August 1 , 1890.] 



KNOWLEDGE 



189 



are connected with the different conditions, as regards the 

 struggle for existence, under which the contrasted species 

 live. There are, according to him, comparatively few 

 species of carrion-feeding flies in our regions, so that they 

 have it all to themselves, with little competition, the 

 numerous beetles that have similar habits being said to 

 feed quite as much on the fly larvie themselves as on the 

 carrion. Their action must, from tlie nature of the case, 

 be rapid, and tlie supply of food lie regards as plentiful. 

 All these conditions fiNour multiplication, and have con- 

 tributed towards |iin;liiciiiu thu extraordinary fecundity for 

 which the insects ;ue noted. TIhtc is one other point 

 which lessens the competition, vi/. that the diti'erent 

 species to some extent succeed one another during the 

 summer in point of time (/.c in the larval form), instead 

 of being contemporaneous. This was proved in the fol- 

 lowing way : he laid out of doors the dead bodies of small 

 vertebrates, such as rats, birds, &c. ; the flies soon laid 

 eggs upon them, and these were then collected and the 

 magLjots bred to maturity, whereby the relative numerical 

 aiiund:ince of the species was ascertained. One of the 

 chief results thus established was that ( 'i/noiin/ia mnr- 

 tuoriim, an out-door flesh-fly, was abundant in spring, but 

 Cidlipliora roinitoria, a bluebottle, did not begin to appear 

 till about the beginning of .Tune, when the former species 

 had begun to decline. Later on, the proportions were 

 reversed, the bluebottle being in excess, and the flesh-fly 

 scarce. Thus everything favours the fecundity of the 

 carrion-feeders. 



But with the dung-feeders the case is dift'erent. Here 

 there is much competition, there being large numbers of 

 dung-feeding flies, as well as beetles, which latter do not 

 feed on the former. The supply of food, too, Portchinski 

 regards, curiously enougii, as less abundant. These are 

 circumstances which place fecundity at a disadvantage, 

 and hence have sprung the more moderate powers of 

 multiplication possessed by the dung-feeding ties. But 

 here, again, a very curious circumstance has arisen, which 

 gives an extra ad\antage to the less proliflc species. There 

 is a fly which is structurally 

 very like M. domestica, differing 

 eliiefly in the brighter colour of 

 its abdomen (Fig. 18), which 

 is of some tint of yellowish or 

 brown. Its name is Musca ror- 

 rina, and it frequently hibernates 

 in houses, and rnay therefore be 

 reckoned iimoiiL;sl ibr household 

 species. Nolw iibstaiiding the 



close resemblance between these 

 two species iu their perfect con- 

 dition, as well as in that of the 

 full-grown larva', there is a most 

 extraordinary dift'erencc in the 

 circumstances of their develop- 

 ment. While the house-fly lays 

 120 eggs. A/, (■(irrittii lays oidy 24, 

 but they are much larger, and lience the larval life is able 

 to be shortened ; this is done at the expense of one of the 

 two transformations. M. iliHncxlira, as before mentioned, 

 passes through three stages in its larval life, while ,1/. mr- 

 riiKi has only two, the second of the three being, in its 

 ca.se, onntled altogether. This enables it to come to 

 maturity sooner than its relative, and hence gives it an 

 advantage which counterbalances its low degree of fecmi- 

 dity. In some such way as this, Portchinski considers 

 that more prolillc flies liave been weeded out by less proliflc 

 ones from aiuongst the dung-feeders, so that tlie majority 

 are now of the less prolific type. But .U. iliuiicsiiai, with 



Fig. 18. — DisTiuiiuTioN 



Ol- COLOim OS AllDOMKN 



OF Musca C'okvixa. The 

 clear parts are y('lli>w, tin; 

 shaded parts lilack. 



a degree of fecundity which, though low as compared 

 with the carrion -feeders, is yet high for a dung-feeder, is 

 apparently an exception amongst the latter, and herein 

 M. Portchinski finds the explanation of its close associa- 

 tion with mankind, the bond of union being, iu fact, in 

 this particular species probably closer than in any other, 

 for the house-fly is said to be rarely found far from human 

 dwellings. According to the above theory, the house-fly 

 has sought the protection and additional resources of 

 man's society to aid it in its struggles with less prolific 

 insects, which, by their shorter larval life, would other- 

 wise have hurried it out of existence. Whatever may be 

 thought of these .speculations, and it would obviously not 

 be difficult to raise objections to them, still the ob.serva- 

 tions on which they are based have i-evealed some very 

 curious facts which require to be accounted for m some 

 way or other, and which invest with special interest the 

 history of the relations between insects and man. Farther 

 researches by the same investigator show that the de- 

 velopmental history of an insect may depend very much 

 upon climate, the same kind of fly developmg in a 

 different way in northern and in southern latitudes. 



However exclusive the tastes of their larvie may be, some 

 at least of these flies seem to be almost omnivorous in 

 their perfect stage, and therefore, possibly, food is one 

 attraction which allures them into our houses ; but then 

 again comes the question " Why these alone '? Why do not 

 the abimdant supplies man's proxidence stores up become 

 equally enticing to other closely allied forms, whose tastes 

 and needs one would suppose to be similar? " Flies gene- 

 rally manage to And out the room in which the provisions 

 happen to be placed, though the aspect of the apartment 

 has certainly quite as much to do with the numbers that 

 And their way thither as the mere presence of eatables ; 

 if the room be bright and sunny, the flies will swarm, 

 while the same provisions in a dull and shady room will 

 be almost ignored. And again, apart from the occasional 

 intrusion of an unwelcome bluebottle making straight for 

 the cold meat, the bringing in of meals does not usually 

 produce any noticeable increase in the number of flies in 

 the room ; often many of those that are there seem 

 supremely indifl'erent to the viands that may be displayed, 

 and continue to amuse themselves by sporting about the 

 windows, mirrors, picture-frames, or gas pendants. We 

 must not forget, however, that wdiat is not food to us may 

 yet be so to them, and they appear to And in these various 

 household objects some sort of nutriment, to judge from 

 the industrious way in which, for exam])le. they will travel 

 over the painted window-frames, dabbing their proboscis 

 down with as much persistence and energy as if they were 

 making a most luxurious feast. What is it they get ? Is 

 it the varnish, or the oil with which the paint was mixed, 

 or is it tlie thin film of miseellaneous matter — dirt we call 

 it- which gradually accumulates on every exposed surface ? 

 On any of these suppositions, one would suppose that there 

 would be at least as much to bo obtained out of doors as 

 inside, and probably a good deal more. 



Such species as hibernate in the liouso, like l'i<U<::i<i 

 niilis, no doubt come in for the sake of shelter from 

 winter's cold. These gradually accumulate, instead of 

 suddenly coming in a swarm. An instance has been 

 recorded of two other species of the same genus s.vanning 

 in the same building, to the exclusion of others, for seven 

 successive years ; tlie suggested explanation was that there 

 were certain conditions which facilitated the cntniucc of 

 the flies, but rendered their exit difficult. 



The Editor ro^rets that want of space has prevented the insertion 

 of any Notices of Books this month. 



