32 AN INTRODUCTION TO ENTOMOLOGY. 



Miall and Denny, however, describe the heart of a cockroach as 

 consisting of thirteen chambers, corresponding to ten abdominal and 

 three thoracic segments. The chambers of the heart are separated 

 by valves, which permit the blood to flow only towards the head. 

 There is in the walls of the heart a pair of lateral openings corre- 

 sponding to each chamber ; these also are furnished with valves, 

 which admit the blood to the heart, but prevent its exit. When, 

 therefore, the chambers contract, a stream of blood is forced towards 

 the head ; and when they expand, the blood rushes into them 

 through the lateral openings. 



The circulatory system has been carefully studied in only a few 

 insects ; and these show a remarkable variation in the form and 

 arrangement of the openings and valves.* But the result 

 of their operation is the same in all. A diagram of a hori- 

 zontal section of the heart of a May-beetle, based on the 

 description by Straus- Durckheim (Graber represents 

 it as more complicated) will serve as an illustration. 

 Fig. 41, S, represents a chamber of the heart during 

 its systole or contraction ; the valves at the caudal 

 FIG. 4 i. Dia- end and at the lateral openings (i) have been 

 hear\ of closed by the pressure of the blood; while the 



May-beetle. , , , .. , 



valves at the cephalic end have been forced 

 open, and the blood, represented by the arrow, is flowing 

 forwards. D repTesents a chamber regaining its natural 

 size, and the blood flowing into it through the lateral 

 openings. 



I have observed a somewhat different arrangement of 

 openings and valves in the nymph of a dragon-fly. A 

 young individual was studied, one in which the skin was 

 sufficiently transparent to allow the beating of the heart 

 to be seen through it. In this insect the most active 

 part of the heart appeared to be the caudal chamber, 

 which is represented in Fig. 42, a. This. chamber was in 

 constant motion, expanding and contracting in rapid suc- 

 cession. With each expansion the valves at I quickly closed and 

 the blood rushed in at 2 ; and when the chamber contracted, the 

 valves at 2 closed and the blood was forced forwards through I. 



* See Dr. V. Graber, Ueber den propulsatorischen Apparat der Insecten, Archiv fUr 

 mikroscopische Anatomic, Band IX. (1873). 

 Straus-Durckheim, Animaux Articules (1828). 

 Miall and Denny, The Cockroach (1886). 



