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.
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.