GRASS THRIPS. 65 



CIRCULATORY SYSTEM. 



The circulatory system consists of a contractile dorsal vessel, 

 the heart, which begins in the sixth abdominal somite and passes 

 forward into the thorax. Here it gives rise to the aorta, which 

 runs forward, ventral to the salivary glands to the head. The 

 heart is very small and lies just below the dorsal wall above the 

 intestine. In almost all of the specimens examined it was so 

 badly collapsed that it was scarcely visible. Its walls are exceed- 

 ingly thin and have very few muscle fibers. The alary muscles 

 are very poorly developed. Four pairs of ostia were found ; 

 these were in the third, fourth, fifth and sixth abdominal somites. 



RESPIRATORY SYSTEM. 



Figs. 14 and 15. 



There are three pairs of stigmata, one at the anterior end of 

 the mesothorax, and one each on the first and seventh abdominal 

 somites. The stigmata are quite large, and have a sieve-like 

 covering to prevent the ingress of solid particles. From the 

 stigmata on the mesothorax tracheae are supplied to the head 

 and its appendages, to the prothorax and fore limbs, and to the 

 organs and appendages of the meso- and metathorax. The two 

 stigmata on each side of the abdomen are connected by a large 

 tracheal trunk, which runs along the abdomen near its lateral 

 wall. From these trunks three branches are given off in each 

 of the six anterior abdominal somites. One of these branches 

 supplies the dorsal part of the somite, another the ventral part of 

 the somite, and a third passes nearly straight into the body, going 

 chiefly to the viscera. The two posterior somites of the abdomen 

 are supplied by long tracheal branches which come from the 

 stigmata on the seventh abdominal somite. The tracheae are 

 very small and thin walled, and the walls have a chitinous lining 

 that shows spiral markings. They are supplied to the viscera 

 very abundantly, and serve the double purpose of respiration and 

 to keep the viscera in place. 



FAT BODY. 



The fat body is large in all stages of development. In the 

 larva it fills all of the space between the viscera. It is made up of 

 a frame-work of large cells, each of which contains a large drop- 



