Morphologically, the ovipositor is composed of three pairs of unjointed styles (_rhabdites_ of Lacaze-Duthiers, _gonapophyses_ of Huxley), which are closely appressed to or sheathed within each other, the eggs passing out from the end of the oviduct, which lies, as Dewitz states, between the two styles of the lowest or innermost pair, and under the cross-bars or at the base of the stylets mentioned; the styles or blades spreading apart to allow of the passage of the egg. [Illustration: FIG. 185.—Saw of Hylotoma: _a_, lateral scale; _i_, saw; _f_, gorget; 7_t_, 7th tergite; 6_s_, 6th sternite; _ov_, oviduct; _in_, intestine.—After Lacaze-Duthiers. ] The ovipositor is best developed in the Thysanura (Fig. 179, Campodea excepted), in Orthoptera (Fig. 184), in the Odonata, Hemiptera, certain Physapoda, Rhaphiidæ, and in the phytophagous Hymenoptera, where it is curiously modified to form a rather complicated saw for cutting slits in wood or leaves (Fig. 185). It is wanting or quite imperfect in Coleoptera, Diptera, and Lepidoptera. Morphologically, the ovipositor appears to be formed out of the abdominal appendages of the seventh, eighth, and ninth segments of the female, which, instead of disappearing in the orders first mentioned, persist as permanent styles. Wheeler asserts from his study of the embryonic development of Xiphidium “there can be no doubt concerning the direct continuity of the embryonic appendages with the gonapophyses.” He goes on to say:— “One embryo, which had just completed katatrepis, still showed traces of all the abdominal appendages. The pairs on the eighth, ninth, and tenth segments were somewhat enlarged. In immediately succeeding stages the appendages of the second to sixth segments disappear; the pair on the seventh disappear somewhat later. Up to the time of hatching the gonapophyses could be continuously traced, since in Xiphidium there is no flexure of the abdomen, as in other forms, to obscure the ventral view of the terminal segments. From the time of hatching Dewitz has traced the development of the ovipositor in another locustid (_Locusta viridissima_), so that now we have the complete history of the organ.” Heymons, however, is inclined to believe that they are simply hypodermal outgrowths. [Illustration: FIG. 186.—Ideal plan of the structure of the ovipositor to illustrate Lacaze-Duthiers’ view: _b_, 8th tergite; _c_, epimerum; _a′_, _a_, two pieces forming the outer pair of rhabdites; _i_, the 2d pair, or stylets; and _f_, the inner pair, or sting; _d_, support of sting; _e_, piece supporting the stylet; _R_, anus; _o_, outlet of oviduct. The 7th, 8th, and 9th sternites are aborted.—After Lacaze-Duthiers. ]