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The smaller tubes contain the spiral thread 

 until they become from 2 to 5 M in diameter. 

 They then divide to form bundles of excessively 

 minute air capillaries, which enter among the 

 tissue cells. The division into capillaries takes 

 place in the substance of large branched cells 

 situated at the termination of the tracheal vessels. 

 The cells often appear cribriform in section from 

 the number of air capillaries. These cribriform 

 cells in connexion with the tracheal endings are 

 well seen in the mid- gut and malpighian tubules. 

 They are, however, seen best of all in the unde- 

 veloped ovary of the newly- hatched mosquito, 

 which is extremely rich in bundles of capillary 

 air tubes. 



The Vascular System. — The dorsal vessel is a 

 delicate walled tuloe composed of longitudinal 

 and oblique fibres with a nucleated inner layer. 

 The fibres may be traced directly from the termi- 

 nations of the branched alary muscle fibres. The 

 alary fibres break up into fibres which pass in 

 close connexion with the large pericardial cells, and 

 eventually form (i) fibres passing into the dorsal 

 vessel as longitudinal fibres, (2) fibres joining in 

 an anastomosis in connexion with the floor of 

 the dorsal vessel. 



The pericardial cells are extremely large cells 

 lying on either side of the dorsal vessel through- 

 out its whole extent. They are by far the largest 

 cells in the mosquito, varying from 30 m to 50 m in 

 longitudinal diameter. They are elongate or pear- 

 shape in form, and contain several nuclei. The 

 nuclei usually show signs of degeneration. The 

 peripheral portion of the cell stains more deeply 

 than the central portion, which contains the 



