PHYLUM ARTHROPODA 



649 



blast 



ser 



amn 



amn.f 



Metamorphosis. — In some instances the young Insect, when 

 it escapes from the egg-mem- 

 branes, has exactly the form of (rast __^^R^^^ r -' vent -P^ 

 the parent, except that, as a 

 rule, the wings have not yet 

 grown. But in most cases 

 there is a metamorphosis. In 

 some this is comparatively 

 slight and gradual, the adult 

 Insect differing from the larva 

 only in comparatively unim- 

 portant points, and the seg- 

 ments and appendages of the 

 latter becoming directly con- 

 verted into those of the former. 

 Such a metamorphosis, in 

 which there is no quiescent 

 stage, is said to be incomplete. 

 The term complete is applied 

 to the metamorphosis of the 

 majority of Insects, in which 

 the larva differs so completely 

 from the imago, or perfect In- 

 sect, in external form, the 

 nature of the appendages, and 

 the internal organisation, that 

 there is need of a quiescent 

 or pupa stage, during which 

 the whole animal, or a con- 

 siderable part of it, undergoes 

 an entire transformation. The 

 metamorphosis is complete in 

 the Diptera, Lepidoptera, Cole- 

 optera, and Hymenoptera, ab- 

 sent or incomplete in the 

 other orders. In the most 

 lowly organised larvae (many 

 Diptera) the body of the larva 

 or "maggot" is completely 

 worm-like, without any ap- 

 pendages, and without any dis- 

 tinct head. In other cases 

 (Lepidoptera, &c), there is a 

 distinct head ; the three thor- 

 acic segments have three pairs 

 of jointed legs, and the abdominal segments short unjointed pro- 

 legs (Fig. 513). In most instances the larva? differ widely from the 



blast 



ect 



amn ser 



Fig. 528. — A — C, transverse sections through the 

 developing ovum of an Insect at successive 

 stages to show the mode of development of 

 the germinal layers and of the amnion. 

 amn. amnion ; amn. f. fold of the amnion ; 

 amn. cav. cavity of the amnion ; blunt, blasto- 

 derm covering the yolk ; ect. ectoderm ; end. 

 endoderm ; yast. invagination of ventral 

 plate ; *er. serosa ; vent. pi. ventral plate ; 

 yk. yolk. (After Korechelt and Heider.) 



