120 Annals Entomological Society of America [Vol. XI, 



Tipula abdominalis, Tabanus sulcifrons and CaUiphora viri- 

 descens of the Diptera; and of the Hymenoptera, Dolerus 

 unicolor and Vespa maculata. 



II. INVESTIGATION. 



General Principles. — Before taking up any anatomical 

 description or comparison of the foregoing forms, a number of 

 preliminary points should be mentioned. 



The distal pair of appendages, as has before been noted, is 

 the cerci, and in the majority of cases in adult insects where 

 these are present they have so changed their position as to be 

 dorso-lateral, or even quite dorsal. As many of the succeeding 

 pairs of appendages as are present are, as a rule, still in a ventral 

 position, and may project beyond the caudal end of the abdo- 

 men. Therefore, it will usually seem convenient to begin 

 descriptions of the genitalia by describing the cerci first. 



Of the two sexes the female is the more generalized. A good 

 example of this point is seen in the female cockroach where all 

 four pairs of genital appendages are present, almost in their 

 primitive position so far as showing their points of attachment 

 to their respective segments is concerned, and where the total 

 number of segments found in adult insects is also present. The 

 saw-fly is another striking example of the generalized female. 

 These are both examples in primitive families of insects, but on 

 the other hand, in certain specialized groups which do not sting, 

 and which, laying their eggs merely on the flat surfaces of leaves 

 or stems, lack ovipositors, the caudal end of the abdomen pre- 

 sents a most generalized condition, the appendages in such 

 cases being, "specialized by reduction." The genital and anal 

 openings in these insects may be easily seen in a caudal view, 

 the former between the ninth and tenth sternites under the 

 edge of the ninth, and the anus in its usual position between 

 the distal tergite and its corresponding sternite. These openings 

 and whatever rudimentary appendages there may be present 

 are enclosed within a cavity formed by the invagination of the 

 segments beyond and including the eighth. From this sac-like 

 invagination, the appendages may be protruded. When ovi- 

 positors and stings are present in specialized forms, these too 

 are often concealed, or at least partially so, in pocket-like 

 cavities similar to the one described above, except that they are 

 usually somewhat longer. 



