554 TEXT-BOOK OF ENTOMOLOGY 



original invagination had become fused, thus forming a continuous 

 tube, the lurnina opening into each other. (Btitschli.) 



The cuticular tracheal intima is differentiated late in embryonic 

 life. The entrance of the air is accomplished in part before the 

 embryo hatches, the air being derived from the tissues and fluids of 

 the body. 



The farther development of the tracheal branches is due to the progressive 

 formation of diverticula. The branches thus arising are intercellular formations. 

 On the other hand, the finest twigs are intercellular structures. However, as 

 Schaeffer states, the differences between the two modes of formation are not 

 important. 



Wheeler mentions the existence of " two pairs of very indistinct tracheal 

 openings in the 10th and llth somites " of the abdomen of Doryphora (Fig. 546, 

 t 19, t -20), and Heider believes that they exist in Hydrophilus. 



The tracheal invaginations as a rule begin to appear after the 

 appendages commence to bud out. An exception is met with in the 

 bee (Apis), where the tracheal ingrowths are seen before the rudi- 

 ments of the legs. Most of the tracheal invaginations appear 

 simultaneously. Only rarely do we see an indication of their suc- 

 cessive development from before backwards. Thus in Hydrophilus, 

 Graber observed that the mesothoracic stigmata appeared somewhat 

 earlier than those of the other segments. 



h. Nervous system 



The rudiments of the nervous and tracheal systems essentially con- 

 tribute to the building up of the relief of the primitive baud of 

 insects. The nervous system is the earliest to appear, being indi- 

 cated very early, in fact before the appendages begin to grow out. 

 The first traces of the nervous system are two ridges extending 

 along the primitive band, the depression between them being called 

 the primitive furrow. At an early period the segmentation is 

 observed in the primitive ridges, while widened spaces (the rudi- 

 ments of the ventral ganglia) alternate segmentally with the narrow 

 places which are the incipient longitudinal commissures (Fig. 

 527, A, g). 



The primitive ridges extend anteriorly into the head-lobes; this 

 part must be regarded as the rudiment of the cesophageal commis- 

 sure. The rudiments of the brain are from their first appearance 

 directly connected with the ventral chain of ganglia. 1 



1 Will (Aphis) and also Cholodkowsky's statement (Blatta), as well as Balfour and 

 Schimkewitch's statements that the brain is at first disconnected from the ventral 

 cord, are apparently erroneous. 



