THE CIRCULATORY SYSTEM 31 



heart is neurogenic; and this pace-maker is affected by acetylcholine, 

 which causes acceleration. Adrenaline, which appears to act upon 

 the post-ganglionic fibres, also causes acceleration; and under the 

 stimulus of feeding the corpus cardiacum of Periplaneta liberates a 

 peptide hormone which causes the pericardial cells to secrete a 

 cardiac stimulator, perhaps an indolalkylamine related to serotonin, 

 which increases the rate and amplitude of the heart beat. 



Composition of the blood 



The blood or haemolymph of insects may be a clear, colourless fluid ; 

 but more often it is tinged with green or yellow (p. 79). The specific 

 gravity varies from 1-03 in the larva of Celerio (Lep.) to 1045 in the 

 larva of Apis. The reaction is usually very slightly acid: pH = 6-4 in 

 Celerio larva, 6-6-6-8 in Bombyx mori, 6-83 in Apis larva; but it may 

 differ slightly in the two sexes and may vary with the age of the 

 insect. Thus, in Bombyx it becomes more alkaline at each moult ; and 

 in Apis, Celerio and other insects it is more acid during the pupal 

 stage. The haemolymph is moderately well buffered; bicarbonates, 

 phosphates, amino-acids and, chiefly, proteins are responsible. The 

 total molecular concentration is rather high. Expressed in terms of 

 sodium chloride concentrations of the same osmotic pressure, we get 

 instead of 0-9 per cent., the usual value in mammals, such values as 

 1-5 per cent, in the bee larva, 1-38 in Pieris larva, 2-12 in the larva of 

 Tenebrio; but lower values occur in aquatic insects, such as 0-75- 

 0-85 in the larvae of mosquitos, 0-69-1-0 in Ephemeroptera. The 

 ionic composition is exceedingly diverse in different groups of insects. 

 Among exopterygotes the concentration of chlorides may approxi- 

 mate of that formed in mammals (65-70 per cent, of the total mole- 

 cular concentration) but in many insects chlorides contribute no 

 more than 15 per cent. On the other hand, the concentrations of 

 phosphate, calcium and magnesium are relatively high as compared 

 with vertebrate blood. A very curious feature of insect blood is the 

 extreme variability of the ratio of sodium to potassium in different 

 species. In many insects the potassium concentration is so high that 

 nerve-fibres will not conduct impulses when exposed to it. In such 

 insects the nerves continue to function only because the perineurium 

 cells below the fibrous sheath that covers the nervous system actively 



