OF WASHINGTON, VOLUME XII, ]<UO. 91 



It is not necessary to go on with other families; it is evident 

 that as far as insects and arachnids of this country are con- 

 cerned, one cannot make a map that will express the faunal 

 areas of all families. 



It is evident, I think, that the distribution of a group does 

 not depend entirely upon the contour of a country, its plants, 

 geological history, or distribution of other groups, but is 

 partly due to the origin of the group and to its methods of dis- 

 persal. Other causes may affect the distribution of a group; 

 such as whether its immature stages are passed under ground 

 or above ground, or whether the winter is passed in egg-stage 

 or as a partly grown insect; whether it feeds in adult condition 

 or not, etc. 



These same facts will, I believe, be found to influence the 

 distribution of a group throughout the world. 



In the family Myrmeleonidre (ant-lion flies) there are a few 

 genera like Myrmelcon and Acanthaclisis that occur in all 

 countries, and Glenurus in all tropical regions. A few gen- 

 era, like Dendroleon, are common to Europe and North 

 America, but our most characteristic genus, Brachynemurus, 

 does not occur in Europe, but in Central and South America. 



The most characteristic African and Indian genus, Pal- 

 pares, extends into Southern Europe, but not in Japan, while 

 several other European genera occur in Africa. But the Aus- 

 tralian forms, while showing affinity to the European fauna 

 by having Formicaleo and Gymnocnemia, do not have Pal- 

 pares and Toinatares or any allied forms. Nor do these latter 

 genera extend into the Malay region, or Insulinde. None of 

 the Australian forms show any particular relation to the 

 South American fauna. However, if we take the family Nem- 

 opterida-, we find that the Australian forms show relation to 

 India, Persia, South Africa, and Chile, as well as less closely 

 to North Africa and to South Europe. 



The flying power of an insect has little to do with its dstrii- 

 bution; the dragon-flies of the Eastern and Western United 

 States are more different than some groups of lesser flying 

 capacity. 



The spread of an insect is not accidental, nor due to a 

 scarcity of food, but to the desire of the fertilized female to 

 deposit her eggs away from the place of her birth. The hin- 

 drances to distribution vary with each species. 



Considering these facts, I think we should not attempt to 

 make maps of the distribution of groups much larger than 

 families, nor draw generalizations from one group expecting 

 they will apply equally as accurately to other groups. 



