It lives in perpendicular burrows, feeding upon those insects which come within its reach. Its usual position is at the upper part of the burrow, with its jaws widely extended, so as to snap up any insect that may venture too near.
When it has secured its prey, it seeks the bottom of its burrow, makes its meal in quiet, and reascends. How it does so we shall soon see. Towards the end of the body, one of the segments is much enlarged, and has a bold prominence upon the back. On the summit of this prominence there are two horn-like hooks, shaped as seen in the illustration. These hooks are used exactly like the boy’s climbing-spurs, the alternate elongation and contraction of the body answering the same purpose as the movements of the boy’s legs. When the larva has seized its prey and wishes to retreat, all that it has to do is to withdraw the hooks, straighten the body, and down it falls by its own weight.
In the nautical branch of this subject I have already treated of the curious pushing-poles by means of which the Serpula protrudes itself from its tube. As all must have noticed who have seen these creatures alive, the Serpula protrudes itself very slowly, but flies back into its tube with such velocity that the eye can scarcely follow its movements. Its difference of motion shows that there must be a difference in the means by which these movements are produced.
Referring to the illustration on page [45], the reader will see that the instruments with which the Serpula propels itself are used just after the fashion of punt-poles, and cannot act with any great swiftness. When, however, the creature wishes to withdraw itself, it employs a curious apparatus, consisting of many rows of little hooks. The points of these hooks readily catch against the lining of the tube, and by their aid the worm jerks itself back with wonderful celerity.
Three rows of these hooks are shown next to the Tiger-beetle larva.
The structure of these remarkable organs is elaborately described by Mr. Gosse in his “Evenings with the Microscope:”—
“If you look again at this Serpula recently extracted, you will find with a lens a pale yellow line running along the upper surface of each foot, transversely to the length of the body. This is the border of an exceedingly delicate membrane, and, on placing it under a high power (say six hundred diameters), you will be astonished at the elaborate provision here made for prehension.”
“This yellow line, which cannot be appreciated by the unassisted eye, is a muscular ribbon, over which stand edgewise a multitude of what I will call combs, or rather subtriangular plates. These have a wide base, and the apex of the triangle is curved over into an abrupt hook, and then this cut into a number (from four to six) of sharp and long teeth.”
“The plates stand side by side, parallel to each other, along the whole length of the ribbon, and there are muscular fibres seen affixed to the basal side of each plate, which doubtless give it independent motion.
“I have counted one hundred and thirty-six plates on one ribbon. There are two ribbons on each thoracic segment, and there are seven such segments. Hence, we may compute the total number of prehensile comb-like plates on this portion of the body to be about one thousand nine hundred, each of which is wielded by muscles at the will of the animal; while, as each plate carries on an average five teeth, there are nearly ten thousand teeth hooked into the lining membrane of the cell, when the animal chooses to descend.”