ANNALS 
OF 
The Entomological Society of America 
Volume VIII DECEMBER, 19:15 Number 4 
A REVISION OF THE NORTH AMERICAN PACHYGAS- 
TERINZ WITH UNSPINED SCUTELLUM (DIPTERA). 
By J. R. MALLocH, 
Urbana, II. 
It is not by any means unusual for taxonomic workers in 
entomology to discover, when classifying genera in families 
which consist of a large number of closely allied species that 
characters which they find of considerable value for their 
purpose, and which apparently are of real generic rank in these 
groups, are found not in a number of species, but 1n individual 
species in other portions of their series. Indeed, the systematist 
almost invariably finds that by a consistent application of 
the test of his separating characters throughout a family he 
has several large groups of species set apart from each other, 
collectively, by characters the use of which in classifying a 
certain residue of species—often more closely related biologically 
than are some of the individuals of the seemingly natural 
groups which his application of certain rules has defined— 
will result in a generic separation giving a single species to 
each genus. I have had this experience in almost every family 
of insects that I have studied, but have no remedy to suggest, 
since without a knowledge of the life history of the species 
we are often unable to decide upon the limits of genera except 
arbitrarily, or-—which amounts to the same thing—by a con- 
sideration of structural details of the adults. In every order, 
differences of opinion exist regarding the status of genera, 
and still more commonly in the case of species, and, apart from 
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