INTRODUCTION. XXI 



study them first. Afterwards we must continue by way of elimi- 

 nation. As it is for the difficult genera, that it is especially 

 important to adopt a good method, let us choose, for instance, the 

 genus Odynerus and its approximates; it surely will answer our 

 purpose thoroughly. Let us suppose that the pedunculiventrated 

 genera have already been separated and that we have remaining 

 only the sessiliventrated, say the genera Alastor, Monobia, Ptero- 

 chylus, Odynerus. 



First we will exclude the Alastor, by the process of viewing 

 of the wings. Scarcely any other will remain than the Odynerus 

 and the Monobia; their species will at first appearance strike 

 one as an inextricable jumble. 



For the moment, we will not trouble ourselves about the 

 Monobia, which are rare and southerly species. We will, there- 

 fore, begin by confining ourselves to setting aside the Ancistro- 

 cerus and the Symmorphus, characterized by the suture of the 

 first abdominal segment, and we can then enter directly on the 

 study of the species. But even here we will advance as far as 

 possible without the assistance of books. 



The next operation consists in sorting out the males and females 

 in order to occupy ourselves exclusively with the latter. With 

 a little practice this separation proceeds rapidly, since the males 

 of the northern regions are distinguishable mostly by their 

 entirely yellow clypeus. For whatever may concern the excep- 

 tions (either from the females offering the same character, or from 

 the males not offering it) one has still as object of consultation, 

 the form of the antennae, organs which in most males are more 

 generally terminated by a hook or a spiral. At last, as a final 

 resource, and as an infallible means, we remove all doubts by 

 counting the number of abdominal segments. The % have only 

 7, the ? only 6. 



We will first occupy ourselves only with the ? and will group 

 the individuals by species on small boards. This is a work of 

 time and patience. To succeed, we will make use of every 

 appreciable characteristic, even of the most empirical ones. In 

 soiling out in this manner, the colors are always of the greatest 

 assistance because they are obvious to the eye. One must, as 

 much as possible, subordinate the importance oi' the spots to a 

 graduated order, in beginning with the most fixed and character- 



