31 



some hours, and if we remember that the power of locomotion is only used when 

 reflexes of quite a definite kind are released according to definite variations in the 

 surrounding medium, it will be understood that the postulate, that the above-named 

 paragraph in Howard, Dyar and Knab only contains half of the truth, is really 

 correct. The pupse are able to move, but commonly they do not for the whole 

 period of their life actively use this power. They are much more stationary than 

 one would think; as stated above, this more especially holds good for the last day 

 before ecdysis, the pupse being then almost glued to the surface. 



If we will try to understand the peculiar pupa stage of the mosquitoes, we 

 must compare it with similar stages in the water insects; it will then be clear, that 

 the pupa is constructed in accordance with the purposes which at all events we 

 are not accustomed to put in the first line, when we discuss the peculiar shape 

 and mode of life of these pupa?. 



If we survey all those aquatic insects which are going to perform the last ec- 

 dysis in the water before leaving this element for ever, we shall see that, just at the 

 time when the last ecdysis is to take place, there appear organs of quite a new 

 type; further we find old organs which are now modified in such a way that it is 

 clearly understood that they have only significance at the moment when the insects 

 leave the water; nay, in several groups of insects we find even more than that, 

 either peculiar biological phenomena or quite new developmental stages intercalated 

 between the old well-known stages. 



In all those aquatic insects which leave the water as larvae, climbing on shores, 

 we find none of these peculiar structures and phenomena. We may only note that 

 some of the Perliclw crawl out of the water, throw their whole anterior part of the 

 alimentary canal out of the mouth, fasten it to stones to which it is glued, now 

 using this organ in this quite uncustomary manner as a cable by means of which 

 they crawl out of the nympha skin. Other Perlidce use their larval gills as glutin- 

 ating organs, by means of which these insects are fastened at the same moment 

 for the same purpose to the slippery stones. 



A second group of aquatic insects live as nymphs or as pupse in the water 

 and complete their last ecdysis upon the surface of the water; to these belong the 

 Ephemeruke, the Trichoptera and some Diptera (Chironomidce). As a transition sta^e 

 between the aquatic and aerial life we find in the Ephemeridce intercalated the highly 

 peculiar subimago stage. In this stage, which only lasts for some hours or, rarely, for 

 a few days, the legs and nervatures of the wings are often covered by a coating of 

 minute tliorns which hinder the wings from getting wet; a remarkable oily appea- 

 rance, f. i. on the wings of Ephemera danica, has the same purpose. These struc- 

 tures are undoubtedly inconvenient when the wings are used as flying org;nis 

 during the mating processes and egg-laying phenomena. Lying in my boat on 

 calm summer evenings, I have seen the large Ephemera danica nymphs rise to the 

 surface from a depth of from four to live meters; with undulating movements of the 

 abdomen they lie for a few seconds below the surface-film fastened to it by the out- 



