THE MOULTING FLUID 613 



Gonin has fully confirmed Newport's discovery of the exuvial 

 fluid. He states that during pupation the outside of the pupa, espe- 

 cially the parts of the head and thorax " is coated with a viscous 

 liquid secreted by special glands." The parts only harden sub- 

 sequent to pupation after exposure to the air (p. 41). His observa- 

 tions were made under the direction of Professor Bugnion, who 

 kindly writes us : 



" M. Gonin has proved the formation of a liquid which passes under the 

 cuticle al the time of the last moult and facilitates exuviation. We think that 

 this liquid is secreted by large cells (unicellular glands) which we see especially 

 on the surface of segments 1-3. These cells form part of the hypoderinis, and 

 their pores open under the cuticle." 



In a subsequent letter enclosing a sketch kindly made for me by M. Gonin 

 (Fig. 500), Professor Bugnion writes me Aug. 24, 1897, regarding the functions 

 of the large hypodermal cells (I. /*//), as follows: "It seems to me, in fact, 

 after having again examined the sections, that the function of these cells is 

 not sufficiently elucidated. Indeed these cells occur only in the section passing 

 through the 1st segment, between the head and 1st thoracic segment. It would 

 seem, if these cells supply the liquid which lubricates the surface at the time 

 of ecdysis, that they should be spread over the entire surface of the body. More- 

 over, these cells have no distinct orifice, and although there is seen at times to 

 issue streams of a substance (coagulated by the reagents), they cannot be com- 

 pared with true unicellular glands like those of the epidermis of fishes, amphib- 

 ians, etc. 



" On the other hand, if it is the blood which oozes out on the surface (accord- 

 ing to your hypothesis), it would seem that the loss of blood would cause the 

 death of the larva. I believe then it is due to the secretion of the hypoderinis 

 which spreads over the whole surface when the cells are still soft (not yet hard- 

 ened from contact with the air). At all events, there is a liquid spread over the 

 surface ; it is this liquid which glues the wings and the legs to the body at the 

 moment the caterpillar issues from the rent in its skin. If at this instant we 

 plunge the pupa in the water the liquid is dissolved, and the feet, wings, etc., 

 are not glued to the body." 



Dr. T. A. Chapman also writes us: "There is no question about the exist- 

 ence of a fluid between the two skins at moulting. In hairy larvae the hairs 

 are always wet at first, or if the skin be renewed rather more quickly than the 

 larva does it naturally, the wetness of both surfaces is obvious. I do not know 

 the nature of the fluid, but it is related to that which hardens into the dense 

 pupal case, and also hardens in a less degree the skin of the larva. I suppose 

 it must contain some chitine in a soluble form. If a newly cast larva skin be 

 taken, there is no difficulty in extending the shrivelled mass to its full length 

 and dimensions, but if a short time elapses, this chitine hardens, and the skin 

 cannot be extended after soaking in water, alcohol, ammonia, or any other sol- 

 vent I have tried." 



It has been stated that there is a subimaginal pellicle in Lepidop- 

 tera, but as Dr. Chapman writes me, "what has been observed has 

 been some of the inner pupal dissepiments, such as the pupal cases 

 of the under wings," etc. They may be observed in the head of the 

 tineid pupae, and other small moths. We have thought that the 



