SAW-FLIES(Tenthredinidae). This subdivision of the Hymenoptera is characterized by possessing a sessile abdomen which hides the base of the posterior legs. The antennae vary in their structure and in the number of their joints. The ovipositor is modified to form two saws, which when at rest lie in a sheath formed of two valves. The larvae resemble caterpillars, but may be distinguished by their greater number of legs; usually 9 to 11 pairs are present. When alarmed they have the habit of rolling themselves up in a spiral fashion; some also discharge a thin fluid from lateral pores situated above the spiracles. The females place their eggs in small incisions made by means of their saws in the soft parts of leaves. Usually one egg is placed in each slit. Some species merely attach their eggs in strings to the exterior of the leaves. With each incision a drop of fluid is usually excreted, which serves to excite a flow of sap to the wounded part. The egg is said to absorb this sap, and so to increase in size. One genus (Nematus) alone forms galls. These occur in the young leaves of the willow, a tree which the true gall-flies do not attack. Nematus ventricosus resembles the bees and wasps in the fact that the parthenogenetic ova produce only males; as a rule in the animal kingdom the absence of fertilization results in the production of females....