135 



plate of the organ. A large muscle arises on each 

 side, and is inserted into each of the lateral plates. 



In the thorax, a small muscular band rises 

 from the neighbourhood of the first pair of legs, 

 and passes upwards close to and outside the sali- 

 vary glands of each side. The contraction of this 

 band must exert pressure upon the salivary glands. 



Anteriorly and posteriorly small muscular 

 bundles pass from the dilated portion of the mid- 

 gut to the abdominal wall. 



The Tracheal Syste 



M 



Respiration is entirely carried on by tracheae. 

 These, take their origin from external openings — 

 the spiracles — and eventually terminate in minute 

 capillaries in the actual tissues of the insect. In 

 the Culicidae there is no development of large or 

 multiple air sacs in connexion with the tracheal 

 system, as in many insects. 



The spiracles are placed both in the thorax 

 and in the abdomen. The thoracic spiracles are 

 two in number, situated in the meso-thoracic and 

 meta-thoracic segment, respectively. Of these the 

 anterior one is the largest in the body. The 

 second thoracic spiracle is also much larger than 

 the abdominal spiracles. The abdominal spiracles 

 are situated in the pleural membrane, one in each 

 segment (Fig. 42). 



The Tracheae. — Very large tracheae pass 

 inwards from the anterior thoracic spiracles. 



I . A large branch passes forwards towards the 

 neck and gives off a branch which passes down on 

 either side of the middle line to the two anterior 

 coxae and the salivary glands. The main branch 



