254 ANIMAL LIFE 



and move actively by reversed figure-of-eight twists 

 of the whole body. The head is small and differs from 

 that of a Culex-larva in having rasping teeth instead of 

 a sifting arrangement. The tail-end is marked by a 

 pair of hooked feet, and between these lies a minute 

 coronal of four gill-plumes. The larva has no opening 

 to its tracheal system, and does not attach itself to the 

 surface-film. On the contrary, it buries itself head 

 downwards in a burrow or case formed by particles 

 of mud glued with saliva, and if the water contains 

 sufficient air the larva respires by gentle undulations 

 of the tail, causing a current that flows over the 

 skin and tail-filaments. It only leaves the burrow 

 at night. If the larvae live on a hard bottom they 

 form a portable case of debris. Larvae which inhabit 

 stagnant or sewage-water acquire a red colour (hence 

 their name, blood- worms), whereas those of aerated 

 water are greenish. This difference of tint is due 

 to the presence of red blood in the one case, and of 

 colourless blood in the other. The red colouring 

 matter is identical with haemoglobin, and its presence 

 confers increased respiratory efficiency on the blood, 

 in virtue of which the harlequin fly can pursue its 

 larval life in foul water and at considerable depths. 

 This is not the only adaptation enabling these larvae 

 to utilise very small quantities of oxygen. The 

 surface-forms which can utilise the dissolved oxygen 

 have no tubules at the hinder end of the body, whilst 

 those which inhabit deeper water have two pairs of 

 vascular tubes on the last segment, and these in all 



