ESPM 134Insects 2

Introduction to the Insects: Structure and Function

Exoskeleton: The rigid integument forms the exoskeleton and consists ofEpidermis (single layer of cells)Procuticle (chitin and protein, 0.1-0.5mm) secreted by the epidermisEpicuticle (lipids and hydrocarbons to prevent desiccation, 0.1mm), maintained by dermalglands in epidermisFlexible intersegmental membranes allow for growth and movement

External anatomy: Adult insects have three distinct body regionsHead bears mouthparts, often adapted for different modes of feeding, sensory organs includingantennae, palps and eyesThorax comprises three segments (pro, meso and meta) designed for movement, a pair of legson each segment, and a pair of wings on the meso and metathoraxAbdomen comprised of 7-11 segments encloses the reproductive system and support thegenitalia

Mouthparts: Adapted for different types of feedingChewing - heavily chitinized laterally moving mandiblesPiercing and sucking - needle like stylets formed from different mouthpart structuresSucking - an extensible proboscis among Lepidoptera, sponging beak among DipteraChewing and lapping - extended labial palps form a tongue among bees

Flight: Greatly enhances mobility and diversification through specializationThe thorax is a box with sides (pleura) and base (sternum) rigidly fused and wings attached tothe sides and a rigid top (tergum) by a flexible membrane (resilin)Direct muscles attached to wings used in primitive insects (dragonflies, cockroaches)Indirect muscles used by most insects using movement of tergum to lever the wings

Digestive system: The alimentary canal extending from mouth to anus consists ofForegut (pharynx, esophagus, crop, proventriculus) used primarily for storageMidgut (gastric caeca, ventriculus, peritrophic membrane) digestion and absorptionHindgut (Malpighian tubules, large & small intestine, rectum) disposal of nitrogenous wastesand resorption of water and valuable solutes (filter chamber for Hemiptera)Symbionts common among wood feeders (termites, beetles) for digestion of cellulose, and sap-suckers for enhancement of nutrition

Circulatory system: An open system consisting ofHemolymph (body fluid) consisting of plasma (aqueous solution of nutrients) and hemocytes(ingest small particles, encapsulate parasites and foreign materials, coagulation, storage anddistribution of nutrients)Circulation maintained by dorsal vessel with ostia (1-way pumps, forward) in abdominalsegments

Respiratory system: Intake of O2 and removal of CO2 via an air-filled tracheal systemClosable spiracles open laterally to the air (2 thoracic and 6-7 abdominal)Trachea (invaginations of the epidermis lined with cuticle) ramify through the bodyGas exchange via diffusion, limits the size of insects

Nervous and sensory system: Consists ofCentral nervous system (brain, subesophageal ganglion, ventral nerve cord with ganglia inthoracic and abdominal segments)Sensory organs (sensilla to detect mechanical, thermal, chemical and visual stimuli)

Endocrine system, voltinism and diapause:Endocrine system based on hormones with release regulated by neuropeptidesBrain produces brain hormone (PTTH), stored and released from corpora cardiaca to controlthe prothoracic glandProthoracic gland produces molting hormone (ecdysone) to control moltsCorpora allata produce juvenile hormone (JH) to control timing of metamorphosis and regulatereproduction and diapauseVoltinism (generations per year) varies from 1 per 2 years (wood borers) to 10-12 per year(some sap suckers)Development rates and voltinism affected by temperature (predictable by degree days)Development may be interrupted by a period of dormancy – hibernation (winter), aestivation(summer), quiescence (controlled direct by unfavorable conditions), diapause (adaptivephysiological change obligatory each generation for univoltine species, facultative inmultivoltine species)Diapause – induced by photoperiod, temperature, food quality, moisture, can extend multipleyears in forest insects

Reproductive system, mate finding, modes of reproduction: Most insects are bisexualFemale system, paired ovaries consisting of ovarioles empty mature eggs into oviducts,subsequently fertilized by sperm from a spermathecaMale system, paired testes produce sperm stored in a seminal vesicle, with accessory glandsproducing a spermatophore (package surrounding sperm)Swarms of males formed by many Diptera some Lepidoptera around landmarksLeks or male defended territories are common in some Diptera (fruit flies)Sex pheromones are odor signals produced by virgin females to attract males of the samespeciesReproduction mostly sexual, but can be hermaphroditic (margarodid scales), parthenogenetic(aphids) or haplodiploid (fertilized→female, unfertilized→male, Hymenoptera) – sex ratiodistortion by Wolbachia

ReferencesEdmond, Agee & Gara (2000) Forest Health and Protection - Chapter 18For additional information seeGullan & Cranston (2000) The Insects: an Outline of Entomology