August 1, 1891.] 



KNOWLEDGE. 



141 



^^^ AN ILLUSTRATED '^n/ 



MAGAZINE OF SCIENCE 



SIMPLY WORDED— EXACTLY DESCRIBED 



LONDON: AUGUST 1, 1891. 



PAGE 



141 



CONTENTS. 



Gnats, Midges, and Mosquitos. — II. By E. A. Butler 

 The Life-History of Filaria Sanguinis Homlnis. By 



JosEPU W. Williams 



Lunar and Terrestrial Volcanos. By Rev. H. N. UuTcniN- 



soN, B.A., F.G.S 



The Chemistry of the Dairy. By Vauohan Cornish, 



B.Sr., F.O.S 



On the Fertilization of two common British Orchids. 



By J, Pentland Smith, M.A., B.Sc, &p., Lecturer on 



Botany, Ilorticulturiil College, Swanley 



Swimming Animals. By R. Lydekkeh, B.A. r,int;ih. 



On the Space-Penetrating Power of Large Telescopes. 

 \\\ .V. C. Ranyahd 



The IVIagic Square of Four. By T. S. Barrett 



The Observation of Red Stars. By Miss A. M. Clerke 



Whist Column. By W. Montagc Gathe, B.A.Oxon. 



Chess Column. By 0. D. Locock, B.A.Oxon 



1 .-> 1- 

 ].5f) 

 156 

 1.58 

 159 



NOTICE. 



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GNATS, MIDGES AND MOSQUITOS.-II. 



]>V \i. A. RuTI.EIt. 



As already intimated, Gnats and Moscjuitos are amon!,'st 

 that section of the " throad-hornod " flies whose 

 early life is aquatic, and a truly remarkable history 

 is theirs. That creatures so lV:iy;iIc should have 

 at any time any connection with so unstable and 

 treacherous an element as water is indeed strange, and un- 

 questionably large numbers perish through the mischances 

 involved in this very association ; nevcrtlieless, so groat is 

 their ii'cuiulity tliat the race runs no risii of extermination, 

 notwithstanding the dangers that beset the path of the 



individual in its advance to maturity. The eggs are long 

 oval objects, and from the time of laying they are entrusted 

 to the water. The female, when about to lay, rests with 

 her first pair of legs on some floating stick or leaf or other 

 support, the second pair gently touching the water, while 

 the third project over its surface. Crossing these like an X, 

 she allows an egg to pass into the angle where they meet ; 

 this is soon followed by another and another, their moist and 

 glutinous surfaces causing them to adhere to one another 

 with the long axis nearly perpendicular. In this way a 

 collection of some 200 or 300 is built up into the form of a 

 tiny raft, concave above — a sort of miniature life-boat, so 

 constructed that no capsizing can take place. The egg-raft 

 once made, the maternal duties are over, and the little craft 

 drifts rudderless away, exposed to sim and storm. This 

 venturesome voyage, however, lasts but a few days, and 

 then, the eggs having been from the first placed upside down 

 in the water, the lower end of the shell is forced off, and the 

 newly-hatched grub finds itself at once in position to take a 

 header into the watery world in which it has to pick up its 

 living. 



These larv® are odd-looking objects, foreshadowing the 

 form of the adult to a somewhat greater extent than is 

 usually the case with those insects which pass through a 

 complete metamorphosis. The three regions of the body 

 are distinctly marked out, quite the reverse of what obtains 

 amongst the " short-horned " flies, whose shapeless 

 "maggots" we described in a former number. If we 

 imagine the full-grown Gnat's body to be bereft of aU its 

 long appendages — wings, legs, antermse, and beak — and to 

 be provided at intervals with tufts of hair instead, we get 

 some idea of the outline of the larva. They move by a series 

 of jerks, accomplished by swaying the body from side to side, 

 and the natural position is head downwards. Though living 

 in the water they inhale air, and hence come to the surface 

 occasionally to breathe. The entrance to the breathing 

 tubes is at the end of a sort of arm or branch jutting out 

 fr'om the hinder — i.e., the upper — end of the body, and aU 

 that is necessary for taking in a fresh breath is to expose 

 this little orifice just above the svu-face of the water. The 

 larva is furnished with biting jaws, and spends a good deal 

 of its time in devouring all sorts of rubbish and decaying 

 matters, such as may be found in abundance in the pond it 

 inhabits. Thus it swims about with tail most appropriately 

 pointed to the sky, and head to the muddy bottom, where 

 he its chief stores of food. 



It is easy to understand that thousands of these larvfB, 

 working away in a pond on the decaying organic matter 

 there, will do a good deal towards arresting the pollution 

 of its waters, and Gnats, therefore, in this stage of their life 

 may be regarded as sanitary agents, of more or less use to 

 society at large. It follows, then, that their extermination 

 from any district might not be altogether an advantage, 

 unless accompanied by other changes, such as drainage, 

 &c. ; and in estimating the influence of Mosquitos, for 

 example, in the economy of Natm-e, one has to set 

 their services as scavengers over against the annoy- 

 ance thoy cause by sucking blood. It might be a 

 philosophical, if not very comforting, reflection for anyone 

 suffering from the persecutions of these pests, that the 

 more Mosquitos there are, the more scavenging work must 

 have been done in bringing them to maturity, and the 

 more must the sanitary condition of the country round 

 have been thereby improved ! There is another curious 

 fact connected with this stage in the life-history of these 

 insects ; when fully grown, as we have already seen, they 

 subsist only on liquid food, their mouth organs being 

 excellently fitted for taking in liquids while they would find 

 it absolutely impossible to make any use of solid food. But 



