122 INVERTEBRATE ZOOLOGY. 



pan. Cover with water, as alcohol exjpels the air from the tubes and renders 

 them hard to see. Look for a system of fine tubes and sacks, which appear 

 silvery- white owing to the air confined within them. These are the tracheal 

 tubes, or tracheae, which extend from the spiracles to every part. They vary 

 greatly in size and shape, and may be thus placed under three categories : 



1. Simple tnbnles. These are the most general, and in this form they 



ramify every organ and form a capillary net-work of 

 extreme fineness. Different branches often anasto- 

 mose with each other. 



2. Dilated tracheae. These are local enlargements of the above and are 



foiTud frequently along the main tubes. An impor- 

 tant branch often begins as a dilated trachea. 



3. Air-sacs or vesicles. These are sac-shaped expansions, connected with 



the main system by small tubes. Their tracheal na- 

 ture is often emphasized by the appearance of 

 branching tubules which spring from the air-sacs 

 themselves. 

 The respiratory system is too complex to be followed throughout by dissec- 

 tion. The following series of dissections is given, from which the student 

 may select for special study according to the condition of the specimen used. 

 Make drawings. 



1. System of abdominal air-sacs. There are five pairs of these connected 

 with abdominal sj^iracles 2-6. They differ considerably in the two sexes, as 

 follows : 



Male. The last three pairs lie along the sides of the testis. Dorsally they 

 often give rise to branches which supply the superficial portion of 

 the testis. Ventrally they may be traced to the spiracles from which 

 they arise. 



Female. Here the sacs are very superficial, fiattened, and often pinkish 

 in color. They lie above the mass of ovarial tubules and connect 

 with the opposite sacs by a series of fine tubes forming a diamond 

 pattern. 



2. Thoracic air-sacs. There are two pairs of these, of which the anterior 

 are enormous. They lie directly beneath the pronotum, and are often seen 

 projecting beyond its anterior edge in specimens from which the head has 

 been violently torn. The posterior pair lie in the meta-thorax and are much 

 smaller. 



