THE HARLEQUIN-FLY 113 



surface, and disports itself in the well-aerated water. Having 

 got its supply of oxygen, it descends again, finds its old tube 

 or else makes another, and can then hold out till darkness 

 makes it safe to venture once more to the surface. The habit 

 of coming to the surface only by night implies that the blood- 

 worm is furnished with some means of storing up oxygen. 

 The oxygen taken in during one night has to last till the 

 following night at least. Where and how is the oxygen stored? 

 An answer to this question is suggested by the fact that the 

 blood-worm is of a red colour ; it shows the bright crimson of 

 arterial blood. In a fish or in ourselves this colour indicates 

 the presence of a peculiar substance named haemoglobin. The 

 haemoglobin of the blood picks up oxygen in the gill or lung, 

 carries it to every part of the body, and gives it out to all 

 the tissues which require a fresh supply. Now if haemoglobin 

 can carry oxygen, it can also store it up for a time. The 

 oxygen taken up by the blood-worm during the hours of 

 darkness is combined with the haemoglobin, or red colouring 

 matter of the blood, and doled out as required. The proof 

 that the red colour of the blood-worm is really due to haemo- 

 globin is got by spectroscopic examination of the blood, which 

 yields absorption-bands precisely similar to those of human 

 blood. 



The tube in which the blood-worm conceals itself is easily 

 and quickly made out of bits of dead leaves, earth, sand, any- 

 thing in short which is lying about. The particles are woven 

 together by threads of a viscid substance, a kind of silk, which 

 is formed in the salivary glands and paid out from the mouth 

 as required. By means of the salivary secretion the blood- 

 worm forms a coherent wall for its tube. If no suitable 

 materials are at hand ; if, for instance, the larva is placed in 

 a saucer with nothing but clean water, it will at length begin 

 to make its tube of silk only, a transparent cylindrical case, 

 through which all the movements and actions of the larva 

 can be observed. By this means we can see that the blood- 

 worm, when concealed within its tube, is hardly ever at rest. 

 It holds on to the walls by its hooked feet (p. iii), and then 

 performs an undulatory movement, the effect of which is to 

 set up a current of water which flows through the case. This 

 is one method of extracting from the water any free oxygen 

 which may be present. At other times the blood-worm sticks 



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