FEEDING AND BREATHING 39 



ends of the hind-gut into the great blood-space. When 

 a tube is examined microscopically its wall (Fig. 13) is 

 seen to be composed of a single layer of glandular cells with 

 a thin basement membrane surrounded in some insects — 

 as shown by L. Leger and A. Dusboscq (1899) and by 

 L. Eastham (1925) — by delicate muscular fibres, which by 

 their contraction may assist the passage of the excretion 

 towards the intestine. The tubes lie freely in the great 

 blood-space, floating as it were in the blood currents and 

 capable themselves of bending and extending movements. 

 Chemical examination shows that the tubes contain such 

 compounds as oxalates and urates besides amino-acids such 

 as leucin. These substances — resulting from the disinte- 

 gration of protein or due to excess of nutrient materials in 

 the blood — are extracted from the blood by the cells of 

 the tubules or elaborated by these cells from other related 

 substances held by the blood in solution. An interesting 

 feature in these excretory tubes of insects is the variation 

 in their number. Moths and butterflies have but two, 

 beetles four or six, produced apparently by a branching 

 of the original pair, while cockroaches, grasshoppers, bees 

 and w^asps have a large number, in some cases over a 

 hundred, also derived by elaboration from the simpler 

 condition. In various insect larvae as well as in some 

 wingless springtails, the fat-body may contain concretions 

 of urates, the waste material being thus segregated and 

 stored, but not actually removed from the body. 



More than once it has been pointed out that the Hbera- 

 tion of energy required for the performance of an animal's 

 activities is connected with a combustion process constantly 

 going on in the tissues. For the support of this com- 

 bustion a supply of oxygen is necessary, while the products 

 of combustion — water-vapour and carbon dioxide — must 

 be ehminated from the body. This important work is the 

 function of the breathing organs or respiratory system. The 

 remarkably characteristic form and mode of action of the 

 breathing organs in insects have already (pp. 4-5) been 

 very briefly described. In the vast majority of animals the 



