THE INTERNAL STRUCTURE OF THE LEPIDOPTEROUS PUPA. 65 



soften their cocoons with a special fluid. Chapman notes that he has 

 examined many specimens of P. cecropia at the moment of emergence, 

 having first taken the pupa from the cocoon. As soon as the pupal 

 skin bursts, the head of the imago appears ; the red hairs at the front 

 of the head are at first moist, but soon become quite wet, and, if 

 removed, the fluid is replenished to the quantity of at least one minim, 

 and it is remarkable that, although the wool on the head is as wet as 

 a sponge, the scales of the collar and prolegs which touch it remain 

 quite dry. The fluid is colourless, slightly alkaline, and when applied to 

 the silk of the cocoon at once softens it, so that the silk can be easily 

 eased out. The fluid appears not only to soften the gum that stiffens 

 and binds the silk, but to a certain extent to destroy or neutralise it, 

 for the silk so affected remains soft and pliable. Chapman says that 

 the fluid comes from a narrow slit, separated from the wool of the face 

 by a narrow naked surface that must be the labrum, and that it is 

 prevented from reaching the eyes by means of two small projections 

 which appear to be mandibles, whilst immediately below the slit are 

 two rounded elevations which appear to be the maxilla?. This orifice 

 from which the fluid proceeds, Chapman considers must be the mouth. 



CHAPTER IV. 



THE INTERNAL STRUCTURE OF THE LEPIDOPTEROUS PUPA. 



The marvellous changes that take place in the anatomy of the 

 Lepidoptera (and other holornetabolous insects) during the larval and 

 pupal stages have been already foreshadowed in the preceding chapters, 

 and are no less important than the external differences that occur 

 during the same periods. The internal organs of an insect are, in 

 many cases, totally different in the larval and imaginal stages, and 

 these changes are usually greatest in those insects whose food require- 

 ments undergo a radical change in the imaginal state. In this respect, 

 Lepidoptera are profoundly modified, and, although less is known of 

 tbe internal changes in this than in some allied orders, considerable 

 progress has been made during the last two or three decades. 



The real nature of the internal changes wrought during the process 

 of metamorphosis was first revealed by Weismann in 1864 when he 

 discovered the " imaginal germs " or " imaginal discs " in the Diptera 

 and formulated his theory of histolysis, i.e., the almost complete 

 destruction of the larval organs by a gradual process of disintegration, 

 and tbe histogenesis or rebuilding of new organs from the imaginal 

 discs, by utilising the nutrient material obtained from the histolytic 

 products present in tbe pupa. The intermediary agents in bistolysis 

 are tbe phagocytes, cells similar to the leucocytes or corpuscles of the 

 blood, whilst the intermediary agents in histogenesis are said to be por- 

 tions of tissue existing in the larval state, incorporated with, or preserv- 

 ing a connection with, the different organs. It is these portions of tissue 

 that form the structures now generally known as the imaginal discs ; 

 they are present in very young larvae, and exist for each part of the body, 

 the appendages, wings, &c. At certain periods of the larval (or pupal) 



