128 ENTOMOLOGY 



merida, Coleoptera, Lepidoptera, etc., the currents trending 

 along the tracheae; this circulation ceases, however, with the 

 drying of the wings. 



The chambers of the dorsal vessel expand and contract suc- 

 cessively from behind forward. At the expansion (diastole) 

 of a chamber its ostia open and admit blood; at contraction 

 (systole) the ostia close, as well as the valve of the chamber 

 next behind, while the chamber next in front expands, afford- 

 ing the only exit for the blood. The valves close partly 

 through blood-pressure and partly by muscular action. 



The rate of pulsation depends to a great extent upon the 

 activity of the insect and upon the temperature and the amount 

 of oxygen or carbonic acid gas in the surrounding atmosphere. 

 Oxygen accelerates the action of the heart and carbonic acid 

 gas retards it. A decrease of 8 or 10 C. in the case of the 

 silkworm lowers the number of beats from 30 or 40 to 6 or 

 8 per minute. The more active an insect, the faster its heart 

 beats. 



The rate of pulsation is very different in the different stages 

 of the same insect. Thus in Sphinx ligustri, according to 

 Newport, the mean number of pulsations in a moderately 

 active larva before the first moult is about 82 or 83 per minute ; 

 before the second moult, 89, sinking to 63 before the third 

 moult, to 45 before the fourth, and to 39 in the final larval 

 stage ; the force of the circulation, however, increases as the 

 pulsations decrease in number. During the quiescent period 

 immediately preceding each moult, the number of beats is 

 about 30. In the pupal stage the number sinks to 22, and 

 then lowers until, during winter, the pulsations almost cease. 

 The moth in repose shows 41 to 50 per minute, and after flight 

 as many as 139. 



8. FAT-BODY 



The fat-body appears (Fig. 163) as many-lobed masses of 

 tissue filling in spaces between other organs and occupying a 

 large part of the body cavity. The distribution of the fat- 

 body is to a certain extent definite, however, for the fat-tissue 



