151 



In certain conditions the malpighian tubule 

 cells may be found quite free from granules, 

 though otherwise unchanged. This change occurs 

 in mosquitoes with large numbers of a flagellate 

 organism (previously noted) in the rectum and 

 hind-gut. 



The Muscular System. — The muscular fibres 

 of the mosquito are without exception striated. 

 Those of the wings differ in structure very much 

 from those of the limbs and body segments. The 

 muscle fibres of the alimentary canal are large 

 fusiform cells, with a single large nucleus with some 

 surrounding protoplasm. ■ The muscle fibres in 

 connexion with 'the heart are much branched. 



Many of the fibres contain a very marked 

 sarcolemma and space between this latter and the 

 fibre. This space is usually seen occupied by 

 extremely delicate branching threads, which stain 

 feebly with haematein. 



In the pupae there exist some large cells of 

 peculiar nature in association with the sheaths of 

 the muscle fibres. 



The structure of insect muscle is described 

 in many works on histology, and does not need 

 repetition here. 



TheTracheal System. — The larger trachealves- 

 sels consist of a single layer of flattened cells with 

 an inner chitinous layer. In smaller tubes the cells 

 embrace the entire vessel, the nucleus frequently 

 being bent around the lumen. The cells of the 

 tracheal vessels contain numerous small clear 

 vacuoles (chitin formation). The chitinous lining 

 possesses a thickening in the form of a spiral thread, 

 which may become unwound and lie stretched as 

 a wavy thread in fresh preparations. 



