THE BOOK OF NATURE STUDY 



peller, and a group of four long, thin, narrow plates surrounding 

 the anus at its apex. These plates contain tracheal tubes, and 

 are probably supplementary respiratory organs capable of taking 

 in the oxygen dissolved in the water. On the tip of the respiratory 



siphon there are hinged five pointed 

 valves ; these can be approximated so 

 as to form a five-sided pyramid, and 

 this is their arrangement when the 

 larva is completely below the water. 

 On reaching the surface the apex of 

 the pyramid pierces the virtual film 

 (due to surface tension) and the five 

 flaps are immediately thrown apart 

 above the surface, so that a shallow 

 basin results in open communication 

 with the air above and with the two 

 large tracheal tubes below. Thus the 

 larva is suspended to the surface film, 

 and uses it for support in a manner 

 analogous to that employed by the 

 whirligig beetle, water-skaters, and 

 other insects which move on but 

 above the surface, without, however, 

 being wetted. Many species of water- 

 snails make the same use of the sur- 

 face film as does the culex larva, 

 applying their " foot " to it and crawl- 

 ing in an inverted position upon it. 



The larva casts its skin three times, 

 and when full grown is about i cm. 

 long and of a brownish-grey colour. 

 At the third moult it passes into the 

 pupa stage, which differs greatly both 

 in form and attitude from the pre- 

 ceding. Since the pupa takes no 

 food, it is no longer necessary for the head to be turned down- 

 ward into the water, whence the larva obtained its supplies. 

 It is thus not surprising to find the animal now resting with 



FlG. II. Side view of larva of 

 culex. (After Miall.) 



