Saw-flies — Part 1

(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 Tenthredinidae--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 pear slug, belongs to the group which has the slimy caterpillars referred to above. In fact, its scientific name, limacina, indicates this fact, since Limax is aslug.

This slimy, dark olivegreen, 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 of the female is moved rapidly with a swinging lateral motion from side to side, forming an irregular cell or pocket of an oval outline. The egg is 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.