296 THE AMERICAN NATURALIST: (VoL. XXXIX. 
mentary to this in function is a row of stiff hairs or bristles on 
each posterior metatarsus, known as the calamistrum (Fig. 5). 
The spider possessing these accessory spinning organs can pro- 
duce a peculiar curled silk by using the calamistrum to comb out 
the threads from the cribellum. In some cases there is a coni- 
cal piece between the lower pair of the spinnerets, termed the 
colulus. In the tarantulas and allied spiders there is a small 
post-abdomen above the spinnerets. Near the base of the spin- 
nerets is a pair of stigmata or tracheal spiracles; these are 
sometimes placed more anteriorly, and 
even as near the base as the tip of the 
abdomen. When so situated a trans- 
verse fold or furrow is formed, known 
as the ventral furrow, or rima ventralis. Near the base of the 
abdominal venter is a transverse slit on each side. These are 
the lung-slits or epigastric furrows, which open into the breath- 
ing organs called lung-books. In some spiders there are two 
pairs of these slits, or there may be a tracheal spiracle behind 
the first pair. The hardened cuticle over the lung-books forms 
the epigastric plates. Between the plates on the median line is 
the opening of the reproductive organs. The genital aperture 
of the female is termed the epigynum or vulva, and that of the 
male, small and inconspicuous, is the epiandrium. The epigy- 
num is often of a very complicated nature, and is of much value 
in the study of species. In some cases there is a pointed piece 
projecting below the abdomen known as the finger of the epigy- 
num. The furrow behind the epigynum is called the rima geni- 
talis. In some forms there are corneous plates, shields, or 
scut€ on the abdomen, either above or below. Small circular 
spots are sometimes called sigille. 
The legs and body are clothed with hairs, bristles, scales, or 
articulated Spines. The arrangement of the latter upon the 
legs is often of great systematic value. If there are two rows 
of these under tibia I, each row of three spines, then tibia I is 
said to have spines 3-3. Sometimes these hairs or bristles form 
tufts, crests, or fringes. On the dorsum of the abdomen there 
Is sometimes a leaflike mark known as the folium. In some 
cases there are short, oblique marks in pairs on the dorsum; 

Fic. 5.— Calamistrum. 
