NO. 8 INSECT ABDOMEN SNODGRASS 105 



and the others on the ninth. The genital processes hold the eggs 

 as the latter issue from the ovipositor, but there is present no 

 mechanism for their movement. 



An ovipositor is well developed in one suborder (Terebrantia) 

 of the Thysanoptera, but it is so minute in the ordinary species that 

 the writer has not been able to make a detailed study of its structure. 

 It consists of a large single dorsal piece presumably the united second 

 valvulae, and of two strongly serrate ventral blades. The first val- 

 vifers appear to be a pair of triangular plates on the sides of the 

 eighth segment immediately below the lateral edges of the eighth 

 tergum. A third pair of valvulae was not observed in the species 

 examined. 



VII. THE OVIPOSITOR OF HYMENOPTERA 



The ovipositor of the Hymenoptera, regardless of the shape, length, 

 or function assumed by the shaft of the organ, has the same basic 

 structure throughout the order. In its general form and in the com- 

 position of the shaft the hymenopterous ovipositor resembles the ovi- 

 positor of the Hemiptera more closely than that of the Orthoptera, 

 but it has one particular feature, namely, the articulation of the second 

 valvifers on the first valvifers, instead of on the ninth tergum, which 

 is a characteristic feature of the ovipositor of Gryllidae, though the 

 mechanism is not exactly the same in the two cases. The salient points 

 in the structure of the hymenopterous ovipositor may be summarized 

 as follows : 



1. The subgenital plate of the female is the seventh sternum, and 

 the base of the ovipositor is contained in a vestibular cavity. 



2. The eighth tergum in lower families is a dorsal plate of the usual 

 form exposed externally ; in the bees it is entirely concealed within 

 the seventh segment, its median part is reduced to a membranous 

 fold over the back, and the lateral parts form a pair of small sclerites 

 bearing the eighth spiracles. 



3. The eighth sternum is completely suppressed in all Hymenoptera, 

 though the venter of the eighth segment may be represented by a 

 fold of membrane beneath the gonopore. 



4. The first valvifers are entirely dissociated from the other parts of 

 the eighth segment and form an intimate part of the basal mechanism 

 of the ovipositor or sting. Their muscles, however, take their origin 

 on the eighth tergum. Each is a small triangular plate bearing the 

 ramus of the first valvula on its anterior end, and articulating pos- 

 teriorly by its dorsal angle with the ninth tergum, and by its ventral 

 angle with the second valvifer. 



