56 POND LIFE 



examination, large numbers practically on the surface, and 

 yet they cannot be discerned, until one notices their air sacs, 

 which are coloured. These sacs, of which there are four ar- 

 ranged in two pairs, act as floats, and possibly also play some 

 part in respiration. Why they should be coloured is not 

 understood, but possibly, as Professor Miall remarks, the pig- 

 ment may f unctionise in the production of gases. 



Corethra is predaceous, and its transparency assists it in 

 the capturing of its prey. It is also possible that the pig- 

 mented air-sacs draw attention, and thus attracted by the 

 coloured spots its victims are less able to make out the 

 motionless creature's form, until it is too late. For the next 

 moment the Daphne or Cyclops, as the case may be, is roughly 

 seized and held in the larva's mouth, from which it is pre- 

 vented from making its escape by strong hairs. In the mouth 

 of its captor the victim is acted upon by digestive juices, until 

 little except the indigestible parts remain. Thus the prey 

 of the young Corethra is not swallowed, but only its juices 

 extracted. 



The larva is so transparent that under the microscope the 

 digestive canal, heart, &c., can be as clearly seen as if they 

 had been dissected out. Peristalsis, the extraordinary in- 

 voluntary movement of the alimentary canal, can be clearly 

 watched. The heart, which is tubular, can be seen " beating," 

 driving the blood along the various canals. 



It is interesting to know that Livingstone mentions a cake 

 (Kungucake) eaten by the natives in Central Africa, which is 

 made of the bodies of gnat-like insects, and from comparison 

 it appears that the appetising sweetmeat is composed of a 

 species of Corethra. 



A little red ' ' worm-like " creature lashing its way through 

 the water, twisting itself into the figure 8, and yet always 

 keeping in the same direction, such is the larva of Chironomous, 

 another gnat-like insect. The larva will be found of all sizes, 

 the largest being three-quarters of an inch long, whilst the 

 younger members will be smaller and smaller, as their ages 

 decrease, until some will be found very minute indeed, but they 

 are all characterised by their method of swimming. The 

 larger and therefore older " blood worms " will be noticed 

 to testify to their increase in age by swimming less violently, 

 as if no longer able to twist and turn as nimbly as the younger 

 members of the family. 



Each little worm, as soon as it settles after a swim, builds 

 for itself a case of decaying vegetable matter, mud, &c. In 

 fact, it chooses anything suitable, and each piece is rapidly woven 



