374 THE ANATOMY OF INVERTEBRATED ANIMALS. 



The septum between the pericardial cavity and the gen- 

 eral cavity of the abdomen thus formed is termed by Graber 1 

 the pericardial septum. From their anatomical relations, 

 therefore, the alary muscles can have nothing to do with the 

 diastole of the heart, the pulsations of which, indeed, go on 

 just as well when the alary muscles are cut through. Graber 

 throws out the very probable suggestion that the contraction 

 of the alary muscles causes the pericardial septum to move 

 toward the axis of the body, and, by thus enlarging the cavity 

 of the pericardium, facilitates the flow of blood to the ostia of 

 the heart. The same investigator ascribes a special respira- 

 tory function to the abundant tracheae which are distributed 

 to the walls of the pericardium, and which, undoubtedly, must 

 tend to facilitate the aeration of the returning blood. 



In many insects, a septum, provided with transverse mus- 

 cles, overlies the abdominal nerve-cord and separates a ven- 

 tral blood sinus, in which the cord lies, from the abdominal 

 cavity. The sinus is open in front, and, as the muscles of the 

 septum contract rhythmically from before backward, they 

 tend to drive the blood which enters it to the posterior end 

 of the body. 



In the respiratory system of insects the number of stig- 

 mata is observed to vary from one to ten pairs. As a rule, 

 none are found in the head, 2 or between the head and the 

 first thoracic somite, and they are usually absent from the 

 terminal somites of the abdomen. A very common number 

 is nine pairs; the first being situated between the mesothorax 

 and the metathorax, and the rest between the following somites. 

 Only two pairs of stigmata are found in the Libellulidm and 

 Ephemeridve, and they are seated upon the thorax. In Nepa 

 and lianatra there is only one pair of abdominal stigmata, 

 in addition to those in the thorax, and in the larvae of Tipu- 

 lidce and of Hydrophilus the stigmata are reduced to one 

 terminal abdominal pair. The stigmatic openings are usually 

 situated upon the sides of the abdomen, but in some Coleoptera 

 (e. g., Dytiscus) they are dorsal, and in many Hemiptera they 

 are situated on the ventral aspect of that region of the body. 

 Either the lips of the stigmatic aperture itself, or the walls 

 of the tracheal trunk which arises from it, are so disposed as 



1 "Ueber den propulsationischen Apparat der Insecten" (Zeitseliriftfiir 

 wiss. Zoologie, 1873), and " Ueber den pulsirenden Bauchsinus der Insecten " 

 (ibid., 181$). 



2 Sir John Lubbock found the two spiracles of Smynthurus to be situated 

 on the under side of the head, immediately below the antennae. 



