SWIFT FLIES. 



765 



common house-fly, the figure having been enlarged in order to 

 show its structure the better. 



The generic term TacMna is formed from a Greek word sig- 

 nifying " rapidity," and is given to the insects in consequence of 

 their swiftness. There are very great numbers of species belong- 

 ing to this genus, and there is scarcely a part of the world in 

 which they are not represented. None of them are brilliant 

 in point of colour, brown, 

 black, grey, and yellow 

 beingthe invariable hues. 



The present species is 

 a native of Mexico. Its 

 general colour is black, 

 and even the long hairs 

 which thickly clothe the 

 body are of the same 

 colour. The thorax is 

 black, but is relieved by 

 some lines of silver-grey, 

 with a very slight tinge 

 of blue in them. This is 

 the usual colouring, but 



the insect is exceedingly variable, both in size and colouring. 

 Some specimens are barely half as large as others, while in 

 some the colour is entirely yellow, in others half yellow and 

 half black, and in others again the colour is entirely black, with 

 the exception of a yellow tip to the tail 



The genus to which belong the next two insects is rather a 

 large one, and contains some of the most splendidly coloured of 

 the Diptera. The first species, Dexia cffulgcns, is a native of 

 New Guinea. The ground colour of the insect is soft velvety 

 black. Upon the abdomen the black is relieved by a number 

 of interrupted bands, which glow with tints so varying that it 

 is almost impossible to decide which is the real colour. One 

 bar, for example, will seem to be glittering emerald-green, 

 another gold, and another azure; but on shifting the position 

 of the insect it will be found that the same colours pass in turn 

 over each bar, the hue depending wholly on the direction of the 



Fig. 510. — Tachina grandis. 

 (Brown and yellow, variable.) 



