September io, 1891] 



NATURE 



461 



that the buoyancy of the larva can thus be regulated, and a 

 larger or smaller quantity of air taken in as desired. 



The pupa has a pair of respiratory tubes, which are carried, 

 not on the tail, but on the thorax, close behind the head. One 

 of these tubes is very long, the other very short. The long 

 tube is twice as long as the body, and tapers very gradually to 

 its free tip. Here we find a curious radiate structure, rather like 

 the teeth of a moss-capsule, which seems adapted for opening 

 and closing. There is, however, no orifice which the most care- 

 ful scrutiny has succeeeded in discovering. A delicate membrane 

 extends between the teeth, and prevents any passage inwards or 

 outwards of air in mass. The tube incloses a large trachea, the 

 continuation of one of the main tracheal trunks. This is 

 stiffened by a spiral coil, but at intervals we find the coil de- 

 ficient, while the wall of the tube swells out into a thin bladder. 

 However the tube is turned, a number of these bladders come 

 to the surface. As the pupa lies on the surface of the mud, the 

 filament floats on the top of the water, and the air is renewed 

 without effort through the thin- walled bladders. 



Why should the position of the respiratory organs be changed 

 from the tail-end in the larva to the head-end in the pupa? 

 Chironomus, the gnat, Corethra, and many other aquatic in- 

 sects exhibit the same phenomenon. Evidently there must be 

 some reason why it is more convenient for the larva to take in 

 air by the tail, and for the pupa to take in air by the head. Let 

 us consider the case of the larva first. Where it floats from the 

 surface, or pushes some part of its body to the surface, it is plain 

 that the tail must come to the top and bear the respiratory out- 

 let, for the head bears the mouth and mouth-organs, and must 

 sweep to and fro in all directions, or even bury itself in the mud 

 in quest of food. To divide the work of breathing and feeding 

 between the opposite ends of the body is of obvious advantage, 

 for the breathing can be done best at the top of the water, and 

 the feeding at the bottom, or at least beneath the surface. Such 

 considerations seem to have fixed the respiratory organs at the 

 tail of the larva. Why, then, need this arrangement be reversed 

 when the insect enters the pupal stage ? There is now no feed- 

 ing to be done, and it surely does not signify how the head is 

 carried. Why should not the pupa continue to breathe like the 

 larva, by its tail, instead of developing a new apparatus at the 

 opposite end of its body, as if for change's sake? Well, it does 

 not appear that, so far as the pupa itself is concerned, any good 

 reason can be given why the larval arrangement should not con- 

 tinue. But a time comes when the fly has to escape from the 

 pupa-case. The skin splits along the back of the thorax, and 

 here the fly emerges, extricating its legs, wings, head, and 

 abdomen from their close-fitting envelopes. The mouth- parts 

 must be drawn backwards out of their larval sheaths, the legs 

 upwards, and the abdomen forwards, so that there is only one 

 possible place of escape, viz. by the back of the thorax, where 

 all these lines of movement converge. If, then, the fly must 

 escape by the back of the thorax, the back of the thorax must 

 float uppermost during at least the latter part of the pupal stage. 

 Otherwise the fly would emerge into the water instead of into 

 the air. Granting that the back of the thorax must float upper- 

 most in the pupal condition, it is clear that here the respiratory 

 tubes must be set. 



I need hardly speak of the many insects which run and skate 

 on the surface of the water in consequence of the peculiar pro- 

 perties of the surface-film. They are able to do so, first, by 

 reason of their small size ; secondly, because of the great spread 

 of their legs ; and thirdly, on account of the fine hairs with 

 which their legs are provided. The adhesion of the surface- 

 film is measured by the length of the line of contact, and 

 accordingly the multiplication of points of contact may in- 

 definitely increase the support afforded by the surface of the 

 water. 



In the case of very small insects, it becomes possible, not only 

 to run on the surface of the water, but even to leap upon it, as 

 upon a table. This is particularly well seen in one of the 

 smallest and simplest of all insects — the little black Podura, 

 which abounds in sheets of still water. The minute and hairy 

 body of the Podura is incapable of being wetted, and the insect 

 frisks about on the silvery surface of a pond, just as a house-fly 

 might do on the surface of quicksilver. This is all very well so 

 long as the Podura is anxious only to amuse itself, or move from 

 place to place, but it has to seek its food in the water, and, 

 indeed, the attractiveness of a sheet of water to the Podura lies 

 mainly in the decaying vegetation far below the surface. But if 

 the insect is thus incapable of sinking below the surface, how 



does it ever get access to its submerged food ? I have endea- 

 voured to arrive at the explanation of this difficulty by observa- 

 tion of Poduras in captivity. If you place a number of Poduras 

 in a beaker half full of water, they are wholly unable to sink. 

 They run about and leap upon the surface, as if trying to escape 

 from their prison, but sink they cannot. I have chased them 

 about with a small rod until they became excited and much 

 alarmed, but they were wholly unable to descend. Even when 

 large quantities of alcohol were added to the water, the dead 

 bodies of the Podura are seen floating at the top, almost as dry 

 as before. It is only when they are placed upon the surface of 

 strong alcohol that the dead bodies become wetted, and after 

 a considerable time are seen to sink. How, then, does the 

 Podura ever descend to the depths where its food is found ? 



I found it an easy matter to make a ladder, by which the 

 Poduras could leave the upper air. A few plants of duck-weed 

 introduced into the beaker enabled them at pleasure to puU 

 themselves forcibly through the surface-film, and climb down 

 the long root hanging into the water like a rope. Once below 

 the surface, the Podura, though buoyant, is enabled, by muscular 

 exertion, to swim downwards to any depth. 



Other aquatic insects, not quite so minute as the Podura, 

 experience something of the same difficulty. A Gyrinus, or a 

 small Hydrophilus, finds it no easy matter to quit the surface of 

 the water, and is glad of a stem or root to descend by. 



To leave our aquatic insects for a moment, we may notice the 

 habit of creeping on the under-side of the surface-film, which is 

 so often practised by leeches, snails, cyclas, &c. I find this is 

 often described as creeping on the air, and some naturalists of 

 the greatest eminence, speak of fresh-water snails as creeping 

 " on the stratum of air in contact with the surface of the water." ^ 

 The body of the animal is, nevertheless, wholly immersed during 

 this exercise, as may be shown by a simple experiment. If 

 Lycopodium powder is sprinkled over the water, the light 

 particles are not displaced by the animal as it travels be- 

 neath. The possibility of creeping in this manner depends, 

 not upon any " repulsion between the water and the dry surface 

 of the body," to quote an explanation which is often given, but 

 upon the tenacity of the surface-film, which serves as a kind of 

 ceiling to the water-chamber below. The body of the leech is 

 distinctly of higher specific gravity than the water, and falls 

 quickly to the bottom, if the animal loses its hold of the surface- 

 film. The pond-snails, however, actually float at the surface, 

 and if disturbed, or made to retract their foot, they merely turn 

 over in the water. 



What is the result of all the expedients which have enabled 

 air-breathing insects to overcome the difficulties of living in 

 water? They have been successful, we might almost say too 

 successful, in gaining access to a new and ample store of food. 

 Aquatic plants, minute animals, and dead organic matter of all 

 kinds abound in our fresh waters. Accordingly the species of 

 aquatic insects have multiplied exceedingly, and the number of 

 individuals in a species is sometimes surprisingly high. The 

 supply of food thus opened out is not only ample, but in many 

 cases very easy to appropriate. Accordingly the head of the 

 larva degenerates, becomes small and of simple structure, and 

 may be in extreme cases reduced to a mere shell, not inclosing 

 the brain, and devoid of eyes, antennas, and jaws. The organs 

 of locomotion also commonly afford some indications of de- 

 generation. Where the insect has to find a mate, and discover 

 suitable sites for egg-laying, the fly at least must possess some 

 degree of intelligence, keen sense-organs, and means of rapid 

 locomotion. But some few aquatic insects, as well as some non- 

 aquatic species which have found out an unlimited store of food, 

 manage to produce offspring from unfertilized eggs, and to have 

 these eggs laid by wingless pupa; or hatched within the bodies of 

 wingless larvae. The development of the winged fly, the whole 

 business of mating, and even the development of the embryo 

 within the egg, have thus, in particular insects, been abbreviated 

 to the point of suppression. This is what I mean by saying 

 that the pursuit of a new supply of food has in the case of 

 certain aquatic insects proved even too successful. Abundant 

 food, needing no exertion to discover or appropriate it, has led 

 in a few instances to the almost complete atrophy of those higher 

 organs and functions which alone make life interesting. 



The degeneration of aquatic insects, however, very rarely 

 reaches this extreme. In nearly all cases the pupa is succeeded 

 by a fly, whose activity is in striking contrast to the sluggishness, 



' Semper's "Animal Life," Eng. trans., p. 205, and note 97. 



NO. 114 1, VOL. 44] 



