632 



CLASS IV. INSECTA. 



number and position are subject to great variations. The air to 

 a slight degree doubtless enters by diffusion, but insects also 

 exhibit very marked respiratory movements ; the abdomen con- 

 tracts and expands many times a minute and these muscular 

 movements doubtless assist the passage of the air up and down 

 the tracheae. Except in certain Thysanura the tracheal system 

 associated with one pair of stigmata communicates with all the 

 others by means of longitudinal passages (Fig. 386). Thus if by 

 accident one stigma be blocked, the parts supplied through it 

 receive air from the neighbouring stigmata. 



5 



FIG. 388. Dorsal view of the lirain 

 of Melanoplus jemur-rubrum mag- 

 nified (after E. Burgess). 1 ocelli ; 

 2 median ocellar nerve ; 3 antenna 

 nerve ; 4 antenna or olfactory 

 lobe ; 5 ganglion of sympa- 

 thetic nerve ; 6 oesophageal com- 

 missure to sub -oesophageal gan- 

 glia ; 7 nerve to labrum ; 8 cross 

 nerve or commissure between the 

 brain and sub-oespphageal gan- 

 glia ; 9 optic ganglia. 



FIG. 389. Anterior (ventral) aspect of the 

 developed brain of Oedi-poda (after Vial- 

 lanes), c circum-oesophageal commis- 

 sure ; c' transverse commissure behind the 

 oesophagus ; dc deutocerebrum ; go optic 

 ganglion ; If labro-frontal nerve ; na' an- 

 tennal nerve ; na" accessory antennal 

 nerve ; no nerves of the three ocelli ; pc 

 protocerebrum ; r root of the paired 

 stomato-gastric ganglion ; tc tritocerebrum. 



In the larvae of many aquatic and certain parasitic insects 

 the tracheal system is closed and this secondary condition is 

 termed apneustic as opposed to the normal state of affairs known 

 as holopneustic. The respiration then takes place through the thin 

 integument of the body or through special tubular or leaf -like 

 gills, which are richly supplied with tracheae and present a large 

 surface to the oxygen-containing water. This general respira- 

 tion is supplemented in the Odonata larvae by a marked anal 

 respiration. When tracheae are present in aquatic larvae they 

 are often much modified in position and their stigmata may 



