460 



NATURE 



[September 10, 1891 



■of flaps, which form an efficient swimming-fan. The body of 

 the pupa, like that of the larva, is abundantly supplied with air- 

 tubes, and a communication with the outer air is still maintained, 

 though in an entirely different way. The air-tubes no longer 

 open towards the tail, as in the larva, but towards the head. 

 Just behind the head of the future fly is a pair of trumpets, so 

 placed that in a position of rest the margins of the trumpets 

 come flush with the surface of the water. Floating in this posi- 

 tion, the pupa remains still, so long as it is undisturbed, but if 

 attacked by any of the predatory animals which abound in fresh 

 waters, it is able to descend by the powerful swimming move- 

 ments of its tail fin. 



Not that the descent is without its difficulties. The pupa is 

 not like the larva, denser than water, but buoyant. There are 

 two respiratory tubes in the pupa, whereas there is only one in 

 the larva, and to these two tubes the surface-film clings with a 

 tenacity of which only experiment can give an adequate idea. 

 Will you allow me to give you a little more borrowed physics? 

 If we take al solid body, capable of being wetted by water, 

 and place it in water, the surface-film will adhere to the solid. 

 If the solid is less dense than the water, it will float with part of 

 its surface out of the water. Under such circumstances the sur- 

 face-film will be drawn upwards around the solid, and will 

 therefore pull the solid downwards. But if the solid is denser 

 than the water, the surface-film around the solid will be pulled 

 downwards, and will pull the solid upwards. Suppose that a 

 ■solid of the same density as water floats with part of its surface 

 in contact with air, and that weights are gradually added to it. 

 The result will be that the surface of the water around the 

 upper edge of the solid will become more and more depressed. 

 The sides of the depression will take a more vertical position, 

 until at last the upward pull of the film becomes unable to with- 

 stand further increase of weight. If this point is passed, the 

 solid will sink. Before this point is attained, we shall have the 

 solid, though denser than water, kept at the surface by the pull 

 of the surface-film. 



This state of things may be illustrated by a model. [Float 

 with glass tube attached to its upper surface.] You will readily 

 see that the float has to be weighted appreciably in order to 

 break the connection of the tube with the surface-film. Now 

 the pupa of the gnat has a pair of tubes which are in like 

 manner attached to the surface of the water. When it requires 

 to descend, the pull of the surface-film would undoubtedly be 

 considerable. Adding weight to the body is, of course, im- 

 possible, and a great exertion of muscular force would be waste- 

 ful of energy, even if it could be put forth. The gnat deals 

 with its difficulty in a neater way than this, and saves its muscular 

 power for other occasions. Let me show you a method of free- 

 ing the float from the surface, which was suggested by observa- 

 tion of what was seen in the pupa of the gnat. A thread wetted 

 with water is drawn over the mouth of each tube. It cuts the 

 connection with the surface, and the float, loaded so as to be 

 denser than water, goes down at once. iVIeinert has described 

 a pencil of hairs which appear to perform the same office for the 

 pupa of the gnat. The hairs draw a film of water over the open 

 mouth of each respiratory tube, and muscular contraction, used 

 moderately and economically, does the rest. When the pupa 

 again comes to the surface the tubes are overspread by a glisten- 

 ing film of water. This is partially withdrawn by a movement 

 of the hairs, so that a chink appears by which air can be slowly 

 renewed. When the insect is completely tranquil, the hairs 

 appear to withdraw more completely, and the tube suddenly 

 becomes free of all film. The act of opening or closing the film 

 is so rapid— like the wink of an eye — that I cannot pretend to 

 have observed more than the closed tube, the slightly open tube, 

 and then the sudden change to a completely open condition. 

 [Living pupae shown by the lantern.] 



Another Dipterous larva described and admirably figured by 

 Swammerdam is the larva of Stratiomys, a larva which, as the 

 structure of the fly shows, belongs to an altogether different 

 group from Chironomus, Simulium, or the gnat. Though only 

 remotely connected with the gnat in the systems of zoologists, 

 the Stratiomys larva has learned the same lesson, and is equally 

 well fitted to take advantage of the peculiar properties of the 

 surface-film. The tail-end of the Stratiomys larva is provided 

 with a beautiful coronet of branched filaments. When this 

 coronet is extended, it forms a basin open to the air and im- 

 pervious to water, by reason of the fineness of the meshes 

 be; ween the component filaments. Were the larva provided 

 with a basin of the same proportions foimed out of continuous 



membrane, it might float and breathe perfectly well, but the 

 old difficulty would come back, viz. that of freeing itself 

 neatly and quickly when some sudden emergency required the 

 animal to leave the surface. As it is, the plumed filaments 

 collapse and their points approach ; the side-branches are folded 

 in, and the basin is in a moment reduced to a pear-shaped body, 

 filled with a globule of air, and reaching the surface of the water 

 only by its pointed extremity. Down goes the Stratiomys larva 

 at the first hint of danger, swimming through the water with 

 swaying and looping movements, somewhat like those of Chiro- 

 nomus. When the danger is past, it ceases to struggle, and 

 floats again to the surface. The pointed tip of its tail-fringe 

 pierces the surface-film, the filaments separate once more, and 

 the floating basin is restored. 



The larva of Stratiomys is extremely elongate. The length 

 of its body has evidently some relation to the mode of life of 

 the larva, but none at all to that of the fly which is formed 

 within it. The pupa is so much smaller than the larva as to 

 occupy only the fore-part of the space within the larval skin.^ 

 The interval becomes filled with air, and during the pupal stage 

 the animal floats at the surface within the empty larval skin. 



Stratiomys, both in its larval and pupal states, floats at the 

 surface of the water. The larva can descend into the water 

 when attacked, but the pupa is too buoyant, and too much en- 

 cumbered by its outer case, to execute any such manoeuvre. 

 Provision has accordingly to be made for the protection of the 

 helpless pupa against its many enemies. It is probable that 

 hungry insects and birds mistake the shapeless larval skin, float- 

 ing passively at the surface, for a dead object. The considerable 

 space between the puter envelope, or larval skin, and the body 

 of the pupa may keep off" others, for the first bite of a Dytiscus 

 or dragon-fly larva would be disappointing. Still further security 

 is gained by the texture of the larval skin itself. The cuticle 

 consists of two layers. The inner is comparatively soft and 

 laminated, while the outer layer is impregnated with calcareous 

 salts, and extremely hard. The needful flexibility is obtained 

 by the subdivision of the hard outer layer. Seen from the 

 surface, it is broken up into a multitude of hexagonal fields, each 

 of which forms the base of a conical projection, reaching far 

 into the softer layer beneath. The conical shape of these cal- 

 careous nails allows a certain amount of bending of the cuticle, 

 while the whole exposed surface is protected by an armour, in 

 which even the pointed mandibles of a Dytiscus larva can find 

 no effective chink. 



The larva and pupa of the Dipterous fly, Ptychoptera paludosa, 

 exhibit some interesting adaptations of the tracheal system to 

 unusual conditions. The larva is found in muddy ditches, where 

 it buries itself in the black ooze to a depth of an inch or two. 

 Here, of course, it can procure no oxygen, either gaseous or dis- 

 solved. When it requires a fresh supply, it must reach the sur- 

 face with part of its body, and to enable it to do so with the 

 least possible exertion, the tail-end of the body is made tele- 

 scopic, like that of another and still more familiar Dipterous 

 larva, Eristalis. The last segments are drawn out very fine, 

 and are capable of a very great amount of retraction or expan- 

 sion. No visible opening for the admission of air has been dis- 

 covered, nor do the hairs form a floating basin, as in the Stratio- 

 mys larva. The larva may be often seen lying just beneath the 

 surface, which is broken by the tip of the tail. Whether air 

 can be admitted here by some very minute orifice, or whether it 

 is renewed by the exchange of gases through a thin membrane, 

 I cannot as yet venture to say. In shallow water the larva may 

 be occasionally found lying on or in the mud, and stretching out 

 its long tail to the surface. In deeper water, it often floats at 

 the surface. 



Two tracheal trunks run along the whole length of the body, 

 including the slender tail, where they are extremely convoluted 

 and unbranched. Towards the middle of the body the tracheae 

 become greatly enlarged in the centre of eachsegnnent, the inter- 

 vening portions, from which many branches are given off, being 

 comparatively narrow. Each tube, therefore, resembles a row 

 of bladders connected by small necks. A cross-section shows 

 that the tubes are not cylindrical, but flattened, and that, while 

 the lower surface is stiffened by the usual parallel thickenings, 

 the upper surface is thrown into two deep, longitudinal furrows, 

 so that it is readily inflated, becoxning circular in section, and 

 readily collapses again when the air is expelled. It seems likely 



' So singular is the disproportion between the larva and the pupa that 

 some naturalists have actually described the latter as a parasite (Wesiwood's 

 " Mod. Classification of Insects," vol. ii. p. 532)- 



NO. 



I 141, VOL. 44] 



