_Cynipoidea._--In this division the ovipositor issues from the ventral surface of the abdomen; the pronotum reaches back to the tegulae; the trochanter has two segments; the fore-wing (fig. 4, 2) has no stigma, but one or two areolets. The feelers with twelve to fifteen segments are thread-like and straight. All the insects included in this group are small and form two families--the Cynipidae and the Figitidae. They are the "gall-flies," many of the species laying eggs in various plant-tissues where the presence of the larva causes the formation of a pathological growth or gall, always of a definite form and characteristic of the species; the "oak-apple" and the bedeguar of the rose are familiar examples. Other flies of this group have the inquiline habit, laying their eggs in the galls of other species, while others again pierce the cuticle of maggots or aphids, in whose bodies their larvae live as parasites. _Chalcidoidea._--This division resembles the Cynipoidea in the position of the ovipositor, and in the two segmented trochanters. The fore-wing also has no stigma, and the whole wing is almost destitute of nervures and areolets, while the pronotum does not reach back to the tegulae, and the feelers are elbowed (fig. 7). The vast majority of this group, including nearly 5000 known species, are usually reckoned as a single family, the _Chalcididae_, comprising small insects, often of bright metallic colours, whose larvae are parasitic in insects of various orders. The "fig-insects," whose presence in ripening figs is believed essential to the proper development of the fruit, belong to _Blastophaga_ and other genera of this family. They are remarkable in having wingless males and winged females. The "polyembryonic" development of an _Encyrtus_, as studied by P. Marchal, is highly remarkable. The female lays her egg in the egg of a small ermine moth (_Hyponomeuta_) and the egg gives rise not to a single embryo but to a hundred, which develop as the host-caterpillar develops, being found at a later stage within the latter enveloped in a flexible tube. The _Mymaridae_ or "fairy-flies" are distinguished from the _Chalcididae_ by their narrow fringed wings (figs. 4, 5) and by the situation of the ovipositor just in front of the tip of the abdomen. They are among the most minute of all insects and their larvae are probably all parasitic in insects' eggs. [Illustration: After Riley and Howard, _Insect Life_, vol. i. FIG. 8.--Ichneumon Fly (_Rhyssa per-suasoria_) ovipositing.]