September io, 1891] 



NATURE 



459 



should be a sudden rush of water of unusual violence, or if the 

 larva should be obliged to quit its hold in order to avoid some 

 dangerous enemy. In the case of such an accident it is not 

 easy to see how it will ever recover its footing. Swept along in 

 a rapid current, we might suppose that there would be but a 

 slender probability of its ever finding itself favourably placed 

 for the application of its sucker and hooks. But such emer- 

 gencies have been carefully provided for. The salivary glands, 

 or silk-organs, which the Chironomus larva uses in weaving the 

 wall of its burrow, furnish to the Simulium larva long mooring- 

 threads, by means of which it is anchored to the leaf upon which 

 it lives. Even if the larva is dislodged, it is not swept far by the 

 stream, and can haul itself in along the mooring- thread in the 

 same way that a spider or a Geometer larva climbs up the thread 

 by which, when alarmed, it descended to the ground. 



When the time for pupation comes, special provision has to 

 be made for the peculiar circumstances in which the whole of 

 the aquatic life of the Simulium is passed. An inactive and 

 exposed pupa, like that of Chironomus, may fare well enough 

 on the soft muddy bottom of a slow stream, but such a pupa 

 would be swept away in a moment by the currents in which 

 Simulium is most at home. When the time of pupation draws 

 near, the insect constructs for itself a kind of nest, not unlike in 

 shape the nest of some swallows. This nest is glued fast to the 

 surface of a water-weed. The salivary glands, which furnished 

 the mooring-thread?, supply the material of which the nest is 

 composed. Sheltered within this smooth and tapering case, 

 whose pointed tip is directed up-stream, while the open mouth 

 is turned down-stream, the pupa rests securely during the time 

 of its transformation. 



When the pupa-case is first formed, it is completely closed 

 and egg-shaped, but, when the insect has cast the larval skin, 

 one end of the case is knocked off, and the pupa now thrusts 

 the fore-part of its body into the current of water. The respira- 

 tory filaments, which project immediately behind the future 

 head, just as in Chironomus, draw a sufficient supply of air 

 from the continually changed water around. The rings of the 

 abdomen are furnished with a number of projecting hooks, 

 which are able to grasp such objects as fine threads. The in- 

 terior of the cocoon is felted by a number of silken threads, 

 and by means of these the pupa gets an additional grip of its 

 case. If it is forcibly dislodged, a number of the silken threads 

 are drawn out from the felted lining of the case. The fly 

 emerges into the running water, and I do not know how it 

 manages to do so without being entangled in the current of 

 water, and swept down the stream. The pupa-skin splits open 

 just as it does in Chironomus, but remains attached to the 

 cocoon. 



The larva of the gnat is perhaps more familiar to naturalists 

 of all kinds than any other aquatic Dipterous insect. The 

 interesting description, and, above all, the admirable engravings, 

 of Swammerdam, now more than two hundred years old, are 

 familiar to every student of Nature. 



The larva, when at rest, floats at the surface of stagnant 

 water. Its head, which is provided with vibratile organs suit- 

 able for sweeping minute particles into the mouth, is directed 

 downwards, and, when examined by a lens in a good light, 

 appears to be bordered below by a gleaming band. There are 

 no thoracic limbs. The hind-limbs, which were long and hooked 

 in the burrowing Chironomus larva, and reduced to a hook- 

 bearing sucker in Simulium, now disappear altogether. A new 

 and peculiar organ is developed from the eighth segment of the 

 abdomen. This is a cylindrical respiratory siphon, traversed by 

 two large air-tubes, which are continued along the entire length 

 of the body, and supply every part with air. The larva ordin- 

 arily rests in such a position that the tip of the respiratory 

 siphon is flush with the surface of the water, and, thus sus- 

 pended, it feeds incessantly, breathing uninterruptedly at the 

 same time. When disturbed, it leaves the surface by the scull- 

 ing action of its broad tail. Once below the surface, it sinks 

 slowly to the bottom by gravity alone, which shows that the 

 body is denser than the water. We have, therefore, to explain 

 how it is enabled to float at the surface when at rest. The larva 

 does not willingly remain below for any length of time. It rises 

 by a jerking movement, striking rapid blows with its tail, and 

 advancing tail foremost. When it reaches the top, it hangs as 

 before, head downwards, and resumes its feeding operations. 



In order to explain how the larva hangs from the surface 

 against gravity, I must trouble you with some account of the 

 jroperties of the surface-film of water. You will readily believo 



that I have nothing new to communicate on this subject, and I 

 venture to show you a few very simple experiments, merely 

 because they are essential to the comprehension of what take* 

 place in the gnat. ^ 



In any vessel of pure water, the particles at the surface, though 

 not differing in composition from those beneath, are neverthe- 

 less in a peculiar state. I will not travel so far from the region 

 of natural history as to offer any theoretical explanation of this 

 stale, but will merely show you experimentally that there is a 

 surface-film which resists the passage of a solid body from 

 beneath. [Mensbrugghe's float shown.] You see (i) that the 

 float is sufficiently buoyant to rise well out of the water ; (2) that, 

 when forcibly submerged, it rises with ease through the water 

 as far as the surface-film ; (3) that it is detained by the surface- 

 film, and cannot penetrate it. The wire pulls at the surface- 

 film and distorts it, but is unable to free itself. In the same 

 way the surface-film resists the passage of a solid body which 

 attempts to penetrate it from above. This will be readily seen 

 if we throw a loop of aluminium wire upon the surface of 

 water. [Experiment shown.] The loop of wire floats about 

 like a stick of wood. Aluminium is, of course, much lighter 

 than iron, but the floating of this little bar does not mean that 

 it has a lower density than that of water. If the bar is once 

 wetted, it sinks to the bottom and remains there. Even a 

 needle may, with a little care, be made to float upon the surface 

 of perfectly pure water. Still more readily can a piece of metallic 

 gauze be made to float on water. [Experiment shown.] Air 

 can pass through the meshes with perfect ease ; water also can 

 pass through the meshes with no visible obstruction. Bat the 

 surface-film, bounding the air and water, is entirely unable to 

 traverse even meshes of appreciable size. These simple experi- 

 mental results will enable us to appreciate certain facts of struc- 

 ture, which would otherwise be hard to understand, and which 

 have been wrongly explained by naturalists of the greatest 

 eminence, to whom thj physical discoveries of this century 

 were unknown. 



We may now try to answer three questions about the larva of 

 the gnat, viz. : — 



(i) How is it able to break the surface-film when it swims 

 upwards ? 



(2) How is it able to remain at the surface without muscular 

 effort, though denser than water ? 



(3) How is it able to leave the surface quickly and easily 

 when alarmed ? 



The tip of the respiratory siphon is provided with three flaps, 

 two large and similar to one another, the third smaller and 

 differently shaped. These flaps can be opened or closed by 

 attached muscles. When open, they form a minute basin, 

 which, though not completely closed, does not allow the surface- 

 film of water to enter. When closed, the air within the siphon 

 is unable to escape. At the time when the larva rises to the 

 surface, the pointed tips of the flaps first meet the surface-film, 

 and adhere to it. The attached muscles then separate the flaps, 

 and in a moment the basin is expanded and filled with air. The 

 surface-film is now pulling at the edges of the basin, and the 

 pull is more than sufficient to counterbalance the greater density 

 of the body of the larva, which accordingly hangs from the sur- 

 face without effort. When the larva is alarmed, and wishes to 

 descend, the valves close, their tips are brought to a point, and 

 the resisting pull of the surface-film is reduced to an unimportant 

 amount. [Living larvae shown by the lantern.] 



Swammerdam found it necessary, in explaining the flotation of 

 the larva of the gnat to suppose that the extremity of its siphon 

 was supplied with an oily secretion which repelled the water. 

 No oil-gland can be discovered here or elsewhere in the body of 

 the larva, and indeed no oil-gland is necessary. The peculiar 

 properties of the surface-film explain all the phenomena. The 

 surface-film is unable to penetrate the fine spaces between the 

 flaps for precisely the same reason that it is unable to pass 

 through the meshes in a piece of gauze. 



After three or four moults the larva is ready for pupation. By 

 this time the organs of the future fly are almost completely 

 formed, and the pupa assumes a strange shape, very unlike that 

 of the larva. 



At the head-end is a great rounded mass, which incloses the 

 wings and legs of the fly, beside the compound eyes, the mouth- 

 parts, and other organs of the head. At the tail-end is a pair 



I A number of other experiments, illustrating the properties of the surface- 

 film of water, are described by Prof. Boys in his delightful book on " Soap 

 Bubbles." 



NO. II4I, VOL. 44] 



