Mr. Westwood, who examined these insects very closely, throws out, in his “Modern Classification of Insects,” the idea which forms the subject of this book. Writing of the cutting weapon of the Saw-flies, he remarks that “from its admirable construction it cannot be doubted that a careful examination of its various modifications might furnish ideas for improved mechanical instruments.”
Mr. Gosse, in his “Evenings at the Microscope,” points out that, beautiful and elaborate as these instruments are, they are but the sheaths of a still finer and more delicate pair of saws. These secondary saws have only a few teeth on the edge, and these near the point, whereas the sides are furnished with a number of sharp blades, set on their edges, slightly overlapping each other, and directed backwards. There is a similar structure on the ovipositor of the Sirex, as we shall see when we come to treat of Boring Instruments.
Although the saws are made expressly so that they shall not stick in the wood, there are many instances known where female Saw-flies have been found dead on the branches, their saws still in the last groove which they have cut. I am inclined to think that these must be females which have deposited all their eggs, and which have died, as do nearly all insects under similar circumstances. This opinion is strengthened by some observations made by Mr. J. K. Lord on the Cicada, the female of which is furnished with a similar ovipositor:—
“I was curious to watch the female depositing her eggs.
“She first clasps the branch on both sides with her legs, and with the ends of the file very carefully slits up the bark. Then, placing the instrument longitudinally, she files away until she has obtained sufficient length and breadth. The small teeth of the files are now used crosswise of this fissure, until a trench is made in the soft pith.
“When large enough, slowly down the groove in the centre of the instrument glides a small pearly egg, pointed at both ends, and so transparent that the little grub within is clearly discernible. Gently she lays it within its bed, and then drops a thin gummy material on it, to secure it from moisture. This finished, she proceeds to deposit another, and so on, until a sufficient number are produced to fill the fissure; then over all she drags the everted bark. It is easy to perceive where the Cicada has been concealing her brood, by the elevation on the branch.
“In this manner she deposits about seven hundred eggs, going from branch to branch, her marvellous instinct teaching her to select the most suitable wood for the purpose. The time occupied in constructing each nest was from fifteen to twenty minutes. Her earthly mission finished, she drops, fainting and exhausted, from the branch, and dies.
“The male, who is always trilling his refrain, goes on, indifferent, or unconscious, that the task of his faithful spouse is finished, singing even, until his time comes—then he too drops beside her. Thus the songs one by one cease,—not only the Cicada’s, but all the forest choir, and give place to blasts that sigh in mournful music through the leafless trees.”
The Sirex and several of the larger Ichneumon-flies are often found dead in like manner, and I have no doubt from the same cause. An elaborate description of the beautiful double saws of the Cicada is given by Mr. Westwood in the work already quoted, together with illustrations.