112 ENTOMOLOGY 



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 molt 

 is about 82 or 83 per minute; before the second molt, 89, sinking to 63 

 before the third molt, to 45 before the fourth, and to 39 in the final 

 larval stage; but the force of the circulation increases as the pulsations 

 decrease in number. During the quiescent period immediately preced- 

 ing each molt, the number of beats is about 30. In the pupal stage 

 the number sinks to 22, and then lowers until, during winter, the pulsa- 

 tions 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. 165) 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 conforms to the general segmentation and is 

 arranged in each segment with an approach to symmetry. Much of 

 this tissue forms a distinct peripheral layer in each segment, and masses 

 of fat-body occur constantly on each side of the alimentary tract and 

 also at the sides of the dorsal vessel, in the latter case forming the 

 pericardial fat-body. 



Fat-Cells. The fat-cells (Fig. 166) are large and at first more or 

 less spherical, with a single nucleus (though there are said to be two in 

 Apis and several in Mused), but the cellular structure of the fat- tissue is 

 often difficult to make out because the cells are usually filled with glob- 

 ules of fat (Fig. 167), while old cells break down, leaving only a disorderly 

 network. The fat-cells sometimes contain an albuminoid substance, and 

 usually the fat-body includes considerable quantities of uric acid or its 

 derivatives, frequently in the form of conspicuous concretions. 



Functions. The physiology of the fat-system is still obscure. 

 Probably the fat-body combines several functions. In caterpillars and 

 other larvae it furnishes a reserve supply of nutriment, at the expense of 



