102 ENTOMOLOGY 



aeration of tissues, that function being relegated to the tracheal sys- 

 tem. 



Circulation. The course of the circulation is evident in trans- 

 parent aquatic nymphs or larvae. In odonate or ephemerid nymphs, 

 currents of blood may be seen (Fig. 162) flowing through the spaces 

 between muscles, tracheae, nerves, etc., and bathing all the tissues; 

 separate outgoing and incoming streams may be distinguished in the 

 antennae and legs ; the returning blood flows along the sides of the body 

 and through the ventral sinus and the pericardial chamber, eventually to 

 enter the lateral ostia of the dorsal vessel. A circulation of blood occurs 

 in the wings of freshly emerged Odonata, Ephemerida, Coleoptera, 

 Lepidoptera, etc., the currents trending along the tracheae; this circula- 

 tion ceases, however, with the drying of the wings. 



The chambers of the dorsal vessel expand and contract successively 

 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, affording 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 car- 

 bonic 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 to her organs and occupying a large part of the body 



