FEMALE OKGANS OF GENERATION. 197 



vagina bivalvis, but it is indicated in the shorter internal valves, which 

 in Locusta viridissima are united to the larger ones by membrane, but 

 in other instances they are found free and separate. The terebra 

 of the Tenthredos is an intermediate form ; it, consequently, does 

 not pierce firm substances, but merely guides the eggs into already 

 existing cavities ; but the aculeus forms the cavity itself for the egg, 

 pierces into bodies not firmer than itself, and as a defensive instrument 

 it wounds very severely. We may therefore distinguish the EXTERIOR 

 SHEATH (vagina aculei) and the inner STING (aculeus, sen terebra) as 

 the chief parts of this kind of ovipositor ; we will first turn our atten- 

 tion to the sheath. 



We have but little to say of the exterior sheath, for its differences 

 are unimportant. It always consists of two valves (PL XXIII. f. 6. 

 a, ), which are united by articulation with the dorsal plate of the last 

 abdominal segment, by which it is partially covered above ; the ventral 

 plate then covers it from below. They are as long as the sting itself, 

 and lying together form a tube, in which the latter is completely con- 

 cealed. If the sting project beyond the apex of the abdomen they 

 accompany it. A thus projecting sting (aculeus exsertus) Latreille 

 calls a terebra. But when the sting lies concealed within the abdomen 

 (as for example, in the bees,) the valves are there also, and they embrace 

 the concealed sting (aculeus abscondilus) precisely in the same way 

 as the exserted one. The exterior upper surface of the sheath is 

 generally rough and uneven, particularly in the projecting aculeus, 

 and entirely covered with short hair; the edges are simple, smooth, 

 and fit closely together. 



The internal sting is differently formed according to the peculiarity 

 of its function. 



In the Tenihredonodea it diverges most in form. In these it should 

 not properly be called a sting, but a saw, and indeed earlier entomologists 

 have compared it with this tool. It consists (PL XXIV. f. 8.), like 

 the sheath, of two valves (a, a, and b, b), between which at their base 

 there lies a short triangular process (c). Each internal valve has the 

 same form as the sheath enclosing it, but it is smaller, so that it can be 

 entirely embraced by it. The inferior edge of the inner valve is finely 

 toothed (PL XXIV. f. 9. ), very sharp and narrow, inwardly sepa- 

 rated by a projecting line from the remaining very smooth surface of 

 the valve. The exterior has likewise a corresponding projecting ridge 

 (the same, I), b}, which, like the ridge, is finely ajid sharply toothed ; 



