33 



Chironomidw they differ only in so far that the pupa? do not ascend from the bottom 

 of the water but live their whole life fastened to the surface film. 



There is yet a third group which are hatched as imagines at the bottom of 

 the water and as such ascend through the water layers to the surface; to these belong 

 the Simuliidce and the few aquatic Lepidoptera. The Simuliidce use the air from the 

 pupal tracheal system as air-bubbles wrapped up in which they ascend as 

 imagines to the surface with an extraordinary rapidity. The moment this is reached 

 the bubble bursts, the fly stands dry upon the* waves, whereupon it takes to 

 its wings. 



The few Lepidoptera which undergo the last metamorphosis in the water, also use 

 an air-bubble by means of which they reach the surface; but these insects are 

 not wrapped up in this bubble; they only hold it between the wings and thorax 

 in a manner similar to that in which the bubble is held by the Culicid pupae. 

 Moreover the whole body of the butterfly, at the moment it leaves the pupa 

 case, is covered by a wax-like substance which prevents the body from getting wet 

 and which for a moment remains as a column in the water after the butterfly has 

 left the water. All the above-named structures and biological phenomena, appearing 

 in the aquatic insects on the threshold between aquatic and aerial life: the peculiar 

 fastening of the alimentary canal in Perlidw; the subimago stage in the Ephemeridw, 

 the free swimming part of the pupa life of the Trichoptera with the peculiar modifi- 

 cation of the second pair of legs; the store of air in the pupa of Chironomida-, the 

 air-bubble of Simuliidce and Lepidoptera, and the wax covering of the last-named 

 insects are in my opinion all phenomena which tend to the same purpose: to carry 

 the insects rapidly and in dry condition up into the new element. Just this perfectly 

 dry condition, more especially with regard to the wings, is a conditio sine qua non 

 for all the insects which are unable to change element if the wings are wet. 



It is from this point of view that the peculiar free-swimming pupa stage 

 of the Culicidce must be interpreted. Like all other pupae the mosquitoes in the 

 pupa stage are quite defenceless; in contradistinction to most other insects which 

 at this very stage hide themselves, the pupae of the mosquitoes live in quite the 

 same localities as the larvae, exposed to quite the same dangers. The pupa is 

 therefore adapted to escape from these dangers by means of active motions, but 

 as the pupa stage is simultaneously a resting stage in which the great alterations 

 from larvae to imago are to take place, the power of active motion is only used if 

 dangers appear or in search of water with the highest temperature. Otherwise the 

 pupa' are at rest, and the stage is also used by these insects as a real resting-stage. 

 As however the pupa? even as such are to pass their lives in an element which 

 is hardly ever perfectly calm, a series of peculiar physiological and anatomical 

 structures must be formed, by means of which the pupa- even in a locality so mo- 

 bile as the surface of the ponds can be fastened to the spot where they have once 

 got hold. 



We shall now study the power of locomotion of the mosquito pupa a little 



D.K. D. Vidensk. Selsk.Skr., naturvidensk. ug mathem. Afd. 8. Rrekke, VII, 1. 5 



