DIPTERA : STRUCTURE. 415 



periments of Sir Everard Home, it has been generally con- 

 sidered that this was effected by the formation of a vacuum, 

 by means of the close application of the margins of these 

 soles of the feet, as we may term them, and the subsequent 

 muscular raising up of their central parts ; but the following 

 remarks by one of our most acute modern observers, Mr. 

 Blackwall, published by him in the Appendix of the last 

 volume of the Linnaean Transactions (in correction of a 

 memoir previously published by himself, in which he had 

 adopted the vacuum system), will be read with interest, as 

 showing what interesting sources of inquiry are opened to 

 the student of nature, even in the commonest objects of the 

 creation. 



In experimenting upon the house-fly, he observed that in- 

 dividualsfrequentty remained fixed to the sides of an exhausted 

 glass receiver after they had entirely lost the power of loco- 

 motion, and an evident distension of the abdomen had been 

 occasioned by the exhaustion of the aeriform fluids it con- 

 tained. To detach them from these stations, the employ- 

 ment of a small degree of force was found requisite. 



" In prosecuting this subject, clean phials of transparent 

 glass, containing spiders and various insects in the larva and 

 imago states, capable of walking on their upright sides, were 

 breathed into, till the aqueous vapour expelled from the lungs 

 was copiously condensed on their inner surface. The result 

 was remarkable ; the moisture totally prevented those animals 

 from obtaining any effectual hold on the glass, and the event 

 was equally decisive if a small quantity of oil was substituted 

 for the aqueous vapour. A similar consequence ensued, also, 

 when the flour of wheat, or finely pulverised chalk or gyp- 

 sum, was thinly strewed on the interior surface of the phials, 

 the minute particles of those substances adhering to the 

 tarsal brushes of the spiders, the pulvilli of the perfect in- 

 sects, and the under side of the feet of the larvae. These 

 facts, far from corroborating the mechanical theory, appeared 



