128 EN TOMOLOGY 
merida, Coleoptera, Lepidoptera, etc., the currents trending 
along the trachez; 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 Sphina 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. Fatr-Bopy 
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 
