i6o THE BIOLOGY OF INSECTS 



in the process following as a response to some internal or 

 external stimulus. The course of embryonic development, 

 like the programme of instinctive behaviour illustrated in a 

 previous chapter (p. 107), suggests to many students the 

 thought of a '' racial memory." 



As already mentioned in this chapter (p. 150), the mode 

 of growth of insects after hatching or birth is an especially 

 characteristic feature of their life, and the transformation 

 (metamorphosis) which in many cases is an accompaniment 

 of this post-embryonic growth, has, from early times, 

 arrested the eager attention of observant people. A brief 

 introductory survey of the subject has been given in our first 

 chapter (pp. 10—12) ; now it requires to be discussed with 

 some fullness and in sufficient detail for appreciation of the 

 essential problems that it presents to the student. It has 

 been noted that an insect's cuticle, being an outer secretion 

 of the skin and not a sheet of living tissue, cannot grow with 

 the creature's growth ; therefore it must be periodically shed 

 and renew^ed. Thus the life-history of an insect is marked 

 by a series of '' moults " (ecdyses), which divide the period 

 of its growth into a series of stages. Before the actual 

 moulting process the skin sinks away from the old cuticle, 

 pouring out a '' moulting fluid," the secretion in some cases 

 at least, as J. Gonin (1894) and W. L. Tower (1906) have 

 shown, of special large unicellular glands of the skin. Thus 

 there is a fluid-filled space between skin and cuticle. The 

 body now grows quickly, the body- wall, as it is still im- 

 prisoned in the cuticle, being of necessity thrown into 

 ridges and furrows to a greater or less degree. Then the 

 skin begins to secrete a new cuticle beneath the old one ; 

 this is at first soft and flexible, and necessarily follows the 

 folds and wrinkles of the skin. By the pressure of the 

 accumulated fluid the old cuticle is burst open, generally 

 along a median suture or " line of weakness " in the dorsal 

 region of the thorax ; and through the slit thus made the 

 creature in its new cuticle emerges ; as a rule the dorsal 

 region of the thorax comes first, then the head with its ap- 

 pendages, then the legs, and lastly the abdomen (Plate I, A), 



