80 MOSQUITOES OF NORTH AMERICA 



among the salivary glands and other viscera. Other smaller masses lie in the 

 head and abdomen." 



THE TRACHEAL SYSTEM. 

 There are two main tracheal trunks, passing along each side of the body near 

 the digestive canal. A branch is sent off to each spiracle and other branches pass 

 to the different organs and ramify through the body. The thoracic spiracles 

 are described on another page in connection with their function in the produc- 

 tion of sound. Both spiracles and tracheas of the thorax are greatly developed. 

 The abdominal spiracles are very small and the tracheal trunks are much more 

 slender in this part of the body than in the thoracic region. No large air-sacs 

 belonging to the tracheal system are present. The branches of the tracheal 

 system are described by Christophers as follows : 



" The Trachece. — Very large trachese pass inwards from the anterior thoracic 

 spiracles. 



" 1. 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 continues on through the neck, and sup- 

 plies the head with numerous large branches. 



" 2. A large branch passes upwards and backwards along the edge of the meso- 

 Bcutum, and gives off branches which supply the wing muscles. A smaller 

 branch also passes forwards and supplies the muscles of the thorax. 



" 3. The largest trachea in the body (main trachea) passes downwards, back- 

 wards, and inwards, so as to lie on either side of the anterior portion of the 

 alimentary canal. Numerous branches are given off from this trunk to the 

 thoracic muscles, the alimentary canal, and legs. Posteriorly the trunk is con- 

 tinuous with a trachea passing forwards from the second thoracic spiracle, thus 

 forming on either side a large tracheal loop. 



" Large tracheae also pass inwards from the posterior thoracic spiracles. 



" 1. Branches pass forwards and join in a loop with the main trachea, also 

 backwards to join the abdominal system. 



" 2. Branches pass downwards to the meta-thorax and posterior pair of legs. 



" 3. Branches pass inwards to the muscles and mid-gut. 



" From each abdominal spiracle a short thick trunk passes inwards which 

 gives rise to the following branches : — 



" A dorsal branch ramifying beneath the tergum and joining the branch of the 

 opposite side. 



" A sternal branch supplying the sternal plate and muscles, also joining the 

 branch of the other side. 



" Loop branches passing to the trunks anterior and posterior. 



" Branches passing inwards and supplying viscera. Branches from the first, 

 second, third, and fourth abdominal trachea supply mainly the mid-gut, those 

 from the fourth and fifth the ovaries, those from the sixth and seventh the 

 genital organs." 



" HISTOLOGY. 



"THE ALIMENTARY CANAL AND APPENDAGES. 



" The epithelial lining differs considerably in the mid-gut from either the 

 fore-gut or hind-gut. In the mid-gut the possession of a marked striated border 

 by the epithelial cells is characteristic. The muscular fibres of the alimentary 

 canal are striated throughout. 



'' The Fore-gut. — The anterior portion of the fore-gut is lined by chitin and 

 does not differ from the cuticle in structure. It consists of a single layer of 

 cubical cells of small size. The oesophageal dilatation and its diverticula re- 



