THE INTEGUMENT. 11 



tcbratcs, but differ in being largely supplied with nerve filaments, 

 which ramify in a branching network amongst them. Following 

 the nomenclature I have already commenced, I think endoderm 

 an appropriate name for this innermost cuticular layer. 



The development of the niesodemi and eudoderm is usually in 

 an inverse ratio, and this is no more than we might expect, 

 seeing that they are really different states of the same tissue. 

 In the fully developed fly the cellular structure of the nieso- 

 derm is almost or entirely lost, but it can be demonstrated in 

 many cases by carefully macerating the integument, drying it, 

 and mounting it in balsam, especially when viewed by the aid of 

 the polariscope. Its cellular structure is, however, beautifully 

 apparent in the insect soon after it emerges from the pupa. The 

 whole coloration, as well as the hardness of the integument, is due 

 to the indurated niesoderm.' As has been already observed it can 

 only be separated from the protoderm in those parts of the 

 insect in which it is but slightly developed. 



The pigment with which the hard pails of the mesoderm are 

 chiefly coloured in the fly is deep blue or violet : whatever its 

 chemical nature may be, the colour depends upon oxidization. 

 It is first to be met with in the fat bodies of the larva. These are 

 perfectly white, but when cut from the larva and exposed to 

 the air they rapidly assume an inky blackness ; and the same is 

 true of the molecular fluid with which the pupa skin is filled. 

 When the perfect insect emerges from the pupa and respiration 

 again commences, the integument is nearly white, or a faint ashy 

 colour prevails. This soon gives place to the characteristic 

 blue or violet tint, first immediately around those portions 

 most largely supplied with air vessels, but soon over all those 

 parts of the insect which assume this colour; light seems to 

 have little to do with the process, but it is more rapid in warm 

 than in cold weather. The fat bodies of the larva are not per- 

 meated by air vessels, which will account for their retaining 

 their whiteness. The pigment is finely granular when examined 

 by high powers. 



