458 



NA TURE 



[September io, 1891 



which becomes dissolved in the abundant blood of the larva. 

 Four delicate tubes filled with blood, which are carried upon 

 the last segment of the body, are believed to be especially 

 intended for the taking up of dissolved oxygen. The tracheal 

 system is rudimentary and completely closed, and hence gaseous 

 air cannot be taken into the body. The dissolved oxygen, pro- 

 cured with much exertion and some risk, must be stored up 

 within the body of the larva, and used with the greatest 

 economy. It is apparently for this reason that the larva of 

 Chironomus contains a blood-red pigment, which is identical 

 with the haemoglobin of vertebrate animals. The haemoglobin 

 acts in the Chironomus larva as it does in our own bodies, 

 as an oxygen-carrier, readily taking up dissolved oxygen, and 

 parting with it gradually to the tissues of the body. 



It is instructive to notice that only such Chironomus larvae as 

 live at the bottom and burrow in the mud possess the red 

 haemoglobin. Those which live at or near the surface have 

 colourless blood, and a more complete, though still closed, tra- 

 cheal system. The larva of the carnivorous Tanypus, which is 

 found in the same streams, but does not burrow, has a much 

 more complete tracheal system, and only enough haemoglobin to 

 give a pale red tint to the body. The larva of the gnat again, 

 which has a large and open tracheal system, and in all stages of 

 growth inhales gaseous air, has no haemoglobin at all. A list 

 of the many animals of all kinds which contain haemoglobin, 

 shows that for some reason or another each of them requires to 

 use oxygen economically. Either the skin is thick, and the 

 respiratory surface limited, or they are inclosed in a shell, or 

 they burrow in earth or mud. We might expect to find that 

 haemoglobin would always be developed in the blood of animals 

 whose respiration is rendered difficult in any of these ways, but 

 any such expectation would prove to be unfounded, and there 

 are many animals whose mode of life renders it necessary that 

 oxygen should be stored and economically used, which contain 

 no haemoglobin in their blood. Hence, while we have a toler- 

 ably satisfactory reason for the occurrence of haemoglobin in a 

 number of animals whose respiratory surface is limited, and 

 whose surroundings make it a matter of difficulty to procure a 

 sufficient supply of oxygen, we have to admit that many similar 

 animals under the same conditions manage perfectly well without 

 haemoglobin. Such admission is not a logical refutation of the 

 explanation. I might fairly put forward the baldness of man- 

 kind as at least the principal reason for wearing wigs, and this 

 explanation would not be impaired by any number of cases of 

 bald men who do not wear wigs. The fact is that the respiratory 

 needs, even of closely allied animals, vary greatly, and further, 

 there are more ways than one of acquiring and storing up oxygen 

 in their bodies. 



Either the storage-capacity for oxygen of the Chironomus 

 larva is considerable, or it must be used very carefully, for the 

 animal can subsist long without a fresh supply. I took a flask 

 of distilled water, boiled it for three-quarters of an hour, closed 

 it tight with an india-rubber bung, and left it to cool. Then 

 six larvae were introduced, the small space above the water 

 being at the same time filled up with carbonic acid. The bung 

 was replaced, and the larvae were watched from day to day. 

 Four of the larvae survived for forty-eight hours, and one till the 

 fifth day. Two of them changed to pupae. Nevertheless, the 

 water was from the first exhausted of oxygen, or nearly so. 



The Chironomus larva is provided with implements suited to 

 its mode of life. The head, which is extremely small and hard, 

 carries a pair of stout jaws, besides a most complicated array of 

 hooks, some fixed, some movable. The use of these minute 

 appendages cannot always be assigned, but some of them are 

 apparently employed to guide the silky threads which issue 

 from the salivary glands. The first segment behind the head 

 carries a pair of stumpy legs, which are set with many hooks. 

 These are mainly used in progression, and help the larva to 

 hitch itself to and fro in its burrow. A similar, but longer pair 

 of hooked feet is found at the end of the body. This hinder 

 pair serves to attach the animal to its burrow when it stretches 

 forth in search of food. 



Creeping aquatic larvae, such as Ephydra, possess several pairs 

 of legs in front of the last pair, but the burrowing species, such 

 as caddis-worms, agree with Chironomus, not only in their mode 

 of life, but also in the reduction of the abdominal legs to a single 

 pair, which are conspicuously hooked. 



The larval head in this, as in many other aquatic insects, is 

 far smaller and simpler than that of the fly. The larval head is 

 little more than an implement for biting and spinning, by no 



NO. I 141, VOL. 44] 



means such a seat of intelligence as it is in higher animals. In 

 Chironomus it contains no brain ; the eyes are mere specks of 

 pigment, and the antennae are insignificant. But the head of 

 the fly incloses the brain, and bears elaborate organs of special 

 sense — many-facetted eyes, and in the male beautiful plumed 

 antennae. This difference in size and complexity probably 

 explains the fact that the head of the fly is not developed within 

 the larval head, but in the thorax. It is only at the time of 

 pupation that it becomes everted, and its appendages assume 

 the position which they are ultimately intended to occupy. 



At length the Chironomus wriggles out of the larval skin, and 

 is transformed into a pupa. It no longer requires to feed, and 

 the mouth is completely closed. It is equally unable to burrow, 

 and usually lies on the surface of the mud. Two tufts of silvery 

 respiratory filaments project from the fore-end of the body just 

 behind the future head, and these wave to and fro in the water, 

 as the animal alternately flexes and extends its body. At the 

 tail-end are two flaps, fringed with stout bristles, which form a 

 kind of fan. The pupa virtually consists of the body of the 

 I fly, inclosed within a transparent skin. The organs of the fly are 

 already complete externally, and even in microscopic detail they 

 very closely resemble those of the perfect animal. These parts 

 are, however, as yet very imperfectly displayed. The wings 

 and legs are folded up along the sides of the body, and are 

 incapable of independent movement. For two or three days 

 there is no outward change, except that the pupa, which origin- 

 ally had the blood-red colour of the larva, gradually assumes a 

 darker tint. The tracheal system, which was quite rudimentary 

 in the larva, but is now greatly enlarged, becomes filled with 

 air, secreted from the water by the help of the respiratory tufts, 

 and the pupa floats at the surface. At last the skin of the back 

 splits, the fly extricates its limbs and other appendages, pauses 

 for a moment upon the floating pupa-case, as if to dry its wings, 

 and then flies away. 



This fly is a common object on our window panes, and would 

 be called a gnat by most people. It can be easily distinguished 

 from a true gnat by its habit of raising the fore-legs from the 

 ground when at rest. It is entirely harmless, and the mouth- 

 parts can neither pierce nor suck. Like many other Diptera, 

 the flies of Chironomus associate in swarms, which are believed 

 in this case to consist entirely of males. The male fly has 

 plumed antennae with dilated basal joints. In the female fly 

 the antennae are smaller and simpler, as well as more widely 

 separated. 



In brisk and lively streams another Dipterous larva may often 

 be found in great numbers. This is the larva of Sinmlium, 

 known in the winged state as the sand-fly. The Simulium larva 

 is much smaller than that of Chironomus, and its blood is not 

 tinged with red. The head is provided with a pair of ciliary 

 organs, fan-like in shape, consisting of many longish filaments, 

 and borne upon a sort of stem. The fringed filaments are used 

 to sweep the food into the mouth. The larva of Simulium 

 subsists entirely upon microscopic plants and animals. Among 

 these are great numbers of Diatoms, and the stomach is usually 

 found half full of the flinty valves of these microscopic plants. 

 The Simulium larva seeks its food in rapid currents of water, 

 and a brisk flow of well-aerated water has apparently become a 

 necessity to it. If the larvae are taken out of a stream and 

 placed in a vessel of clear water, they soon become sluggish, 

 and in warm weather do not survive very long. It matters 

 little, however, to the larvae whether the water in which they 

 live is pure or impure ; and streams which are contaminated 

 with sewage often contain them in great abundance. There are 

 no externally visible organs of respiration, but the skin is sup- 

 plied by an abundant network of fine tracheal branches, which, 

 no doubt, take up oxygen from the well-aerated water in which 

 the animal lives. From this network at the surface, branches 

 pass to supply all the internal organs. The Simulium larva is 

 found upon aquatic weeds, and the pair of hind- feet, which in 

 Chironomus were shaped so as to enable the larva to hold on 

 to its burrow, here become altered, so as to furnish a new means 

 of attachment. The two feet are completely united into one. 

 The two clusters of hooks found in the Chironomus larva form 

 now a circular coronet, and the centre of the inclosed space 

 becomes capable of being retracted by means of muscles which 

 are inserted into it from within. The larva is thus enabled to 

 adhere to the smooth surface of a leaf, holding on by its sucker, 

 which is, no doubt, aided by the circle of sharp hooks. Efficient 

 as this adhesive organ undoubtedly is, it must be liable to de- 

 rangement by occasional accidents, as, for instance, if there 



