22 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 Dolichopodida?. 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 Dolichopodidas far exceeds the 

 European in the variety of forms and in the number of species. 

 A striking circumstance 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 only 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 Europe and North America, it is not improbable that 

 some of them should have been accidentally imported in ships from 

 the former. 



The second part, including the supplement, consists of 371 pages, 

 and is illustrated by five plates. 



