52 



METAMORPHOSIS 



progressive development of metamerism in insect embryos. In 

 a broad general way, segmentation commences anteriorly and 

 extends backwards as development proceeds. The head and 

 thorax, for example, are usually clearly defined, along with their 

 developing segmental appendages, before the abdomen has 

 undergone any marked differentiation. Although embryonic 

 development is a continuous process, and none of its phases are 

 sharply marked off from another, three important stages or 

 " landmarks " in the process can be recognised in many insects. 

 These are termed by Berlese the protopod, polypod and oligopod 



phases respectively. 



In the protopod phase (Fig. 31, A) 

 the embryo, as a whole, is as yet 

 incompletely differentiated both 

 externally and internally. Although 

 the head and thorax bear evident 

 limb rudiments, the abdomen as a 

 rule is imperfectly segmented at 

 this stage, and is always devoid of 

 appendages. In the earliest aspect 

 of the phase the abdomen may 

 betray no evidences of metamerism, 

 but as development advances a 

 greater or smaller number of its 

 segments become defined : in 

 Berlese's terminology, growth pro- 

 ceeds from an oligomerous to a polymerous condition. The 

 internal organs are not fully differentiated, and even their 

 fundaments may be, as yet, very rudimentary, particularly as 

 regards the digestive, circulatory and nervous systems, while the 

 tracheal invaginations have yet to be formed. 



The polypod phase (Fig. 31, B) follows upon the protopod, and its 

 chief characteristic is the completely segmented abdomen, each 

 somite bearing a pair of rudimentary limbs. The tracheal system 

 now becomes evident by the presence of spiracular invaginations, 

 while the digestive, nervous and circulatory organs are clearly 

 defined and more or less completely formed. 



Fig. 31. Embryonic 

 according to Berlese. 



topod 

 oligopod. 



B, polypod 



phases 

 A, pro- 



