12 ON THE ANATOMY OF THE FLY. 



The chemical composition of the indurated mesodenn is but 

 partially understood. It is commonly said to be composed of 

 chitine, from its resemblance to a coat of mail. The induration 

 is partly due to animal tissue and partly to deposits of earthy 

 salts, especially phosphate of lime, which form beautiful double 

 refracting crystals in the wing cases of many beetles.* The animal 

 tissue is unlike the horny matter of vertebrates in being insoluble in 

 a hot solution of caustic potash, thus resembling vegetable cellulose 

 from which it is said to differ, however, in containing nitrogen. 



The hairs with which certain parts of the body are clothed 

 are developed from the three layers of the integument. The bulb 

 is a single cell of the eiidoderm, and its cavity is continuous with 

 the hollow cavity of the hair. The hair itself is composed of 

 fibres which run through its entire length, giving it a channelled 

 appearance when sufficiently transparent. These are probably 

 developed from cells or nuclei of the mesodenn; whilst the 

 whole is invested by the protoderm. The hairs in the pupa at 

 the end of the first week consist only of hollow processes of 

 the protoderm filled with very transparent cells or granules. 

 No hairs arc developed on the pupa skin. 



The base of each hair is surrounded by a thickened ring; this 

 is quite apparent from its earliest appearance, when the ring 

 exists only as a fold of the protoderm. 



* I have used the term Crystal for these circular bodies , because I 

 believe they are similar to those remarkable bodies which were first pro- 

 duced artificially by the late Mr. Rainey by the crystalization of salts in 

 viscid fluids, and which are now known to abound in the animal kingdom. 

 Some naturalists think these sppts are only due to tension of the organic 

 substance of the elytra, but Mr. Hislop, who has paid the subject much 

 attention, tells me that their out-line is far more definite, when seen with 

 polarized light, than that of any spots he has seen produced by tension 

 alone, and that they disappear entirely when the wing-cases are treated 

 with chlorine or strong acids, whilst the lines of tension, which are distinctly 

 visible in the same elytra, are not effected by these reagents. 



