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Saw-Fly

1911 Encyclopedia Britannica

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The name given to the members of a well-known subdivision (Symphyta) of the Hymenoptera 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. Two of the processes of the ovipositor are modified to form saws, which when at rest lie in a sheath formed of two other processes which are modified into protective structures or valves. The larvae are usually caterpillars, but may be distinguished from the caterpillars of Lepidoptera (moths and butterflies) by the greater number of their abdominal prolegs; usually 6 to 8 pairs are present. When alarmed they roll 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.

The injury which the saw-flies inflict upon crops or young trees is almost entirely brought about by the voracious habits of the larvae. These possess well-developed mouth-appendages, by means of which they gnaw their way out of the leaf in which they have been hatched, and then eat it. In this way the turnip saw-fly (Athalia spinarum ), not to be confused with the turnip' ` fly," a beetle (Phyllotreta nemorum ), attacks the leaves of the turnip, often completely consuming the leafage of acres at a time. The pine saw-fly (Lophyrus pini ) causes great damage to plantations of young Scotch firs, devouring the buds, the leaves and even the bark of the young shoots. Other species infest currant and gooseberry bushes, consuming the soft parts of the leaves, and leaving only the tough veins. The only remedy in most cases is to collect and kill the larvae when they first appear, or to spray the plants with some arsenical wash. The best known family of saw-flies is that of the Tenthredinidae, most of whose caterpillars feed on leaves. The larvae of other families - the Cephidae and Siricidae - are internal feeders, burrowing in succulent Turnip Saw-Fly ( Athalia spinarum). SawFly (magnified, with lines to left showing natural size), caterpillars, pupa and pupa-case.

or woody stems, and their limbs are in an extremely reduced condition.