AND ON THE HOVERING HABIT OF ERISTALIS TEN AX. 519 



readily attained its equilibrium as a cork does in water. One of 

 the opposing forces was external and independent, and therefore 

 not quite to be compared with a hovering fly, which has to create 

 and control opposing forces. 



We all know that the gossamer spider has the ability to emit 

 filaments of web w^hich, by the aid of its legs, it subsequently 

 gathers into a flocculent mass, and thus constructs an aerial 

 chariot capable of sustaining and transporting it on the zephyrs 

 of autumn for considerable distances. Here the support is the 

 direct product of the spider, but there is no permanent connec- 

 tion between them ; the conveyance once made lasts as long as 

 it is required, without further expenditure of labour, whereas 

 the hoveling fly has to be constantly expending energy in order 

 to maintain its position in the air. 



Now, we may ask, can we discover anything in connection with 

 Eristalis teiiax — and for that matter other hovering flies also — 

 that would probably assist it to maintain its equilibrium ? Are 

 any organs more highly developed than similar parts in flies that 

 cannot hover ? Is it provided with auxiliary organs absent in 

 the case of non-hovering species ? 



An affirmative answer can, I think, be given to each of these 

 questions. 



In the following descriptions I shall adopt the modern terms 

 " alulets or scales," the equivalents of " winglets " of the old 

 authorities. 



The fifth slide contains portions of the membranes which con- 

 stitute the two alulets or scales ; these are formed by the folding 

 of the parts upon themselves ; the deep depression in the margin 

 indicates the point where the fold occurs ; they are of unequal 

 size. The scales are spread apart, the better to display the 

 remarkable appendages which border their free margins. 



The fringing appendages are of two distinct kinds, and will well 

 repay very careful examination. Those which border the smaller 

 scale appear at first sight like simple flat plates ; but if examined 

 carefully, they will be found to be hollow, or tubular, organs. A 

 central tube passes either completely through, or stops short of 

 the free ends — which, I have been unable to determine, as some 

 appear to favour one view, others the reverse. Probably they 

 are subject to wear, but until a further supply of insects can 

 be obtained this must remain an open question. 



The basal portions of these organs can, I think, be traced 

 for some distance within the margin of the scale, and in this 



