INSECTA 437 



chemically as uric acid, inside the cells and in the lumen of the tubule. 

 A mass, mainly of uric acid, is found in the hind gut of pupating in- 

 sects, having been deposited there by the tubules. But in addition 

 nitrogenous end products are found in the nephrocytes (cells found 

 commonly associated with the fat body and the pericardium), the 

 fat body and the hypodermis in quantities which increase with age, 

 and in the hollow wing scales of certain butterflies, e.g. Pieridae, so 

 that it appears that the mechanism of the Malpighian tubules for 

 ridding the body of the insect of nitrogenous excreta is by no means 

 efficient. 



Of non-nitrogenous excretory products may be mentioned the 

 carbonates of calcium, potassium and magnesium. Calcium car- 

 bonate may be excreted in the integument, but in many cases it is 

 eliminated by the Malpighian tubules, either gradually, e.g. Droso- 

 phila, or expelled en masse by way of the blood and the hypodermis 

 during pupation, e.g. Ascidia, the celery fly. In this latter example 

 the recrystallization of the compound on the inner wall of the 

 puparium (see p. 509) may serve to strengthen the weakness of the 

 latter. 



To the majority of insects the matter of water conservation is of 

 considerable importance. It is interesting, therefore, to find that in 

 a blood-sucking bug (Rhodnius) the proximal part of the Malpighian 

 system is concerned with the withdrawal of water from the lumen of 

 the tubules, and its return to the body cavity. In this insect, the 

 distal parts of the tubes secrete into their lumen potassium and 

 sodium urate, water and base returning to the blood through the 

 walls of the proximal parts of the tubes (Fig. 306). A circulation of 

 water and base exists, therefore, within the system, similar to that 

 obtaining in the vertebrate kidney. 



The circulatory system. There is, first, a heart, primitively con- 

 sisting of thirteen chambers, each corresponding to a segment, with 

 a pair of ostia guarded by valves precluding outflow, at the base of 

 each chamber. The blood is driven forward in these by muscular 

 action of the heart wall, and passes into an anterior aorta which opens 

 into the general body cavity in the head region. The haemocoelic body 

 cavity is very spacious and the blood bathes all the organs. There is a 

 dorsal horizontal diaphragm perforated by many holes, which separ- 

 ates off the pericardium in which the heart lies, and attached to this 

 are paired alary muscles, the outer ends of which are inserted in the 

 terga (Fig. 307). By their contraction the passage of blood from the 

 body cavity into the pericardium and heart is facilitated. Though the 

 circulatory system is usually simple, accessory vessels are known, 

 which direct blood backwards along the nerve cord and upwards 

 towards the pericardium in the metathorax (in the moth Protoparce) 



