10 



ON THE ANATOMY OF THE FLY. 



This layer is usually called tlie cuticle or epidermis, but the 

 term is unfortunate as it is quite unlike the cuticle in vertebrates, 

 or, indeed, any structure found in them ; it is persistent and not 

 deciduous. The cells from which it is formed appear early in 

 the development of the insect and coalesce into an apparently 

 structureless membrane of extreme toughness, which has the 

 remarkable property of being quite insoluble in a hot solution 

 of caustic potash; I shall, therefore, substitute the term proto- 

 derm for this layer. 



The continuity of the protoderm is a fact well worth con- 

 sideration at the commencement of the study of the anatomy of 

 insects, as the terms insect, articulate, and articulate animal, are 

 extremely liable to mislead, and to induce one to believe that the 

 various segments are distinct and separable. Such is not really 

 the case, but certain portions of the protoderm are thickened by 

 the induration of the epithelial cells beneath it, which become 

 inseparably united to it and to each other, and so produce the 

 hard parts of the integument. 



The two layers beneath the protoderm consist of epithelial 

 cells in various stages of development ; these are best observed 

 in those parts of the integument which are transparent, as in 

 the lips of the proboscis, and between the different plates of the 

 pectus. The more superficial layer in these parts in the young 

 fly will be found to censist of a single layer of flattened angular 

 cells, containing well marked nuclei and beautifully coloured 

 with bright orange-coloured pigment. These cells are about 

 1- 1000th of an inch in diameter, and have a great tendency to 

 adhere at their edges. In the adult fly they form a continuous 

 membrane, but their nuclei become apparent on the addition 

 of a little acetic acid, I shall call this layer the mcsodcrm. 



Beneath this, numerous spheroidal nucleated cells, slightly 

 angular by mutual pressure, are disposed in a layer of several 

 cells in thickness ; they seldom contain pigment, and correspond 

 closely to the rete-mucosum or deep layers of the cuticle in vcr- 



