68 BULLETIN 66, UNITED STATES NATIONAL MUSEUM. 



by force of circumstances has been the last to be studied. (See pis. 

 3,4,9, 11, and 12.) 



The stylopid, xenid, and hylechthrid heads occupy only a small 

 apical portion of the cephalothorax. The mandibles in all three 

 families are somewhat prominent. In the Stylopidse rudiments of 

 another pair of mouth parts are discernible. The shape of the mandi- 

 bles furnishes a good character in these families. The transverse slit 

 is generally curved with the bow forward. It is never very wide. 

 The shape of the entire cephalothorax is a valuable character. The 

 character of the metasternal spiracle also furnishes great aid. 



The Halictophagida? and Elenchidoe form another grouping. In 

 these the mandibles do not appear at all. The cephalothorax appears 

 as a disk witli several lobes and a transverse slit. In Halictophagidse 

 the lobes are still apical. The transverse slit is much farther back 

 than in the preceding families and in most of the genera is merely a 

 slit, but in Agdlliaphagus a very considerable area is depressed, causing 

 a deep emargination of the head. In the Elenchidoe the lobes are all 

 ventral and considerably removed from the apex. In Deinelenchus 

 these areas are very prominent and the transverse slit is narrow trans- 

 verse. In Mecynocera the areas are obsolete and the transverse slit 

 becomes a very large area, causing a deep emargination of the thorax. 

 A thin transverse veil extends forward over the cavity from the 

 thoracic edge of the slit. 



CLASSIFICATION. 



The writer has based his system of classification upon the following 

 considerations : 



1. Four types of tarsi and seven types of antenna? occur in the 

 order as known at present. Each of these types of antennae the 

 writer considers of family value. At first it seemed that the antennae 

 should be used as the first character, because of the importance of 

 their function in perpetuating the species. The coordination of 

 characters finally compelled an alteration of the scheme, following 

 Perkins's (1905) lead. 



2. It was found that as the number of tarsal joints decreased the 

 females became likewise more degenerated or differentiated from the 

 typical form; hence these two characters are the basis of the super- 

 families. The specialization of characters is also paralleled in the 

 reduction of the number of veins in the wings. Biologically the diffi- 

 culty of gaining access to new hosts becomes greater in the progression 

 of the superfamilies. 



3. The subfamily when used designates a difference in the character 

 of mouth parts. The character of the number of median genital tubes 

 in the female is also of subfamily value. 



