46 GENUS NEMATUS. 



other, owing to some species combining to a more or 

 less extent the characters of two or even three of the 

 groups ; hence the arrangement and classification of 

 the species is a work of very great difficulty. 



The first author who separated and arranged the 

 species into groups distinguished by coloration or 

 structural peculiarities, or by both, was C. G. Thom- 

 son (Hymen. Scand., i), and his work is certainly a 

 vast improvement on anything that existed before. 

 Zaddach (Schr. Ges. Konig., xiv) followed him in this, 

 and in some respects arranged the species into more 

 natural groups, for Thomson, relying too much on 

 certain minute structures or on sculpture, separated 

 widely, it seems to me, species which agree very 

 closely in general coloration and habits. 



I have followed Zaddach generally in the grouping 

 of the species. Some of the groups might (and no 

 doubt will hereafter) be united, for with not a few of 

 the species it is difficult to say into what group they 

 should fall, and the differences between some of the 

 sections are by no means great. I believe, however, 

 that after a little study the student will have no great 

 difficulty in finding out, with the definitions I have 

 given, the section to which a given species belongs ; 

 and I have noted the points in which the groups vary 

 from, and agree with, their nearest allies. 



In specific discrimination the points of greatest im- 

 portance to be noted are the length, thickness, clothing 

 and colour of the antennae, the form of the head, of its 

 clypeus and sutures, the sculpture and clothing of the 

 thorax, the form of the abdomen and especially of the 

 last segments. The neuration of the wings is of great 

 importance, especially the relative size of the cubital 

 cellules and the position of the transverse cubital and 

 recurrent nervures. Still, too much reliance must not 

 be placed on the position of the nervures, for they 

 are not constant in any of the species, and in some 

 species vary not a little in both wings. In the legs the 

 length of the calcaria, of the tarsal joints, and the 



