Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

Lecture no. 5

Structure and functions of insect cuticle and molting (definition & importance)

Structure of insect cuticle

The insect exoskeleton is usually called a cuticle. The cuticle is the characteristic feature of arthropods and is to a large extent responsible for the success of insects as terrestrial animals. The body wall or integument is the outer layer of insect and bends inwards at various points to form supporting ridges or braces. The body wall of insect is composed of three principal layers: the cuticle, epidermis and basement membrane.

General structure of the insect cuticle

Cuticle: The cuticle is an outer layer which contains a characteristic chemical compounds called chitin, proteins and pigments. The cuticle is made up of three principal layers: epicuticle, exocuticle and endocuticle. The exo- and endo- cuticle together referred as procuticle. Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

A. Epicuticle: Epicuticle is the outer, thin, delicate layer without any chitin and may have four sub- layers

I. Cement layer: The cement layer is very thin layer outside most of wax layer and consists of mucopolysaccharide. It protects underlying wax and not produced by all insects. It is secreted by dermal glands and protects the body from external damage. II. Wax layer: The wax layer contains many different compounds but wax comprises over 90%. It is consisting of long chain hydrocarbons, esters of fatty acids and alcohols. It serves as water proof layer preventing water loss from the body. III. Polyphenol layer or Inner epicuticle: it is the thickest layer of 0.5 to 2.0 µm immediately outside the procuticle and chemically consists of tanned lipoproteins. It is a non-static layer containing various types of phenols which are mainly used in resistant to acids and organic solvents. IV. Cuticulin layer or outer epicuticle: This is very thin trilaminar layer only of about 15 nm. It is the first formed layer of new cuticle product at each molt protecting the new procuticle from the molting enzymes. The material forming the outer epicuticle is highly polymerized lipid is often referred to as cuticulin. It serves the purpose of permeability and also acts as growth barrier.

B. Exocuticle: It is a thicker layer below epicuticle and is often darker and harder than the rest.

 It contributes rigidity and toughness to the cuticle. The exocuticle is wanting or considerably reduced in the regions of integument which are more flexible.  The exocuticle structurally consists mainly of chitin and protein.  Chitin is a polymerized nitrogenous polysaccharide linked to protein.  It is pigmented by a hard brown material as is referred to as tanned. Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

 Other constituents of cuticle include quinones which polymerized to form dark brown or black pigment known as melanin and in others quinones are utilized in forming sclerotin.

C. Endocuticle: The inner undifferentiated part below the exocuticle is endocuticle and is characterized by the presence of chitin. Endoskeleton of insect cuticle provides space for attachment of muscles of antenna and mouthparts, called as Tentorium.

Epidermis: It is a cellular layer of one cell thick beneath endocuticle. The epidermal cells are glandular and secrete cuticle and the enzymes for the cuticle formation and digestion at the time of molting. Some of the epidermal cells have specialized glandular function.

Basement membrane: The basement membrane is also called basal lamila. It is a noncellular layer beneath epidermis and serves as stable platform where epidermal cells are anchored and muscles are attached.

Functions of insect cuticle

Being an interface between a living animal and an environment, the cuticle of an insect serves many functions.

1. It limits the dimensions of an exoskeleton and is a basis for muscle insertions (mechanical function and function of locomotion). 2. It is an important element in organism defence against a variety of external factors, such as mechanical stresses, dry, wet, cold or hot environments. 3. It takes part in the transport of diverse epidermal secretions, and serves as a chemical reservoir for the storage of metabolic waste products. 4. A variety of cuticular structures are parts of mechano- and chemoreceptors. 5. The cuticle, its coloration pattern, and chemical components are important for thermoregulation, and are often involved in diverse communication systems. 6. Specialised cuticular protuberances may serve a variety of functions, such as oxygen retention, food grinding, body cleaning (grooming), etc.. Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

Molting

The molting process is triggered by hormones released when an insect's growth reaches the physical limits of its exoskeleton. Each molt represents the end of one growth stage (instar) and the beginning of another. In some insect species the number of instars is constant (typically from 3 to 5), but in others it may vary in response to temperature, food availability, or other environmental factors. An insect is known as an imago (adult) when it becomes sexually mature. At this point, molting stops and energy for growth is channeled into production of eggs or sperm.

An insect cannot survive without the support and protection of its exoskeleton, so a new, larger replacement must be constructed inside the old one -- much like putting an overcoat under a sweater! The molting process begins when epidermal cells respond to hormonal changes by increasing their rate of protein synthesis. This quickly leads to apolysis -- physical separation of the epidermis from the old endocuticle. Epidermal cells fill the resulting gap with an inactive molting fluid and then secrete a special lipoprotein (the cuticulin layer) that insulates and protects them from the molting fluid's digestive action. This cuticulin layer becomes part of the new exoskeleton's epicuticle.

After formation of the cuticulin layer, molting fluid becomes activated and chemically "digests" the endocuticle of the old exoskeleton. Break-down products (amino acids and chitin microfibrils) pass through the cuticulin layer where they are recycled by the epidermal cells and secreted under the cuticulin layer as new procuticle (soft and wrinkled). Pore canals within the procuticle allow movement of lipids and proteins toward the new epicuticle where wax and cement layers form.

When the new exoskeleton is ready, muscular contractions and intake of air cause the insect's body to swell until the old exoskeleton splits open along lines of weakness (ecdysial sutures). The insect sheds its old exoskeleton (ecdysis) and continues to fully expand the new one. Over the next few hours, sclerites will harden and darken as quinone cross-linkages form within the exocuticle. This process (called sclerotization or tanning) gives the exoskeleton its final texture and appearance. Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

An insect that is actively constructing new exoskeleton is said to be in a pharate condition. During the days or weeks of this process there may be very little evidence of change. Ecdysis, however, occurs quickly (in minutes to hours). A newly molted insect is soft and largely unpigmented (white or ivory). It is said to be in a teneral condition until the process of tanning is completed (usually a day or two).

Summary of Molting

Step 1 Apolysis -- separation of old exoskeleton from epidermis Step 2 Secretion of inactive molting fluid by epidermis Step 3 Production of cuticulin layer for new exoskeleton Step 4 Activation of molting fluid Step 5 Digestion and absorption of old endocuticle Step 6 Epidermis secretes new procuticle Step 7 Ecdysis -- shedding the old exo- and epicuticle Step 8 Expansion of new integument Step 9 Tanning -- sclerotization of new exocuticle

Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

Lecture no. 6

Body segmentation: Structure of head & its types

Insect have segmented bodies with certain segments fusing to form three usually well defined regions: head, thorax and abdomen. The grouping of segments into functional regions is known as tagmosis.

Head consists of 6 segments. Head comprises of mouthparts, compound eyes, simple eyes (ocelli) and a pair of antennae.

Thorax consists of 3 segments i.e. prothorax, mesothorax and metathorax. Meso and metathorax are together known as pterothorax. All the three thoracic segments possess a pair of legs and meso and meta-thorax each possess one pair of wings.

Abdomen has 7-11 segments with genital appendages. The 8 and 9 segments in female and 8 segment in male insects are modified to bear genital appendices.

The outer parts of every segments of body is hard, colored and complex in nature. There are hard, colored and complex plates in each segments of insect body which is called sclerites.

 The sclerites present on dorsal side is called tergides,  Lateral side is called pleurites and  Ventral side is called sternites. Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

Segments except in head (6 segments fused together to form a capsule) joint together with a flexible soft and white membrane called inter segmental membrane or suture.

Insect Head

Insect head is a hard and highly sclerotized compact structure. It is the foremost part in insect body consisting of 6 segments that are fused to form a head capsule. The head is connected to the thorax by a flexible neck or cervis strengthened by small cervical sclerites. The head segments can be divided in to two regions i.e. procephalon and gnathocephalon (mouth).

Sclerites of Head

I. Vertex: Summit of the head between compound eyes. II. Frons: Facial area below the vertex and above clypeus. III. Clypeus: Cranial area below the frons to which labrum is attached. IV. Gena: Lateral cranial area behind the compound eyes. V. Occiput : Cranial area between occipital and post occipital suture. Sutures of Head The linear invaginations of the exoskeleton between two sclerites are called as suture (some times referred as sulcus). I. Epicranial suture/ ecdysial line: Inverted `Y' shaped suture found medially on the top of head, with a median suture (coronal suture) and lateral sutures (frontal suture). II. Epistomal suture/ Fronto clypeal suture: Found between frons and clypeus. (epi –above; stoma- mouth parts) III. Clypeo-labral suture: Found between clypeus and labrum (upper lip). IV. Postoccipital suture: Groove bordering occipital foramen. Line indicating the fusion of maxillary and labial segment.

Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

Types of head or orientation of Head

The orientation of head with respect to the rest of the body varies. According to the position or projection of mouth parts, the head of the insect can be classified as:

A) Hypognathous (Hypo – Below: Gnathous – Jaw) The head remain vertical and is at right angle to the long axis of the body and mouth parts are ventrally placed and projected downwards. This is also known as Orthopteroid type. Eg: Grasshopper, Cockroach

B) Prognathous: (Pro – infront: Gnathous – Jaw) the head remains in the same axis to body and mouth parts are projected forward. This is also known as Coleopteroid type. Eg: beetles

Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

C) Opisthognathous: (Opistho – behind: Gnathous – Jaw) It is same as prognathous but mouthparts are directed backward and held in between the fore legs. .This is also known as Hemipteroid or Opisthorhynchous type. E. g: bugs

Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

Lecture no. 7

Body segmentation: Structure of thorax and abdomen

Structure of thorax

The thorax is the middle body region between the head and abdomen. The insect thorax has three segments: the prothorax, mesothorax, and metathorax.

The anterior segment, closest to the head, is the prothorax; its major features are the first pair of legs and the pronotum.

The middle segment is the mesothorax; its major features are the second pair of legs and the anterior wings.

The third, the posterior, thoracic segment, abutting the abdomen, is the metathorax, which bears the third pair of legs and the posterior wings.

Each segment is delineated by an intersegmental suture.

Thoracic segments are made up of three sclerites namely, dorsal body plate tergum or nota, ventral body plate sternum and lateral plate pleuron. Each segment has four basic regions

 One thickened dorsal plate on each segment of the body of an insect is called the tergum (or notum) to distinguish it from the abdominal terga.  Two lateral regions are called the pleura (singular: pleuron). Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

 One ventral aspect is called the sternum.

In turn, the notum of the prothorax is called the pronotum, the notum for the mesothorax is called the mesonotum and the notum for the metathorax is called the metanotum. Continuing with this logic, there is also the mesopleura and metapleura, as well as the prosternum, mesosternum and metasternum. The inter-segmental membrane absent between pronotum and prosternum on prothorax.

The legs arise on pleura and wings articulated between notal and pleural region.

Structure of abdomen

The insects abdomen is the third region of the insect body and is often longer than head and thorax. Insects abdomen usually consists of six (Collembolas) to ten segments and terminates in the paraproct, where the anus opens. This paraproct may form a lobe like epiproct above the anus and a pair of leteral paraprocts around it. The dorsal and ventral abdominal segments are termed terga (singular tergum) and sterna (singular sternum), respectively. Spiracles usually can be found in the conjunctive tissue between the terga and sterna of abdominal segments 1-8. Reproductive structures are located on the 9th segment in males (including the aedeagus, or penis, and often a pair of claspers) and on the 8th and 9th abdominal segments in females (female external genitalia copulatory openings and ovipositor). Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

Ovipositor

The ovipositor is the egg-laying device found only in female insects. In some insects, the ovipositor is highly modified and conspicuous. In others, the apparatus may be needle or blade-like.

Abdominal Appendages

Abdominal appendages of insects can be grouped as

Pre-genital appendages,

Genital appendages and

Post genital appendages.

Pre-genital appendages: Appendages prior to eight segments

a) Styliform appendages: In primitive apterygotes or in non-insect hexapods, there are pairs of small, unjointed styli each inserted on a basal sclerite called coxa along with eversible vesicles b) Collembolan appendages: On the third abdominal segments, a median lobe called ventral tube projects and at its tip long and tubular eversible vesicles are present in pair. Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

c) Cornicles or siphunculi: Paired secretory structures located dorsally on the 5th abdomen of aphids. It project from the dorsum of 5 or 6 or from between them and have defensive function in most cases. The cornicles produce substances that repel predators or elicit care- giving behavior by symbiotic ants. d) Gills: Respiratory organs found in the nymphs (naiads) of certain aquatic insects. In Ephemeroptera (mayflies), paired gills are located along the sides of each abdominal segment. In Odonata (damselflies), the gills are attached to the end of the abdomen. Many larvae of aquatic insects like mayflies may have 6 or 7 pairs of plate like or filamentous gills. The gills have direct role in gaseous exchange and more importantly to maintain a flow of water over the body. e) Prolegs or pseudolegs: Except to the three pairs of thoracic legs, leg like outgrowths of the body wall in abdominal region are the feature of many holometabolous insects and are called prolegs or pseudolegs or non-thoracic legs or temporary legs. They are fleshy, locomotory appendages found only in the larvae of certain orders (notably Lepidoptera, but also Mecoptera and some Hymenoptera).

Genital appendages: The reproductive openings and genitalia are found on the ninth abdominal segment in male and on eight and ninth abdominal segments in the female are commonly called genital appendages

The insect's genital opening lies just below the anus: it is surrounded by specialized sclerites that form the external genitalia.

 In females, paired appendages of the eighth and ninth abdominal segment fit together to form an egg-laying mechanism called the ovipositor. These appendages consist of four valvifers (basal sclerites with muscle attachments) and six valvulae (apical sclerites which guide the egg as it emerges from the female's body).  In males, the genital opening is usually enclosed in a tube-like aedeagus which enters the female's body during copulation (like a penis). The external Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

genitalia may also include other sclerites (e.g. subgenital plate, claspers, styli, etc.) that facilitate mating or egg-laying.

The structure of these genital sclerites differs from species to species to the extent that it usually prevents inter-species hybridization and also serves as a valuable identification tool for insect taxonomists.

Post genital appendages

Cerci: Cerci (singular cercus) are paired appendages on the rear-most segments of many Arthropoda, including insects and Symphyla. Many forms of cerci serve as sensory organs, but some serve as pinching weapons or as organs of copulation. In many insects they simply may be functionless vestigial structures. Most cerci are segmented and jointed, or filiform (threadlike), but some take very different forms. Some Diplura, in particular Japyx species, have large, stout forcipate (pincer-like) cerci that they use in capturing their prey. The cerci of two sexes may differ in many insect species or have dimorphism. Crickets have particularly long cerci while other insects have cerci that are too small to be noticeable. However, it is not always obvious that small cerci are without function; they are rich in sensory cells and may be of importance in guiding copulation and oviposition.

Median caudal filament: This is a thread like projection arising from the center of the last abdominal segment between the cerci. Some insects such as mayflies, silverfish Course name: Fundamentals of Entomology 1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP and bristletails have an accompanying third central tail filament which extends from the tip of the abdomen.

Pseudocerci : In Coleoptera larvae, a pair of outgrowths of the tergum of segment 9 in the form of short spines or multiarticulate processes also referred as pseudocerci; corniculi.

Terminal spines: The 10th abdominal segment of hawk moths and jet moths bear spine like structures Used for defense

Course name: Fundamentals of Entomology,1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

Lecture no. 8

Structure and modifications of insect antennae

Structure of insect antennae

Antennae are located between or behind the compound eyes. All insect except Protura possess a pair of antennae. Antennae function almost exclusively in sensory perception. Some of the information that can be detected by insect antennae includes: motion and orientation, odor, sound, humidity, and a variety of chemical cues. Antennae are well developed in adults and poorly developed in immature stages. The antenna is set in a socket of the cranium called antennal socket. The base of the antenna is connected to the edge of the socket by an articulatory membrane. This permits free movement of antennae. Although antennae vary widely in shape and function, all of them can be divided into three basic parts: Scape Pedicel Flagellum Course name: Fundamentals of Entomology,1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

Scape: It is first segment or basal or proximal segment of antennae are articulate to the head capsule. It is often conspicuously longer than the other segments. Pedicel: It is second segment that follows scape. Specialised sensory organ known as Johnson’s organ situated on pedicel with insect can hear the sound. In honey bee, wasps pedicel forms the pivot between scape and flagellum. Flagellum: It is third division formed by a single or many segments. Flagellum is further divided into three parts. Ring joints: It is basal segment of flagellum are small and ring like form. Club: It is swollen or enlarged distal segments of the antenna. Funicle: segments between ring joints and club. Functions of insect antennae  Antennae function almost exclusively in sensory perception.  To serve as secondary sexual characters in male Lepidoptera and male mosquito.  To find it’s way.  To detect the danger.  To find the food.  To find the opposite sex.  To communicate with each other e.g. Ants.  It bear olfactory organs ( smell) e.g. Housefly.  It bears chordotonal organs (sound hearing) e.g. Male Mosquito.  To detect humidity.  To hold the opposite sex at the time of mating.  To catch the prey e.g. Mantids.

Modifications of insect antennae

1. Aristate

Aristate antennae are pouch-like with a lateral bristle. Examples: House and shore flies (order Diptera). The antennae are important sensory structures used to detect air Course name: Fundamentals of Entomology,1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP movement and odors.The antenna is three-segmented with a branched arista projecting dorsally from the third segment.

2. Capitate or Knobbed or head shaped

Capitate antennae are abruptly clubbed at the end. The antenna starts narrow at the base and gets bigger toward the tip, but only right near the tip. Examples: Butterflies (order Lepidoptera), sap beetles (Coleoptera). Course name: Fundamentals of Entomology,1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

3. Clavate or club shaped

Clavate antennae are gradually clubbed at the end. Technically, the capitate antennae of butterflies are also clavate because they are also clubbed, but they’re a special kind of club and get their own name (they’re capitate clavate antennae). Examples: Carrion beetles (order Coleoptera).

4. Filiform

Filiform antennae have a thread-like shape. All the segments are of about the same thickness and have no prominent constrictions at the joints. Examples: Ground and longhorned beetles (order Coleoptera), cockroaches (order Blattodia). Grasshoppers (Orthoptera).

Course name: Fundamentals of Entomology,1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

5. Geniculate or Elbow like Geniculate antennae are hinged or bent like an elbow.Bents abruptly at an angel at the distal end of scape forming bent like knee or elbow. Examples: Bees and ants (Hymenoptera), Weevils ( Coleoptera).

geniculate

6. Lamellate or plate like The last three segments extend in one side forming a leaf like structure. Examples: Scarab beetles (order: Coleoptera).

8. Pectinate or comb shape The segments of pectinate antennae are longer on one side, giving each antennae a comb-like shape. The term pectinate derives from the Latin pectin, meaning comb. Examples: Fire-colored beetles and fireflies (order: Coleoptera).

Course name: Fundamentals of Entomology,1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

9. Plumose or feather like Segments produce bunch of hairs from each joints. Also called pinnate or Bipectinate Examples: Moths (order Lepidoptera)and male mosquitoes (order Diptera).

10. Pilose: (Hairy) Antenna is less feathery with few hairs at the junction of flagellomeres. e.g. Female mosquito.

Course name: Fundamentals of Entomology,1st Year 2nd Sem. R.K. Panse, Asstt. Prof. (Entomology), College of Agriculture, Balaghat, MP

11. Serrate Serrate antennae have a saw-toothed shape. Examples: Click beetles, mango stem .borers (order Coleoptera).

2. Setaceous or bristle like The size of segments decrease from the base to apex. Examples: Dragonflies and damselflies (order Odonata), cicada (Hemiptera), mayflies (order Ephemeroptera).