The term 'inside opening' can refer to two different crossword solutions: ENTO and ENDO. ENTO is an abbreviation for the word 'entomology, ' which is the scientific study of insects. ENDO is an abbreviation for the word 'endodontics, ' which is a branch of dentistry that deals with the diagnosis and treatment of diseases of the dental pulp. Both terms refer to the study of something from the inside, hence the phrase 'inside opening.'