3 - REPORT OF THE SECRETARY. 
diptera, a sketch of the systematic arrangement of the order, with 
the genera found in North America. It occupies 221 pages, and is 
illustrated by two plates. The second part is occupied with a mono- 
graph of the American Dolichopodide. For a large portion of the 
materials on which both parts of this work are based the Institution 
is indebted to the liberal assistance of Baron Osten Sacken, though 
some interesting species were communicated to Mr. Loew by Mr. 
Le Baron, of Illinois, and by Professor Macklin, of Helsingfors, col- 
lected by Mr. Sahlberg. The types of a collection were also lent to 
him by the directors of the Hof Naturalien Kabinet, of Vienna. 
Although the materials placed at the disposal of the author were 
large, they did not reach the extent desired for the preparation of a 
complete monograph. The hope is, therefore, expressed that ad- 
ditional collections will be made to complete the work, and for this 
purpose the request is earnestly urged on all North American col- 
lectors who take an interest in this order of insects, to favor the 
enterprise by sending specimens to the Institution, which may be 
transmitted to Dr. Loew. 
The fauna of North American Dolichopodide far exceeds the 
European in the variety of forms and in the number of species. 
A striking circemstance connected with this class of insects as found 
in North America is their remarkable analogy to the remains of the 
fossil fauna of the same family preserved in amber. In both there 
is the same abundance of species of a particular genus, difficult to 
distinguish on account of their close resemblance. It would appear 
from this, that if there is a gradual variation of species under varying 
conditions of existence, this variation has been less in regard to 
American insects of this class than in those of Europe. It is import- 
ant in the progress of science not ouly to trace the limits of different 
faunas, but to compare those of a similar class in different countries. 
At present, however, this cannot be done with any degree of pre- 
cision, except in the case of the American and European insect 
fauna. In this case it is distinctly perceived that the two approach 
each other in the species of several genera, while in others the 
species are identical, and again those which are identical in both are 
very unequally represented in the two countries. Of the species 
common to Hurope and North America, it is not improbable that 
some of them should have been accidentally imported in ships from 
the former. a 
The second part, including the supplement, consists of 371 pages, 
and is illustrated by five plates. 
