LETTERS ON ENTOMOLOGY. 107 
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LETTER XI. 
MY DEAR FRIEND, 
There is a tribe of insects more 
numerous and more varied than almost any 
other, and which furnishes a great branch of 
their natural history — I mean the tribe of flies. 
The number of species greatly exceeds that of 
butterflies, but in general they are very much 
smaller ; there are, however, some which greatly 
surpass them in size ; dragon-flies, for instance, 
whose bodies are longer than those of the largest 
butterflies. Grasshoppers belong to the class of 
flies, and some are of considerable size ; but the 
greatest part are comparatively very small, and 
some so diminutive, that the different species 
cannot be distinguished from each other. 
The principal distinction of flies from other 
winged insects is the transparency of their wings, 
which are neither powdered like the butterfly's, 
nor enclosed in sheath-cases like the beetle's. 
There are two general classes, according to Reau- 
mur, those with two wings, and those with four. 
The two- winged flies have, in place of the under 
