The males are usually smaller than the females, and have, in proportion to their size, smaller abdomens and longer legs. They are usually darker colored, especially on the head and front part of the body; and markings which are distinct in the female run together and become darker in the male. In most species these differences are not great; but in some no one would ever suppose, without other evidence, that the males and females had any relationship to each other. The most extreme cases of this kind are _Argiope_ and _Nephila_, where the male is about a tenth as large as the female. Fig. 43 represents male and female of _Nephila plumipes_ described by Wilder. [Illustration: Fig. 43.] [Illustration: Fig. 44.] [Illustration: Fig. 45.] The female of one of the common crab spiders is white as milk, with a crimson stripe on each side of the abdomen; while the male is a little brown-and-yellow spider, with dark markings of a pattern common in the family to which it belongs. In the genus _Erigone_, which includes the smallest known spiders, the males often have curious humps and horns on their heads, Fig. 44. The most extreme example is Fig. 45, where the eyes are carried up on the end of the horn. The females of all these species have plain round heads; and what use the humps are to the males nobody knows. The peculiar organs by which the adult males and females can always be distinguished are, in the males, the palpal organs, on the ends of the palpi; and, in the females, the epigynum, Fig. 1. 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. [Illustration: 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. [Illustration: 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. [Illustration: Fig. 48.]