VIII 



ARTHROPODA 



271 



Galerucella, but it has been carefully worked out iu Doryphora by 

 Tower (1909). The first traces of the wings appear before the 

 embryo is hatched as small oval piguiented areas situated on the 

 sides of the mesothorax and metathorax, just below the spot where, 

 later, the tergum ends aud the pleuron begins. They are therefore 

 about the same level as the 



stigmata which are situated 



the 



imoeb 



1C 



FIG. 216. Portion of the abdominal muscles of 

 the larva of (Jalentcelln. itfini undergoing 

 histolysis. (After Poyarkotf. ) 



on the segments of 

 abdomen. When the areas 

 thus marked out are ex- 

 amined by sections they are 

 found to consist of elongated 

 columnar ectoderm cells- 

 distinguished from the other 

 ectoderm cells merely by 

 their depth and their slender 

 character, so that they appear i' 

 crowded together. In each 

 area a circular pit appears 

 which lengthens to form a 

 longitudinal furrow. This 

 furrow deepens till the whole 

 imaginal disc consists of a 

 longitudinal fold of ectoderm 

 with merely a virtual cavity. 

 The dorsal limb of this fold 



1 ,i -i ,i ,1 iini'ii'h, amoebocytes which ingest the larval muscles; 



Clie i.c, imaginal cells, which build up adult muscle; Z.miwc, 

 Ventral, and from this dorsal larval muscle fibres ; l.n, larval nuclei. 



limb, during larval life, the 



actual wing rudiment grows out as a protuberant fold of ectoderm. 

 The thinner ventral limb of the imagiual fold, which forms what 

 we may term the wing sac, is passively stretched during the process ; 

 but not until the moult takes place which transforms the larva into 

 a pupa do the limbs of the original fold come apart and allow the 

 wing to protrude as an external appendage. 



The wing consists at first merely of two layers of pillar-shaped 

 ectoderm cells each underlaid by a thin layer of mesoderm, such as 

 everywhere underlies the ectoderm, forming a kind of dermis. The 

 layer of mesoderm secretes a strong basement membrane on which 

 the inner ends of these cells rest. The two basement membranes, 

 corresponding to the two sides of the wing fold, are mostly in contact 

 with one another, but in certain places they remain separated, leaving 

 in this way spaces, bounded on both sides by basement membrane, 

 some of which are in open communication with the general body- 

 cavity. 



These spaces are the primary veins of the wing ; they become 

 much swollen during the pupal moult, for the abdomen contracts 

 and its contained blood is necessarily forced forwards. At the same 



