moths and butterflies and other soft-bodied insects. Types of Seasonal Cycle 3. Delayed Voltine cycles – refer to more than one year for the life cycle to complete. An example of delayed voltine cycles is the June beetles with the larvae requiring 2 to 3 years to mature. Cicada, on the other hand, requires 17 years to develop Seasonal Adaptations Seasonal Adaptations 1. Dormancy is a seasonally recurring period in the life cycle of insects where growth, development, and reproduction are halted. Dormancy may occur when conditions are not favorable for survival. Particularly in countries with temperate climates, dormancy can occur during summer, fall, winter, or spring. When dormancy falls during summer, it is called aestivation; when it happens during fall, it is called autumnal dormancy; when it occurs during winter, it is called hibernation. When dormancy happened during spring is called vernal dormancy. Seasonal Adaptations Here, the insect survived a freezing environment through physiological processes such as supercooling and freezing tolerance. Supercooling – is resistance to freezing by insects through the lowering of the body temperature at which freezing of the body fluids begins. Freezing tolerance – is the ability to survive even the body fluids freeze. Seasonal Adaptations 2. Diapause – is a form of survival through low metabolism, little or no development, increased resistance to harsh conditions, and altered behavioral activities Thank you for listening!!! Thank you for listening!!!
