172 AN INTRODUCTION TO ENTOMOLOGY 



Coincident with the formation of the new epidermis, new setae 

 are formed beneath the old epidermis; these lie closely oppressed to 

 the outer surface of the new epidermis until released by the molting 

 of the old epidermis. 



In the above account only the more general features of the process of molting 

 are indicated, the details, according to the observations of Tower ('06) are as 

 follows. (See Figure 113, p. 99). In the formation of the new epidermis it appears 

 as a thin, delicate lamella, spread evenly over the entire outer surface of the 

 hypodermis; it grows rapidly in thickness until finally, just before ecdysis takes 

 place, it reaches its final thickness. After ecdysis the epidermis hardens rapidly 

 and its coloration is developed. As soon as ecdysis is over the deposition of the 

 dermis or secondary cuticula begins. This layer is a carbohydrate related 

 to cellulose, and is deposited in layers of alternating composition, through the 

 period of reconstruction and growth, during which it reaches its maximum thick- 

 ness. Preliminary to ecdysis a thin layer of molting fluid is formed, and through 

 its action the old dermis is corroded and often almost entirely destroyed, thus 

 facilitating ecdysis. This dissolving of the dermis, is, according to Tower, a most 

 constant phenomenon in ecydsis; and has been found in all insects examined by 

 him in varying degrees. 



It is said that the Collembola molt after reaching sexual maturity, 

 in this respect agreeing with the Crustacea and the "Myriapoda," and 

 differing from the Arachnida and from all other insects (Brindley '98). 



The molting fluid. As indicated above, the process of molting is 

 facilitated by the excretion of a fluid known as the molting fluid. This 

 is produced by unicellular glands (Fig. 113, p. 99) which are modified 

 hypodermal cells. These glands are found all through the life of the 

 insect and upon all parts of the body ; but are most abundant upon 

 the pronotum, and are more abundant at pupation than at any other 

 period. 



The number of postembryonic molts. A very wide range of vari- 

 ation exists as to number of molts undergone by insects after they leave 

 the egg-shell. According to Grassi ('98, p. 292), there is only a single 

 partial molt with Campodea and Japyx, while the May-fly Chloeon 

 molts twenty times. Between these extremes every condition exists. 

 Probably the majority of insects molt from four to six times; but 

 there are many records of insects that molt many more times than this. 



Stadia. The intervals between the ecdyses are called stadia. In 

 numbering the stadia, the first stadium is the period between hatching 

 and the first postembryonic ecdysis. 



Instars. The term instar is applied to the form of an insect during 

 a stadium; in numbering the instars, the form assumed by the insect 

 between hatching and the first postembryonic molt is termed the first 

 instar. 



