544 Professor L. C. Miall [March 4, 



our notice on account of its minute size, for it is less than one- 

 quarter of an inch in length, is the egg-raft of the gnat. This 

 was beautifully described 150 years ago by Eeaumur. The eggs 

 of the gnat are cigar-shajjed, and 250 or 300 of them are glued 

 together, so as to make a little concave float, shaped like a shallow- 

 boat. The upper end of each egg is pointed; the lower end is 

 provided with a lid, through which the larva will ultimately issue 

 into the water. The gnat in all stages, even while still in the egg, 

 requires an ample supply of air. It is therefore necessary that the 

 egg-raft should float at the surface ; it is also necessary that it 

 should always float in ^he same position, so as to facilitate the 

 escape of the larva. This is efi'ectually secured by a provision of 

 almost amusing simplicity. Let us first notice how efficient it is. 

 If we take two or three of these tiny egg-rafts, and place them in 

 a jug of water, we may pour the water into a basin again and 

 again ; every time the egg-rafts float instantly to the surface ; and 

 the moment they come to the top, they are seen to be as dry as 

 at first. The fact is that the surface-film cannot penetrate the fine 

 spaces between the pointed ends of the eggs. The cavity of the 

 egg-raft is thus overspread by an air-bubble, which breaks the 

 instant. it comes to the top. The larva of the gnat, when it escapes 

 from the egg, floats at the surface, and it is enabled to do so in 

 consequence of the properties of the surface-film. When the larva 

 changes to a pupa, it becomes buoyant, and floats at the surface, 

 excejit when alarmed. To enable it to free itself without unneces- 

 sary eff'ort from the surface of the water, the respiratory tubes of 

 the pupa are furnished with a valvular apparatus, which can cut 

 the connection with the air in a moment, and restore it at pleasure, 

 when the pupa again floats to the surface.* 



Another Dipterous insect, whose larva inhabits rapid streams, 

 makes an ingenious use of the properties of the surface-film. This is 

 the larva of Simulium, of which I have given some account in the 

 lecture just quoted. At the time of the delivery of that lecture, I 

 was wholly unable to explain how one difficulty in the life of the 

 insect is surmounted. The larva clings to the water-weeds found in 

 brisk and lively streams. The pupal stage is passed in the same 

 situation. But a time comes whea the fly has to emerge. Now the 

 fly is a delicate and minute insect, with gauzy wings. How does it 

 escape from the rushing water into the air above, where the remainder 

 of its life has to be passed ? This was a question upon which I had 

 spent much thought, but in vain. It appeared to me for many months 

 completely insoluble. However, I was informed last year by Baron 

 Osten Sacken of a paper written by Verdat, seventy years ago, in 

 which the emergence of the fly of Simulium is described. Guided by 

 Verdat's description, I had little difficulty in seeing for myself how 



* The larva and pupa of the gnat are more fully described in my British 

 Association lecture on " Some Difficulties in the Life of Aquatic Insects," 

 reported in ' Nature,' vol. Ixiv. p. 457. 



