July 1, 1891.] 



KNOWLEDGE 



123 



organs, the maxillary palpi, representatives of the two 

 iinjointed red clubs which are such conspicuous appendages 

 of the mouth of the blow- fly. This straight, unjointed 

 spike is, at first sight, as different as could well be 

 imagined from the elbowed, broad-tipped apparatus with 

 which the house-fly and the blow-fly sip their hquid 

 nutriment ; jet both are but extreme modifications of the 

 same plan, the rasping and sucking elements being carried 

 to the summit of perfection in the one case, and the boring 

 or piercing ones in the other. Many intermediate forms 

 may be seen, as in the drone-flies, breeze-flies, wasp- 

 flies, and others which have no popular names, and a very 

 interesting series showing the gradations might without 

 much difticulty be prepared. 



Now how is this collection of weapons used ? The little 

 insect drops gently and daintily down on to the spot it has 

 selected for its attack, and the descent of so light and airy 

 a being is likely to leave the victim unconscious of its 

 presence, unless he has actually seen it settle. Then the 

 proboscis is pointed downwards, and the tiny lips that form 

 its tip pressed against the flesh. The bristles withia the 

 gutter-like sheath, being then pressed together into one 

 solid boring implement, their common tip is forced down 

 on the flesh, and as they enter the wound, the trough in 

 which they were lying separates from them in the middle, 

 and becomes bent towards the insect's breast, the two little 

 lips all the while holding on tight. The greater part of 

 the length of the stilettos is then plunged into the victim's 

 flesh, and the blood is drawn up the flne interstices of the 

 composite borer. The wound, though six instruments are 

 concerned in making it, is extremely minute. 



8o far, our description has concerned the proboscis of 

 the female gnat or mosquito only. That of the male is 

 somewhat different. There is still the straight stick-like 

 labium, but the palpi are greatly elongated, running along 

 by the sides of the tubular proboscis as far as, or even 

 beyond, its tip, and tufted at the end. A fine rod-like 

 organ may be separated from the labium, but whatever 

 else the insect may have in this way, it does not use for 

 sucking blood, being in fact perfectly harmless. 



In their earlier life these insects inhabit ponds and stag- 

 nant water generally. The larva and pupa are shown in 

 the accompanying photograph. The former, an odd- 

 looking, big-headed, wriggling creature, swims about head 

 downwards, devouring all sorts of organic refuse in the 

 water, coming to the surface to breathe through the 

 opening at the end of the tail-branch. The pupa also 

 swims about, but with its head upwards, and though 

 active, it takes no food. It requires to come to the surface 

 occasionally for air, which is taken in at the two little 

 projecting horns on the thorax. Fuller details of these 

 early stages and of the entire life-history must be reserved 

 for our next paper. 



{To be continued.) 



ON THE PLAN OF THE SIDEREAL SYSTEM. 



By J. R. Sutton," B.A., Cantab. 



ANY lines of circumstantial evidence goto show 

 that the Milky Way is a ring-shaped formation, 

 roughly circular in section, one of the most 

 important depending upon the almost obvious 

 connection between the lucid stars lying on or 

 near the galactic belt and the nebulous looking matter of 



M 



• Mr. Sutton is at present in South Africa. Hi 

 Kiniburley with a letter iu which ho apuh>Ki/os fn 

 in the ul.acnrc of books of reterem-o. lli-i i.liM> 

 tion of tho belt of i^eut stars with the Milky 

 indeiwudeutly and before he had seen my paper iV 



1 jiaper was forwarded from 

 ■ wiifinif ou such a subject 

 wilh regard to the flssocia- 

 NN'iiy were formed eutirely 

 tho May number. — A.C.R. 



which it consists. It scarcely needs demonstration that 

 all these lucid stars are not necessarily ijidm-tii- (under 

 which term we include all stars actually within the 

 stream, or which, though outside it, are so near as to 

 influence it to an appreciable extent), nor is it Ukely that 

 the relation when it does exist will always be made out. 

 Nevertheless, in many, perhaps the majority of cases, so 

 far as the (optically) extra-galactic stars are concerned, 

 the problem is not a diflicult one for any person ou whom 

 the shadow of -John Michell's mantle has fallen. When- 

 ever, for example, a star or group of stars lies opposite or 

 inside a gap in the profile of the Milky Way ; or whereve. 

 any of its numerous branching lateral offsets terminate in 

 the immediate vicinity of a bright star or star-group, we 

 are entitled to assume, if the doctrine of chance has any 

 credit at all, that these stars are intimately associated 

 with, if they are not the agents which have determined 

 the conformation of the stream, and they may therefore be 

 considered a part of it. 



When we come to deal with the stars optically upon 

 the stream the problem is not quite so simple. Such 

 stars may be either within the stream or without it, and, 

 if the latter, may or may not be galactic. We know that 

 the parts of the sky traversed by the Milky Way contain 

 very many more bright stars than would be the case if the 

 stars were uniformly distributed over the whole celestial 

 sphere ; and Probability interprets this to mean that the 

 chances are enormously against a general dissociation 

 between the two. This, however, is not exactly the same 

 as provini/ a relationship. If it were, it would mean that 

 all, or nearly all, the stars so situated are galactic. On 

 the contrary, although the statement may be true in 

 general, it is not possible to indicate at random any star 

 or star-group as therefore forming part of the Milky Way. 

 Such details have to be decided independently, from 

 particular and not from general considerations. The only 

 instances we can be tolerably sure of are those (1) in 

 which a stream of lucid stars and a nebulous streamer 

 branch out together from the main course of the Galaxy, 

 and turn to the right or left up to the apex of either with- 

 out parting company ; (2) in which a star or star-group 

 lies in the midst of a small dark space surroimded by 

 fields of normal brightness ; and (8) in which stars are 

 seen significantly mixed up with clustering aggregations 

 of nebulous matter. In all cases of this nature there is no 

 reason to doubt that the Galaxy and the lucid stars are 

 mutually dependent. 



There is no necessity just now to discuss these points at 

 any greater length ; that has been done elsewhere. They 

 have been introduced, in brief, simply to indicate the 

 principal lines of evidence made use of in the attempt to 

 prove the rmg-fonn of the Milky ^^'ay, and to avoitl con- 

 tinual repetition iu the course of this tliscussion. 



It is clear that some of the clusters in the Milky Way 

 might be streams of stars seen in projection. 



Suppose an observer to select some star-group lying at 

 the extremity of a straight galactic streamer, and then to 

 take up such a position iu space that his line of sight 

 should pass through the group and along the axis of the 

 streamer. If the streamer be supposed dinded into 

 sectional lamime of the same thickness, these, taken 

 singly, would be of the same intrinsic brightness however 

 far off they might be ; and in the case of a cylindrical 

 stream their apparent size would vary inversely as the 

 square of their distance from the observer. Hence the 

 streamer would have, from the assigned position in space, 

 the characteristics of a close globular cluster, its brilliancy 

 decreasing gradually but rapidly towards its edges. 

 Moreover, the lucid star-group at its apex would be pro- 



