RESPIRATION IN INSECTS. 343 



buried among the muscles. Besides the ordinary air- sacs, 

 there is in the end of the abdomen, behind the ovaries, a 

 plexus of six dilated air-sacs (Fig. 287, I, II, III), which 

 are long, spindle-shaped, and are easily detected in dis- 

 secting. 



There is a system of dilated tracheae and about fifty air- 

 sacs in the head. 



In the legs two tracheae pass down each side of the femora, 

 sending off at quite regular intervals numerous much-branch- 

 ing, transverse twigs ; there is one large and a very small 

 trachea in the tibia, and the main one extends to the ex- 

 tremity of the last tarsal joint. 



By holding the red-legged locust in the hand, one may 

 observe the mode of breathing. During this act the por- 

 tion of the side of the body between the spiracle and the 

 pleurum (Fig. 278, A) contracts and expands ; the contrac- 

 tion of this region causes the spiracles to open. The gen- 

 eral movement is caused by the sternal moving much more 

 decidedly than the tergal portion of the abdomen. When 

 the pleural portion of the abdomen is forced out, the soft 

 pleural membranous region under the fore and hind wings 

 contracts, as does the tympanum and the membranous por- 

 tions at the base of the hind legs. When the tergum or 

 dorsal portion of the abdomen falls and the pleurum con- 

 tracts, the spiracles open ; their opening is nearly but not 

 always exactly co-ordinated with the contractions of the 

 pleurum, but as a rule they are. There were sixty-five con- 

 tractions in a minute in a locust which had been held be- 

 tween the fingers about ten minutes. It was noticed that 

 when the abdomen expanded, the air-sacs in the first ab- 



Fig. 286. Showing distribution of air-tubes (tncheae) and nir-sacs side view of 

 the oody. v, main ventral trachea (only one of thu two shown) ; *, left stigmatal 

 trachea, connecting by vertical branches with D, the left main dorsal trachea; c, left 

 cephalic trachea ; oc, ocular dilated trachea. From the first, second, third, and fourth 

 spiracles arise the first four abdominal air-pace, which are succeeded by the plexus 

 of three pairs of dilated tracheae, I, II, III, in Fig. 287. Numerous air-sacs and 

 tracheae are represented in the head and thorax. The two thoracic spiracles are rep- 

 resented, but not lettered. 



Fig. 287 D, left dorsal trachea; S, left stigmata! trachea ; T, II, III, first, second, 

 and third pairs of abdominal dilated trachea?, forming a plexus behind the ovaries- 

 1, pair of enormous thoracic air-sacs ; 2, pair of smaller air-sacs ; 3-7. abdominal 

 air-sacs; oc, ocular dilated trachea and air-sacs- c, cephalic trachea. The relations 

 of the heart to the dorsal tracheae are indicated. Drawn by Emerton from dissec- 

 tions by the author. 



