The insects play important role in conserving ecosystems. Insects like fruit fly, Drosophila, are used in the laboratory to study genetics; commercially used to produce important products like dyes, silk, and honey and the deliberate introduction of insect diseases into populations of insect pests involves knowledge of microbiology and biochemistry as well as entomology. Entomology has made contributions to almost all life sciences, from ecology and taxonomy to genetics, molecular biology and medicine. The science comprises many sub-disciplines including, among its more theoretical aspects, insect morphology, taxonomy, ecology, ethology, medical, agriculture, physiology and biochemistry. The present book has been designed to understand an overall structure of entomology. The book includes the Section I: Basic Origin of Insects from Arthropods, Section II: Insect Morphology, Section III: Insect Physiology and Section IV: Insect Systematic and Classification. The book has been well illustrated and photographs have been included where ever it is necessary. The book also includes selected bibliography and glossary of entomological terms. The book appended with various kinds of agricultural pests for students.