30 INSECT PHYSIOLOGY 



sinus, where the pressure is higher, while the afferent stream (in 

 the anterior chamber) communicates with the perivisceral sinus, 

 where the pressure is lower : the blood in the limb is thus subject to 

 the aspirating action of both the heart and of the pulsatile organs of 

 the thorax (Fig. 6, B). Finally, in those insects in which the ventral 

 diaphragm is well developed (many Orthoptera, Hymenoptera, Lepi- 

 doptera), it may contain muscle fibres, and by means of wave-like 

 contractions may actively assist in driving the blood backwards and 

 laterally. 



Control of the heart-beat 



The pulse-rate of the dorsal vessel varies from many causes. It is 

 slower in the less-active stages, such as the pupa : in Sphinx ligustri it 

 falls from around 40-50 per minute in the larva to 20 or 10 per 

 minute in the pupa. The rate is increased as the temperature is 

 raised. It increases also when the insect becomes active : in the imago 

 of Sphinx it is 40-50 per minute during rest, 1 10-140 during activity. 

 And the activity of the accessory pulsating organs shows similar 

 variations. 



If the heart of the cockroach may be taken as a normal example, 

 the heart of insects has a very complicated nerve-supply. Two lateral 

 nerves, containing ganglion cells, and apparently constituting an 

 intrinsic cardiac ganglion, run along its entire length and are con- 

 nected in front with the ganglia of the visceral nervous system. In 

 addition, the heart receives sensory and motor nerve-fibres from the 

 segmental ganglia of the ventral chain. There is no doubt, therefore, 

 that the heart enjoys a large degree of nervous control ; experimen- 

 tally, faradic stimulation in the region of the neck gives rise to 

 accelerating impulses which reach the heart both by the lateral nerves 

 and by the segmental branches from the ventral cord. 



But though influenced by the nerve-supply in this way the property 

 of rhythmical contraction resides in the heart-muscle ; the heart-beat 

 is 'myogenic'. For in many insects, such as Aeschna, ganglion cells 

 are absent, and yet the isolated heart or even fragments of the heart 

 continue to beat rhythmically. This rhythmical activity is greatly 

 influenced by the normal tension exerted upon the heart by the alary 

 muscles. But it is probably true to say that the pace-maker of the 



