GROWTH AND TRANSFORMATION 159 



Folsom, 1900) between the mandibles and maxillae. The 

 bearings of such facts on speculations about the racial history 

 of insects will be discussed in later chapters. 



The embryonic development of an insect is closed by 

 the experience of hatching (Plate VI, B) which introduces 

 the young creature into the outer world A necessary pre- 

 liminary to hatching, as a rule, is the rupture of the egg- 

 case or shell. This may be brought about by inflation of 

 the cuticle of the neck region just behind the head, so as to 

 burst the egg- case, as in the emergence of the young grass- 

 hopper, or by the young insect biting a hole through the 

 shell with its mandibles or, as in the case of certain fly- 

 maggots, with its mouth- hooks or with a hard and sharp 

 mouth-spine ; or special spinose processes or ridges — 

 " hatching spines " — may be present on the head or 

 prothorax. Such structures, the purpose which they serve, 

 and the manner in which they work are striking instances of 

 what is constantly noticed by students of the life of insects — 

 an apparent prevision of the needs of the creature in succeed- 

 ing stages of its development, so that it finds itself furnished 

 beforehand with the instruments needed for the next act in 

 its life-drama. The whole course of embryonic develop- 

 ment, which has been briefly sketched in the preceding 

 pages, may be regarded as a series of successive events each 

 leading on and preparing for that next to follow, and tending 

 to the construction of the young insect which is to be 

 hatched in due season. Remembering that the process is 

 essentially one brought about by the division and specialisa- 

 tion of cells, we are able to catch some glimpse of the 

 mechanism of the process as we remember that all these 

 cells are derived from the fertilised egg- cell with its complex 

 of inherited factors that render possible the continuity of 

 the racial characters through an innumerable series of 

 generations. The nature of the germ-plasm is such that 

 it can serve as the means for ensuring that the embryo 

 passes through a series of progressive stages that are generally 

 in the same order and of the same nature as those through 

 which its parents and its ancestors passed, each fresh step 



