Insect guide > Flies > Saw Flies

Saw Flies

(Super-family Tenthredinoidea.)
The saw-flies derive their name from the fact that the ovipositor of the female is peculiarly constructed, so as to act like a saw. There are two saws set side by side in a groove underneath the body and can be shoved out and moved up and down. They are used to make the proper aperture in leaves or other vegetable tissues in which the eggs are placed. The head and thorax are wide and the base of the abdomen is not slender. The front shanks bear two spurs. The eggs are, as just indicated, laid in plant tissues, in apertures made by the female saws.

The larvae as a rule are remarkable from their resemblance to caterpillars. Some of them look so much like cutworms that one might almost expect to breed moths from them instead of flies. They have, however, from twelve to sixteen prolegs, instead of ten, which is the usual rule with the caterpillar. Many of them also have the habit of curling the body around so as to embrace the twig upon which they may be walking. Many of them feed exposed upon the leaves of plants in much the same way as do caterpillars. Others, however, are covered with slime and look more like slugs than like insect larvae, while still others are covered with a white, waxy excretion which completely disguises them.

There are a few leaf miners in this group, while in the family Nematidae are many gall makers. A few make cases in which they live. Nearly all descend to the surface of the ground to transform to pupae, and spin silken cocoons about themselves. This super-family is particularly well represented in the United States, and its species in fact seem to be more abundant in temperate and cold regions than in the tropics. About two thousand species have been described.

Many saw flies are so injurious to vegetation as to possess much economic importance; the larch saw fly (Nematus erichsonii), in certain years, has destroyed large sections of larch forests in northern New England through the work of its larvae; the imported currant worm (Nematus ribesii) is a famous enemy of currants in most parts of the United States ; the common rose slug (Monostegia rosae), next to the so-called green flies and the rose chafer, is the most abundant enemy to rose bushes in different parts of the country, while the large and handsome Cimbex americana, known as the American saw fly, is frequently found upon elms, willows and birches in sufficient numbers to almost entirely defoliate them. The yellow-spotted willow slug (Nematus ventralis, Say) is a common enemy to willows in the United States.

The life histories of all of the species just mentioned are well known, but there are very many forms which need careful study. Several of the species of the genus Pontania which make the curious galls on willow leaves are convenient forms for study, and the full and careful life history of any one of them would be a valuable contribution to science.

The super-family Tenthredinoidea was formerly considered a family - the Tenth redinidae - but it has been justifiably separated by Mr. Ashmead into eleven distinct families.

Life History of the "Pear Slug"
(Eriocampoides limacina, Retzius.)
This insect, the larva of which is commonly known as the has the slimy caterpillars referred to above. In fact, its scientific name, limacina, indicates this fact, since Limax is aslug.

This slimy, dark oliveĀ­green, slug-like creature occurs commonly upon the leaves of pear, cherry, plum and allied fruit trees during most of the summer. Frequently it occurs in such extraordinary numbers with the later broods that the leaves of the tree turn brown, die and fall to the ground in midsummer. Sometimes when the slugs are very abundant, the sound of the eating of myriads of mouths resembles the falling of a fine rain upon the leaves. There are sometimes thirty or more feeding upon a single leaf.

The adult insect is a small glossy black, four-winged fly, about one-fifth of an inch in length. The eggs are laid in April and May. The ovipositor of the female is thrust obliquely through the skin of the leaf from below, not reaching through the upper surface, however. The saw is moved rapidly with a swinging lateral motion side, forming an irregular cell or pocket of an oval quickly passed down between the plates of the ovipositor, and dropped into the pocket thus made, the time occupying little more than a minute for the operation.

A single saw fly usually deposits only one egg in the same leaf, and after laying the egg she goes around to the upper side of the leaf and examines it carefully, rests awhile, and then flies to another leaf and repeats the operation. The egg is oval, slightly flattened on one side, and remains in the leaf about two weeks. It increases in size apparently by absorbing the plant juices.

The young larva on hatching makes cut through the upper surface and crawls on the top of the leaf. At first it is nearly white in color, with a yellowish brown head. Almost immediately a slimy, olive-colored liquid begins to exude over the whole body; the head appears black under the slime, and the body becomes dark. The anterior segments of the thorax swell out and the head is retracted, so that the little larva appears club-shaped. It begins feeding on the upper surface of the leaf, eating out small holes the size of a pinhead. This work continues and increases as the larva grows until the leaf becomes entirely skeletonized. Full size is reached in less than a month. The larva casts its skin four times, and usually eats its cast skin for its first meal after each molt.

When full grown it molts a fifth time, leaving its cast skin as a slender line of slime attached to the leaf. It now appears as a light orange-yellow worm, perfectly clean and dry, with no slime. It then crawls down the plant to the ground, penetrating for half an inch or more and forming a little cell the sides of which it moistens with saliva, thus forming a kind of cocoon of firm texture, more or less impervious to water. Near Washington the first generation of larva; leaves the trees by the end of June, and a second generation begins to appear soon after; but in New York State many of the individuals of the first generation pass the winter in surface of the their cocoons. The insect hibernates below the ground, and the flies appear the following April or May.

No insect is easier to destroy than the pear slug. All of the insecticide mixtures kill it readily, and even throwing dust over the leaves will destroy it.