21 



the larva sinks to the bottom, rests here for a moment and swims upwards to the 

 surface again. I have often been surprised at the facility, with which the apex of 

 the sipho, when it touches the surface-film, pierces it, and the rapidity with which the 

 flaps are folded out. I suppose that the apex of the sipho is oiled and that the oil 

 comes from the pore of the above-named tube. I have vainly tried to see if there 

 is any real connection between tube and sipho and do not think that this is pos- 

 sible to observe, but I have often seen that the sipho is pierced into the buccal 

 cavity, and that the flabellae are folded round it. As far as I can understand, we 

 have here to do with two processes: a cleaning and an oiling one, either one of 

 them or most probably both: first a cleaning and then an oiling process. 



With regard to the tracheal system, thoroughly described by Meinert (1886 

 p. 391) and by RASCHKE (1887 p. 16), I shall restrict myself to the following short 

 remarks. The thickness of the tracheae and the form of the transversal cuts differ 

 widely in the different species; in some species they are circular, in others elliptical, 

 the tracheae being then converted into broad flattened bands; this more especi- 

 ally holds good for F. geniculata. In the thorax, in particular, the tracheal system 

 presents great variations from species to species. 



The tracheal system of the mosquito larva has a double function, respiratory 

 and hydrostatical; the last-named in almost all mosquito larvae is but slight. In 

 recent years the respiration of the mosquito larvae has been subjected to many 

 valuable explorations (BABAK 1912; KOCH 1919; KROGH 1920). 



Referring to this literature, dealing with problems which only touch the ex- 

 ploration which is here published, I shall only deal with a few points. BABAK (1912 

 p. 84) as well as many others remark that during the ventilation of the tracheal system 

 respiratory movements are wholly wanting. The most recent observations, (KROGH 

 1920 p. 95) confirm this. Some authors (BABAK 1912 p. 85) suppose that the re- 

 novation of the air in the large tracheal trunks takes place owing to the strong move- 

 ments of the abdomen, by means of the pulsation of the heart and owing to the 

 movements of the intestine. KROGH (1920 p. 96) has however pointed out that all these 

 movements do not suffice for the necessary ventilation and that we must try to 

 find other ways if we are to understand the ventilation here and in all those insects 

 where respiratory movements are not visible. Basing on his excellent studies on 

 some large insect larva? he has pointed out that diffusion alone is sufficient to pro- 

 duce the air transport in the tracheal trunks. 



Babak (1912 p. 90) has shown that the two main tracheal trunks, when the larvae 

 are forced away from the surface, are flattened and emptied of air. With regard to 

 the summer larvae I have made quite the same observation. It is however in con- 

 tradiction to an observation of KOCH (1919 p. 467) according to which, instead 

 of gradually emptying during submersion, the tracheae are filled still more, and 

 when they are quite full, small gas bubbles escape from the sipho which there- 

 fore is not always completely closed under water. The author supposes that this 

 air cannot be oxygen, and that it must be regarded as air that has already been 



