634 



ZOOLOGY 



SECT. 



structure undergoes, together with the mode of formation of the 

 appendages, are similar in most members of the class, except that 

 in most Insects the formation of the mesoderm and endoderm is 

 associated with a more or less distinct invagination (Fig. 538). The 

 same holds good of the formation of the amnion and the further de- 

 velopment of the mesoderm and endoderm. In some cases there is 

 developed between the serosa and the true amnion a space filled 

 with yolk, and the ventral plate appears sunk within the yolk. 

 The nervous system is developed from the ectoderm in the manner 

 indicated in the account of the Cockroach (p. 615). The tracheal 

 system is derived from a series of pairs of segmentally arranged 

 ectodermal involutions (Fig. 540, st). 



* Metamorphosis. In some instances the young Insect, when 

 it escapes from the egg-membranes, has exactly the form of the 





-of 



an 



BW 



FIG. 539. A E, ventral view of five stages in the development of Hydrophilus. a and b, 

 points at which the blastopore first closes ; a/, edge of the amnion fold ; a/', caudal fold ; 

 a/", paired head-fold ; an. antenna ; es, terminal segment ; g, pit-like invagination to form 

 the rudiment of the amnion cavity; k, procephalic lobes ; r, groove-like medio-ventral in- 

 vagination ; s, germinal bands covered by the amnion. (From Lang, after Heider.) 



parent, except that, as a rule, the wings have not yet grown. But 

 in most cases there is a metamorphosis. In some this is com- 

 paratively slight and gradual, the adult Insect differing from the 

 larva only in comparatively unimportant points, and the segments 

 and appendages of the latter becoming directly converted 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 Insect, 

 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 considerable part of it, undergoes 

 an entire transformation. The metamorphosis is complete in the 

 Diptera, Lepidoptera, Coleoptera, and Hymenoptera, absent or 

 incomplete in the other orders. In many Diptera the body of the 



