Insect Anatomy Lecture Notes PDF

The insect head and its appendages: The insect head is a strongly sclerotized capsule joined to the thorax by a flexible membranous neck. It bears the mouthparts and antennae. The mouthparts: The mouthparts are the organs concerned with feeding. They are comprised of: unpaired labrum (upper lip)...

The insect head and its appendages: The insect head is a strongly sclerotized capsule joined to the thorax by a flexible membranous neck. It bears the mouthparts and antennae. The mouthparts: The mouthparts are the organs concerned with feeding. They are comprised of: unpaired labrum (upper lip) in front, a hypopharynx (Tongue-like, bear's openings of salivary ducts), and a pair of mandibles (first jaws) and maxillae (second jaws) laterally, and a labium forming lower lip. The form of mouthparts is related to diet, but two types can be recognized: one adapted for biting and chewing solid food, and the other adapted for sucking up fluids. The mouthparts of the mandibulate or chewing type An example of this type is the mouthparts of grasshoppers  The labrum: It is a broad, transverse lobe attached to the head by a then articular membrane, which allows freedom of movement up and down. It covers the mandibles, prevents the escape of food, and forms a closed passage for the intake of liquids. On its inner side, it is membranous and produced into a median lobe, the epipharynx, equipped with taste organs. The mandibles (first jaws): They are strong, sclerotized and provided with sharp cutting edges. They lie below the labrum. Their action is from side to side. In Apterygota, the mandibles have only a single point of articulation with the head capsule, while in Pterygota, the mandibles articulated with the cranium at two points. The maxillae (second jaws): They are composed of several articulated units. They lie behind the mandibles, composed of: the cardo, which is a triangular basal segment by which the maxilla is attached to the head, the stipes; next to the cardo and somewhat longer carrying distally two lobes: an inner one the lacinia and an outer galea. The maxilla posses a 5-jointed palp attached to the stipes at the place of attachment of the galea. The maxillae help to push food into the mouth. The lacinia, being toothed, may help in chewing. The maxillary palp may be sensory in function. The labium ( lower lip): It forms the posterior wall of the gnathal region of the head. It consists of a board plate, the post-mentum that is divided into proximal submentum and a distal mentum. Attached to the mentum are a bilobed prementum bearing laterally a pair of 3-segmented labial palp, a median glossa and a lateral paraglossa. The labium helps in keeping the food in the mouth cavity. The labial palps are sensory in function. The hypopharynx (tongue): It is a prolongation of the floor of the mouth and is attached to the inside wall of the labium. The ducts of the salivary glands open near the hypopharynx. It seems certain that the chewing or biting type of mouthparts is the generalized one, from which the other types frequented in insects have been modified. The moth parts in various insect groups are modified to suit the type of food they subset on. The antennae: The antennae are paired segmented appendages located on the head usually below or between the compound eyes. In general, the antenna consists of three regions: the basal scape, pedicel, and a flagellum. The flagellum is divided into a number of similar annuli. Insect antennae vary greatly in size and form, depending on its precise function and are much used in classification. Most modification occurs in the flagellum. The following terms are used in describing the forms of the antennae: 1) Setaceous, bristle–like, the segments narrow up from the base to the tip, ex. the antennae of the cockroach. 2) Serrate, saw–like, the segments have short triangular projections on one side than the other ex. the antennae of the click- beetle. 3) Filiform, thread-like, the segments nearly uniform in size, ex. antenna of grasshopper. 4) Moniliform, bead-like, the segments are similar in size and more or less spherical in shape ex. the antenna of white ants. 5) Clavate, The segments increasing in diameter gradually ex., the antenna of butterfly. 6) Geniculate, elbow-like, the scape is elongated; the following segments are small and bend at a right angle to the first segment ex. the antenna o honey bee.  Geniculate Clavate Setaceous Moniliform Serrate Filiform The insect thorax: The thorax consists of three distinct segments known as prothorax, mesothorax and metathorax. In most insects all three segments bear a pair of legs. In addition, winged insects have a pair of wings on the meso- and metathoracic segments. Sclerites that comprise the thorax are given specific names; each may be preceded by the prefixes pro-, meso-, or meta-. Each thoracic segment is composed of four groups of sclerites, these are the notum: or dorsal plate, the pleuron or lateral plates and the sternum: or ventral plate. Basic structure of legs:(walking leg) coxa is in the form of cone and articulates basally with the wall of the thorax. There may be only a single articulation with the pleuron; in this case the movement of the coxa is very free. femur is the largest part in most adult insects. It is more or less fixed to the trochanter, which is a small segment. The trochanter is with a small articulation with the coxa, it can only move vertically. tibia is long articulating with the femur, it moves in a vertical plane. In most insects the tarsus consists of 2-5 tarsomeres. The basal tarsomere articulates, with the distal end of the tibia. The pretarsus consists of a median lobe and a pair of claws. ex. Walking leg in cockroaches Modifications of the basic leg structure: 1) Digging: Digging legs are best known in the mole cricket. The fore legs are very short and broad, the tibia and tarsomeres bearing stout lobes, which are used in digging in the soil or excavation. 2) Grasping: Modifications of the legs for grasping are frequent in predatory insects. The coxae are very long. Both femora and tibia are armed with spines and fitted for grasping insect preys. This occurs in the fore legs of mantis. 3) Swimming: The legs of aquatic beetles are modified in such a way that facilitates swimming. The middle and hind legs bear two rows of swimming hairs on the edges of the flattened tibia and tarsi. 4) Jumping: As the hind legs of grasshoppers. The hind femora are greatly enlarged, housing the muscles. 5) Collecting: The legs of honey bee worker bear structures that are used during their pollen- collecting activities. The tibia is broad and has a concave surface forming the pollen basket, where the pollen collected by a worker (bee bread) is stored for transport back to the hive. The first tarsal segment is large and carries rows of hairs forming the pollen brush. The pollen brushes combs off the pollen from the body and appendages. Jumping Swimming Digging Grasping Collecting The wings: The wings are outgrowth of the exoskeleton located dorso-laterally between the nota and pleura. The wings arise as sac-like outgrowth. Fully developed and functional wings occur only in adult insects. Most insect wings are more or less triangular in outline. They, therefore, present three margins: the anterior margin or costa, the outer margin or apical margin, and the inner or anal margin. Three angels are also recognized: the humeral angle at the base of costal margin, that between costal margin and the outer margin is the apex, and that between the outer margin and the inner margin is the anal angle. Each wing consists of a thin membrane supported by a system of veins. The wing form: The shape of the wings varies considerably. Examples of the shapes of wings are: 1. Membranous wing: i. The wings with narrow, petiolate bases are found in damsel flies. ii. In dragon flies as example of membranous wing, the two pairs of wings are roughly elliptical in shape and similar in form. 2. Halter In flies the hind wings are modified to form halters, which are sense organs, concerned with the maintenance of stability in flight. Each halter consists of a basal lobe, a stalk and an end knob which projects backwards from the end of the stalk.  3. Hairy The wings of small insects are reduced to straps with one or two supporting veins and long fringes of hairs. Ex. thrips 4. Elytra. The fore wings of beetles are hard, vein less and shell-like and form protective armor for the membranous hind wings when not in use. Ex. The fore wings of beetles 5. Tegmina The fore-wings are parchment-like or leathery. This condition affords protection to the hind membranous wings. Ex. is the fore wings of grasshoppers. Membranous wing: Halter Elytra. Tegmina Hairy