Entomology: Insect Anatomy and Mouthparts PDF

1 CPP 303: Entomology MOULTING PROCESS cont’d The newly emerged, or teneral, adult has soft cuticle, which permits expansion of the body surface by swallowing air, by taking air into the tracheal sacs, and by locally increasing hemolymph pressure by muscular activity. The wings normally hang down,...

1 CPP 303: Entomology MOULTING PROCESS cont’d The newly emerged, or teneral, adult has soft cuticle, which permits expansion of the body surface by swallowing air, by taking air into the tracheal sacs, and by locally increasing hemolymph pressure by muscular activity. The wings normally hang down, which aids their inflation. Pigment deposition in the cuticle and epidermal cells occurs just before or after emergence and is either linked to, or followed by, sclerotization of the body cuticle. APPENDAGES ON INSECT HEAD The antennae and mouthparts are appendages found on insect head. Insect antennae: This is a pair of mobile jointed appendages that articulate with the head either below or between the compound eyes. Antennae are present in most adults and in many immature insects. They are, however, often inconspicuous in the latter. The antennae have a strong olfactory (smelling) function, though they may also be used for taste (gustatory) and in some insects are actually their „ears‟. An insect antenna is divided into three segments; the basal segment known as the scape is often elongate and separated by an often distinct segment called the pedicel from the rest of the antenna, which forms the flagellum (the segments of which are called flagellomeres). A generalized insect antenna There are two basic types of antennae; (a) in Collembola and Diplura (apterygotes), the antennal segments, except the last one, contain intrinsic muscles. The antennae continue to grow after the embryonic stage and the segments that are closer to the head are the oldest; (b) the presence of Johnston’s organ distinguishes the second type of antennae. This organ is present in the second antennal segment (pedicel) of most insects and it responds to movement of the distal part of the antenna relative to the pedicel. Numerous sensory organs, or sensilla (singular: sensillum), in the form of hairs, pegs, pits, or cones, occur on antennae and function as chemoreceptors, mechanoreceptors, thermoreceptors, and hygroreceptors. Antennae of male insects may be more elaborate than those of the corresponding females, increasing the surface area available for detecting female sex pheromones. Insect antennae are very far from uniform in design and their variety is a great aid to  2 identification. Structures can be modified from a basic plan in adaptation to different life styles. Various types of antennae are recognizable as below: filiform - a string of almost identical segments, each rather tubular in outline moniliform - a string of beads, spherical with a clear constriction between the segments setaceous - very narrow segments, which together look more like a single bristle serrate - an analogy to saw-teeth; the segments are expanded at their tips on one side only pectinate – comb-shaped, an extreme form of „serrate‟. The expansions of each segment are extremely long clavate - the segments gradually increase in thickness towards the tip clubbed - similar to „clavate‟, but the segments are of even diameter until the last few are suddenly expanded to form an obvious „club‟ geniculate - simple or clubbed, but with a long pedicel from which the flagellum arises at nearly a right angle (elbow-shaped) lamellate - often „geniculate‟, with the flagellum segments adpressed together like the parts of a fan, and capable of opening and closing in a similar way plumose - whorls of hairs project from the joints between the segments forming a feather-like structure aristate - these antennae have a small scape and pedicel, with the flagellum represented by an obvious lozenge-shaped segment and by the bristle (arista) set back on it and projecting from it. aristate  3 Examples of insect antennae: (a) clavate; (b) filiform; (c) aristate; (d) geniculate; (e) moniliform; (f) pectinate; (g) serrate. Examples of insect antennae: (a) filiform – linear and slender; (b) moniliform – like a string of beads; (c) clavate or capitate – distinctly clubbed; (d) serrate – saw-like; (e) pectinate –  4 comb-like; (f) flabellate – fan-shaped; (g) geniculate – elbowed; (h) plumose – bearing whorls of setae; and (i) aristate – with enlarged third segment bearing a bristle. Insect mouthparts: Recall from our class discussion that the first 6 segments of the 20- segmented wormlike ancestral prototype of Phyla Arthropoda and Annelida fused together to form the insect head. The segmental origin of the head is most clearly demonstrated by the mouthparts. From anterior to posterior, the head regions gave rise to the following areas: 1 labral (from which the labrum developed); 2 antennal, with each antenna equivalent to an entire leg; 3 postantennal, fused with the antennal segment; 4 mandibular (which gave rise to the mandibles); 5 maxillary (from which the maxillae were formed); and 6 labial (which formed the labium). NOTE: The mouthparts are formed from appendages of all head segments except the second. Just like the antennae, the mouthparts serve sensory functions in addition to feeding activities. The mouthparts, other than the mandibles, are well endowed with chemoreceptors and tactile setae. The neck is mainly derived from the first part of the thorax and it is not a segment. Mouthpart structure is categorized generally according to feeding method and often, feeding structures are characteristic of all members of a genus, family, or order of insects. Thus, the knowledge of mouthparts is useful for both taxonomic classification and identification, and for ecological generalization. Types of mouthparts (I) Biting and chewing type (Mandibulate): This is found among omnivorous insects, such as cockroaches, crickets, and earwigs. It resembles the probable basic design of ancestral pterygote insects more closely than the mouthparts of the majority of modern insects such as Lepidoptera, Diptera, Hymenoptera and Hemiptera which have greatly modified and specialized mouthpart components. There are five basic components of the mouthparts: (i) labrum, or “upper lip”, with a ventral surface called the epipharynx; (ii) hypopharynx, a tongue-like structure; (iii) mandibles, or jaws; (iv) maxillae (singular: maxilla); and (v) labium, or “lower lip” The labrum forms the roof of the preoral cavity and mouth and covers the base of the mandibles; it may be formed from fusion of parts of a pair of ancestral appendages. In cockroaches, the lower dorsal area of the labrum has hair-like sensilla. The labrum has two points of attachment with the head (around points A and B in the diagram below).  5 The hypopharynx is a lobe that projects forward from the back of the preoral cavity, dividing the cavity into a dorsal food pouch, or cibarium, and a ventral salivarium into which the salivary duct opens. The mandibles, maxillae, and labium are the paired appendages of segments 4–6 and they are highly variable in structure among insect orders. The mandibles cut and crush food and may be used for defense; generally they have an apical cutting edge and the more basal molar area grinds the food. They can be extremely hard and thus many termites and beetles have no physical difficulty in boring through foils made from such common metals as copper, lead, tin, and zinc. Each mandible has two points of attachment with the head i.e. dicondylic (via the abductor and adductor tendons, C and D). Behind the mandibles lie the maxillae, each consisting of a basal part composed of the proximal cardo and the more distal stipes and, attached to the stipes, two lobes – the mesal lacinia and the lateral galea – and a lateral, segmented maxillary palp, or palpus (plural: palps or palpi). Functionally, the maxillae assist the mandibles in processing food; the pointed and sclerotized lacinae hold and macerate the food, whereas the galeae and palps bear sensory setae (mechanoreceptors) and chemoreceptors which sample items before ingestion. Each maxilla has one point of attachment with the head capsule i.e. monocondylic (at the tip of cardo, E). The appendages of the sixth segment of the head are fused with the sternum to form the labium. They are similar in structure to the maxillae but in this case, the two appendages on either side of the insect are fused along the midline to form a median plate. In prognathous insects, such as the earwig, the labium attaches to the ventral surface of the head via a ventromedial sclerotized plate called the gula. There are two main parts to the labium: the proximal postmentum, sometimes subdivided into a submentum and mentum, and the free distal prementum which bears a pair of labial palps lateral to two pairs of lobes, the mesal glossae (singular: glossa) and the more lateral paraglossae (singular: paraglossa). The glossae and paraglossae are known collectively as the ligula. The prementum with its lobes forms the floor of the preoral cavity (functionally a “lower lip”), whereas the labial palps have a sensory function, similar to that of the maxillary palps. Each half of the labium has one point of attachment with the head capsule (point F). (II) Chewing and lapping type: This type is found in bees. Lapping is a mode of feeding in which liquid or semi-liquid food adhering to a protrusible organ, or “tongue”, is transferred from substrate to mouth. The elongate and fused labial glossae form a hairy tongue, which is surrounded by the maxillary galeae and the labial palps to form a tubular proboscis containing a food canal. In feeding, the tongue is dipped into the nectar or honey, which adheres to the hairs, and then is retracted so that adhering liquid is carried into the space between the galeae and labial palps. This back-and-forth glossal movement occurs repeatedly. Movement of liquid to the mouth apparently results from the action of the cibarial pump, facilitated by each retraction of the tongue pushing liquid up the food canal.  6 C D A B E F Imaginary midline Generalized mandibulate mouthparts  7 (III) Haustellate typte: Most adult Lepidoptera and some adult flies obtain their food solely by sucking up liquids using sectorial (haustellate) mouthparts that form a proboscis or rostrum. Pumping of the liquid food is achieved by muscles of the cibarium and/or pharynx. The proboscis of moths and butterflies, formed from the greatly elongated maxillary galeae, is extended by increases in hemolymph (“blood”) pressure. It is loosely coiled by the inherent elasticity of the cuticle, but tight coiling requires contraction of intrinsic muscles (IV) Piercing/biting and sucking type: A few moths and many flies combine sucking with piercing or biting. For example, moths that pierce fruit and exceptionally suck blood (species of Noctuidae) have spines and hooks at the tip of their proboscis (modified galeae) which are rasped against the skins of either animals or fruits. In blood-feeding mosquitoes, black flies and horse flies, the labium of the adult fly forms a non-piercing sheath for the other mouthparts, which together contribute to the piercing structure. In contrast, stable flies and tsetse flies lack mandibles and maxillae and the principal piercing organ is the highly modified labium. (V) Piercing and sucking type: Other mouthpart modifications for piercing and sucking are seen in the true bugs (Hemiptera), thrips (Thysanoptera), fleas (Siphonaptera), and sucking lice (Phthiraptera: Anoplura). In each order different mouthpart components form needle-like stylets capable of piercing the plant or animal tissues upon which the insect feeds. Bugs have extremely long, thin paired mandibular and maxillary stylets, which fit together to form a flexible stylet-bundle containing a food canal and a salivary canal. Thrips have three stylets – paired maxillary stylets (laciniae) plus the left mandibular one. Sucking lice have three stylets – the hypopharyngeal (dorsal), the salivary (median), and the labial (ventral) – lying in a ventral sac of the head and opening at a small eversible proboscis armed with internal teeth that grip the host during blood-feeding. Fleas possess a single stylet derived from the epipharynx, and the laciniae of the maxillae form two long cutting blades that are ensheathed by the labial palps (VI) Filter feeding type: This occurs in aquatic insects. Larval mosquitoes and black flies obtain their food by filtering particles (including bacteria, microscopic algae, and detritus) from the water in which they live. The mouthparts have an array of setal “brushes” and/or “fans”, which generate feeding currents or trap particulate matter and then move it to the mouth. In mosquito larvae the lateral palatal brushes on the labrum generate the feeding currents. (VII) Atrophied mouthparts: In some insects with short adult lifespan such as mayflies (Ephemeroptera), some Diptera (warble flies), a few moths and male scale insects (Hemiptera: Coccoidea), mouthparts are greatly reduced and non-functional. In most mayfly species the adult only lives for 1-2 days. There is no need for the insect to feed within this short lifespan and, therefore, the adult does not possess functional mouthparts.