PALPAL ORGANS.
As the male spider gets nearly full grown, the terminal joints of the palpi become swollen, and, after the last moult, the palpal organs are uncovered.
Fig. 46.
The simplest form of palpal organ is found in the large Mygalidæ, [Fig. 6]. It consists of a hard bulb, [Fig. 46], drawn out to a point, in which is a small hole leading to a sac within.
Fig. 47.
In most spiders the terminal joint is flattened, and has a hollow on the under side, in which the palpal organ is partly concealed. The bulb is flattened to fit this hollow; and the point of it is prolonged into a distinct tube of various shapes, furnished with numerous spines and appendages. [Fig. 47] is the palpal organ of Epeira vulgaris flattened out, and made transparent. The internal sac is shown at a; and the tube from it b runs round the inside of the bulb, and ends at the opening c. The outside of the organ has various horns and appendages. [Fig. 48] is the palpus of another spider, where the outer tube is so long, that it is coiled up over the basal part of the bulb; and the end rests on a strong spine at one side of the palpus.
Fig. 48.
Not only the terminal joints of the palpi, but also the next, and sometimes next two joints, are modified in shape, with the development of the palpal organ, [Fig. 48]. The shape of these organs is very constant in the same species of spider, and thus becomes one of the most convenient marks of such a group.