CHAPTER VI. 



THE TOPOGRAPHICAL ANATOMY OF THE MUSCLES AND 

 VISCERA OF THE IMAGO (PL XI.).* 



The cavities of the head, thorax and abdomen are sharply 

 defined in the adult fly by the narrow apertures of communica- 

 tion between the head and thorax and the thorax and abdomen ; 

 but, as has been already observed, the thoracic cavity does not 

 correspond with that of less modified insects, as it is separated 

 from the abdomen by the mesophagma, so that the upper part 

 of the metathorax, much reduced in magnitude, is merged with 

 the abdominal cavity. 



The Cephalic Cavity is divided incompletely by the tentorium 

 into two parts — an upper portion, which contains the brain, 

 the optic nerves, retinae of the compound eyes, the compound 



* The intention of the present chapter is to give a brief account of the 

 viscera, and a somewhat more detailed one of the muscular system of the 

 imago. I have purposely avoided all details, and the discussion of doubtful 

 points. My object is to give those who are not familiar with the anatomy 

 of insects the information needful before entering upon a consideration of 

 the developmental history of the embryo and the imago ; a knowledge 

 of which will materially assist the student in determining the value of many 

 of the observations, which will find their place in the detailed description 

 of the several organs and systems, to form the subject of a second 

 volume. The seventh chapter of this volume will be devoted to so 

 much of the embryology of the insect in the egg as is involved in the 

 general formation of the body, and will serve as an introduction to the 

 ninth chapter, which will treat of the development of the nymph and 

 the imago, not including the development of the various systems of organs. 

 The eighth chapter will treat of the histology of the tissues ; the remainder 

 of this work will be devoted to a detailed description of the several organs 

 and systems of organs ; the development of these in the egg, larva, pupa and 

 imago ; and, as far as possible, the functions which they perform in the 

 various processes of digestion, secretion, and sensation. 



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